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The distribution of Simulium damnosum theobald in Northern Nigeria
379 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE. Vol. 50. No. 4. July, 1956.
THE DISTRIBUTION OF S I M U L I U M D A M N O S U M THEOBALD IN NORTHERN NIGERIA BY R. W. CROSSKEY*
Entomologist, Medical Department, Northern Nigeria. Simulium damnosum Theobald was unknown from Northern Nigeria until 1944, when Dr. T. A. M. NASH collected some~adult female material from the K u d a r u area, on the border of eastern Zaria Province, and forwarded specimens to the British M u s e u m (Natural History) for confirmatory identification. SIMPSON(1912) had earlier collected some man-biting Simuliidae at four localities in the north of Nigeria, from two areas of Niger Province, from the upper Niger River near Bussa, and from Lokoja ; Simpson's records almost certainly relate to S. damnosum, as this is the only truly anthropophilie species of Simulium in Nigeria, and has now been collected from each of the above localities. I have lately published a few locality records of the fly together with some discussion of its bionomics (CROSSKEY, 1954, 1955). Knowledge of S. damnosum and its r61e in the transmission of onchocerciasis in Northern Nigeria was until recently extremely meagre, and the historical outline above gives all that was known until 1951. It should be noted, however, that a n u m b e r of literature records had appeared of S. damnosum from Southern Nigeria and the British Cameroons ; complete references are given by FREEMAN and DE MEILLON (1953). Since 1951, onchocerciasis surveys of much of Northern Nigeria have shown that the distribution of the disease is far wider than was previously supposed, and that several highly endemic loci exist in scattered areas of the country. T h e disease and its occular effects are discussed by BUDDEN (1955, 1956). It was thought that investigation of the entomological aspect of the disease would contribute to a proper understanding of the onchocerciasis problem in Northern Nigeria, and the mapping of the distribution of the vector was considered to be an essential preliminary for onchocerciasis control. For this purpose a survey of the geographical distribution of S. damnosum was carried out, and this paper seeks to present the information obtained. * I am greatly indebted to the late Professor P. A. Buxton for his helpful advice ; to my colleague Dr. F. H. Budden for providing information on onchocerciasis distribution and for criticizing the manuscript of this paper ; to my wife for identifying pupal material in the course of the survey ; and to Mallam Tsofo Makarfi, African assistant, for collecting material, sometimes under difficult circumstances. The permission of the Director of Medical Services, Northern Nigeria, to publish this paper is gratefully acknowledged.
380
D I S T R I B U T I O N OF
Simulium damnosum
IN
NORTHERN
NIGERIA
Table of local names for S. damnosum.
Language Hausa
Name
District or village areas in which used
Province
Kuskuru
Kudaru and Saminaka
Zaria
Kurukuru
Zungeru area
Niger
Kurukundi
Izom, Bwari
Niger
Kurukundu
Kagara, Kusheriki, Tegina
Niger
Serti Zema
Adamawa Niger/Sokoto
Shakwibi*
Abuja, Izom, Madalla
Niger
Cikwibi*
Abuja
Niger
Fulani
Mukwai
Serti, Beli
Adamawa
Nupe
Makurgi
Lafiagi, Ebba, Kutiwenji
Niger
Jukun
Yandori
Takum
Benue
Tiv
Kyowa
Katsina Ala
Benue
~J
Abunbu
Manor
Benue
Bumpwi
Abuja Uke
Niger Benue
Bumpu
Lafiagi, and Kataeregi
Niger
Bopi, Bopwi
Izom, Gwachipe
Niger
Chamba
Dori
Kogin Baba
Adamawa
Gflra
Ita
Ogugu
Kabba
Bangawa
Luntulumu
Zema
Niger/Sokoto
Koro
Akurundi
Abuja Emirate
Niger
Bassa
Angabu
Gwagwalida
Niger
Gwandara
Akirindi
Abuja Emirate
Niger
Filfiluwa
Gwari
. These two names, meaning waist-drinker in view of the biting habits of the fly.
and
waist-biter,
are singularly inappropriate
T h e S. damnosum survey was c o m m e n c e d in 1952 and completed in 1955. Each province of N o r t h e r n Nigeria was toured, but more emphasis was laid u p o n those areas in which topographical conditions m a d e the presence of the fly likely, and on those areas where the occurrence of onchocerciasis was already established. T h e area of N o r t h e r n Nigeria is 281,000 square miles, and the survey covered approximately three-quarters of the country ;
tl.
~V.
CROSSKEY
381
a general description of the country in relation to its Simuliid fauna has been given elsewhere (CROSSKEY, in press). LOCAL KNOWLEDGE AND NAMES OF S .
damnosum
Little notice of S. damnosum is taken by the native inhabitants of Northern Nigeria, even in areas where it is particularly prevalent. But nowhere is the fly a sufficient scourge to make farming or fishing irksome activities. In my experience the fly is not associated by the natives with onchocerciasis, and local knowledge extends little beyond an association of the fly with the "damina" or wet season. Nevertheless most tribes living in S. damnosum foci give the insect a vernacular name, though as a rule such names are only of extremely localized application ; even in Hausa, the lingua franca of the north, no universal name exists in spite of the wide distribution of the fly. I
*ol
~ol
~ol
JoOl
,2oi
,4ol
,o°t
,2°k
,4. t
,~o
MAP ] . - - D i s t r i b u t i o n of S. damnosum in relation to t h e river system and the positions of t h e chief towns. The stippling represents t h e general p a t t e r n of fly distribution, and the uns h a d e d p a r t s of t h e m a p r e p r e s e n t t h e negative areas. T h e area enclosed w i t h i n t h e triangle is discussed in t h e text a n d r e p r e s e n t s a heavily infested area of N i g e r P r o v i n c e w h e r e fly foci are n u m e r o u s a n d f o r m a n e t w o r k a l o n g t h e river system. 4q
601
DISTRIBUTION OF 8 .
e°]
~e
damnosum
S. damnosum, by virtue of the unusual and specialized requirements of the immature stages, shows a sporadic distribution, each focus being generally distinct and centred upon a suitable river or river-system. Only in a few areas, notably north-eastern Niger Province and southern Adamawa Province (in the Cameroons Trusteeship Territory), is the fly more or less continuous over hundreds of square miles. On Map 1 each S. damnosum focus is shown by a solid black circle, and the over-all picture of fly distribution is indicated by the stippled areas of the map. There are many areas within the stippled parts of Map 1 in which 8. damnosum either does not occur or has not been found ; nevertheless the stippled areas with their boundary lines represent broadly the pattern of fly distribution in Northern Nigeria. The loci discovered in the course of the survey are listed below province by province ; the name of the infested river is followed by its situation within the province, and where known by the approximate limits of the infested reaches.
382
DISTRIBUTION OF S i m u l i u m d a m n o s u m IN NORTHERN NIGERIA
(a) List of foci. ADAMAWA PROVINCE R. Hawal from Garkida to Kwafatu, border with Biu Division of Bornu Province. R. Song, Song town, Adamawa Division. R. Fan-Manga 30 miles south-west of Jalingo, Muri Division. R. Kam at crossing of Yola-Wukari road, Muri Division. R. Taraba near Bell, Muri Division. R. Taraba near Kogin Baba and Garbabi, Cameroons Trusteeship Territory. R. Kogin Baba, Kogin B a b a , Cameroons Trusteeship Territory. R. Serti and R. Jim, near Serti, Cameroons Trusteeship Territory. P r o v i n c i a l s u m m a r y : the fly occurs both north and south of the Benue River, with heavy infestations in the south of the province (Cameroons Trusteeship Territory), and in hilly country in the north of Adamawa Division. No loci are known from N u m a n Division. BAUCHI PROVINCE R. Kano, boundary of Bauchi and Kano Provinces. R. Bunga, nine miles above Bunga town, Bauchi Division. R. Balanga, 13 miles east of Tula, Tangale-Waja, Gombe Division. P r o v i n c i a l s u m m a r y : a very lightly infested province, with only three widely-scattered loci known. BENUE PROVINCE R. Katsina Ala, Guguwu rapids, eight miles above Katsina Ala town, Tiv Division. R. Ayia, near Vande-Kya, T i v Division. R. Sonkwala, Manor, T i v Division. Streams on the border of T i v Division with Ogoja Province (Eastern Nigeria). R. Gamana near Fikyu, 16 miles south of Takum, Wukari Division. R. Uke near Uke village, 13 miles west of Keffi, Nassarawa Division. P r o v i n c i a l s u m m a r y : loci mostly in the hilly south-eastern parts of the province, on the rivers which drain off the Adamawa Massif into the plain of the Benue River. A few very minor loci elsewhere. BORNU PROVINCE R. Hawal from Yelwa to Jelaviruwa and Lokwoja, Biu Division, boundary with Adamawa Province. Provincial summary : only the single focus above is so far known from Bornu Province, but the Hawal River is very intensely infested for 15 miles downstream from the bridge carrying the BiuYola road, and a heavy intensity of onchocerciasis occurs. ILORIN PROVINCE R. Niger from Bussa to Kurwasa, including the Bussa rapids, Borgu Division. R. Niger above and for two miles below Jebba, Ilorin Division. R. Oshin, mile 17 Ilorin-Jebba road, Ilorin Division. R. Egwa ( = R . Oro), south of Lafiagi town, Lafiagi-Pategi Division. P r o v i n c i a l s u m m a r y : heavy fly infestations occur on the Niger River in the vicinity of Bussa and Jebba ; there is little doubt that similar infestations exist on the rapids between these two points, but these were not reached in the course of the survey. A single pupa was collected from the Niger near Pategi, but it was found on trailing grass that had almost certainly been swept down from somewhere upstream, probably Jebba. Below Jebba the Niger runs over sedimentary deposits and nowhere provides conditions suitable for fly-breeding. Foci other than those on the Niger are of minor extent. It is probable that undiscovered loci exist on the large rivers of the remote parts of northern and western Borgu Division, towards the border with French Dahomey ; this country was not surveyed, partly because of its inaccessibility, but also because it is so sparsely inhabited as to be of little practical importance. KABBA PROVINCE R. Oyi east of Egbe, Kabba Division. R. Osara between Okene and Lokoja, Igbirra Division. R. Mimi, four miles west of Lokoja town, Igbirra Division. R. Obelle near Ogugu, R. Owe, and R. Ajonye near Adoru, Igala Division. R. Okulu near Allomo, Igala Division. R. Ofu near Ojifu, Idah-Ejule road, Igala Division. P r o v i n c i a l s u m m a r y : minor and dispersed loci on small, heavily shaded, rivers exist in Kabba Province both east and west of the Niger. A moderate to heavy breeding infestation exists along the Mimi River to within two miles of the provincial headquarters at Lokoja, though no breeding appears to occur in the Niger Rivet:. KANO PROVINCE R. Galma valley in the vicinity of Doguwan Giginia, Dadin Kowa, and Unguwan Mallam-Ali,
R.
W.
CROSSKEY
3.83
east of Kudaru, on the boundary of T u d u n Wada District and Zaria Province. R. Kano east of Ririwei and near Zenabi, boundary with Bauchi Province. Provincial s u m m a r y : two moderate infestations exist in the southern half of the T u d u n Wada District, on the Kano and Galma rivers and the watershed between. KATSINA PROVINCE NO known loci. NIGER PROVINCE R. Ka at Zema, 28 miles north of Zuru, Kontagora Division, border with Sokoto Province. R. Sakaba at Gulbin Boka, Kontagora Division. R. Malendo near the crossing of the Yelwa-Kontagora road, Kontagora Division, border with Sokoto Province, arid R. Malendo 16 miles north of Kontagora town. R. Kontagora 13 miles west of Kontagora town. R. Niger opposite Garafini and near the confluence of the Kontagora River, Kontagora Division. R. Mariga on the Kontagora-Tegina road, Kontagora Division. R. Kaduna above Zungeru and for five miles south of Zungeru, Kontagora Division. R. Suka, Minna town, and R. Chanchaga seven miles south of Minna, Minna Division. R. Mando, four miles west of Kusheriki ; R. Durumi, five miles east of Katako and Tegina, Minna Division. R . K a d u n a at Maikkakaki, Gunugo, Manta, Layi, Guwa, Shiroro, Galadima Kogo and Madna, Minna Division. R. Sarldn Pawa near Sarkin Pawa railway station and at the confluence with R. Kaduna, Minna Division. R. Dinya near Guni, Minna Division. R. Eba near Kutiwenji, Bida Division. R. Chanchaga near Lafiagi and R. Jatau at Ebba, Bida Division. R. Niger east bank above Jebba town, Bida Division. R. Gurara at and near Izom, including Gurara Falls, Abuja Emirate. R. Iku in and near Abuja town, and R. Tapa from Daku to confluence with R. Gurara, Abuja Emirate. R. Usuman and tributaries from Ushapa to Gwagwalida, Abuja Emirate. Provincial s u m m a r y : Niger ProVince is more highly infested than the other provinces of Northern Nigeria, although infection rates with onchocerciasis are often lower than elsewhere, for instance Adamawa Province. T h e Kaduna River valley of Minna Division and the northern half of the Abuja Emirate are more or less infested throughout, but the loci in northern Kontagora Division and in Bida Division are more isolated. Bida is much less infested than the other divisions of the province, owing to the much flatter nature of the country. T h e Kaduna River is infested almost continuously from Kaduna, in Zaria Province, to Zungeru, a distance of approximately 140 miles ; other infested rivers, the Sarkin Pawa, Dinya, Gurara, Chanchaga, and the rivers of northern Abuja Emirate, either drain to the Kaduna River or to the Niger, and together with the Kaduna River may be regarded as forming one very large fly area of several thousand square miles. T h e form of this fly area is triangular (see Map 1) and bounded approximately by the lines Kaduna-Abuja : Abuja-Minna-Zungeru : Zungeru-Kaduna. A few other minor fly loci exist near the periphery of this area. T h e north-eastern part of the fly area lies in Zaria Province, with the apex of the triangle at Kaduna ; the Kaduna River follows the north-western border of the area. T h e area as a whole appears to be the most concentrated region of S . damnosum in the country, although similar conditions may exist in the remote parts of southern A d a m a w a Province which at present are not quite so well known. PLATEAU PROVINCE. R. Mafara near Zagun, about 20 miles west of Jos, Jos Division. R. Garle, Dawaki, east of Pankshin, Pankshin Division. R. Dundai, Panyam, Pankshin Division. R. Zandur and R. Takkos, Gindiri, Pankshin Division. R. Sherigia, west of Wamba, Southern Division. River at mile 54 WambaKeffi road, three miles west of Wamba town, Southern Division. R. Mada, mile 32 Wamba-Kefiq road, Southern Division. R. Mayel, mile 70 Jos-Wamba road, Southern Division. R. Assob near Assob Falls, plateau escarpment, Jos Division. R. Kogum near Jagindi, Jema'a Division. R. Matsirga two miles north of Kafanchan town, Jema'a Division. Provincial s u m m a r y : much of Plateau Province is formed by an upland massif lying about 4,000 feet above sea-level, the Bauchi ( = J o s ) Plateau. Many of the great rivers of Northern Nigeria, for instance the Kaduna and the Gongola, rise on the Plateau and flow either north-eastwards to Lake Chad, south-eastwards to the Benue River, or south-westwards to the Niger. These rivers begin life as small streams running down the Plateau escarpments, and it is on these escarpments that most of the fly loci occur: T h e loci are generally of small extent, and of very minor importance, and give rise only to light or moderate intensities of onchocerciasis. SOKOTO PROVINCE R. Ka, at and near Dan Gulbi, Sokoto Division. R. Ka south of Bukwium, Sokoto Division, boundary with Niger Province. R. Zamfara near Takare, between Anka and Bukwium, Sokoto
384
DISTRIBUTION OF Simulium damnosum IN NORTHERN NIGERIA
Division. R. Zamfara at Jega (motor-road ford), Gwandu Division. R. Niger north and south of Yelwa, Gwandu Division. R. Malendo at Makris, east of Bin Yauri, Gwandu Division, boundary with Niger Province. Provincial summary - scattered minor foci occur in the Ka and Zamfara valleys, with more extensive areas on the Niger and its tributary the Malendo. T h e focus at Jega on the Zamfara River appears to be entirely " man-made." T o enable motor vehicles using the Sokoto-Yelwa road to cross the Zamfara, boulders have been laid in the river bed ; these boulders constrict the water through two channels, and thereby considerably accelerate the flow. Silt accumulating in the crevices between the boulders has enabled grass to gain a hold in a river-bed that is otherwise entirely of sand. T h u s swift, broken water and vegetation ideal for the breeding of S. damnosum have been artificially provided, and the fly has gained a hold. Breeding is confined to the Zamfara ford, and removal of the boulders would probably eradicate this minute focus. At present the focus is of no importance, as no onchocerciasis has been found in Jega town ; this could perhaps be due to the focus being newly established, or because no Onchocerca-carrying persons have resided in Jega. Furthermore, the fly may in this area be a nonman-biter, or it may not live sufficiently long as the adult to act as the vector, in this latitude (12 ° 18' N). The fact that onchocerciasis in moderate intensities, though producing ocular effects, occurs on the same latitude in the Zamfara valley 80 miles east of Jega makes the latter two possibilities unlikely. ZARIA PROVINCE
R. Galma, in the Kudaru area of Lere District. R. Kaduna, Kaduna township area. R. Sarkin Pawa, Sarkin Pawa town, boundary with Niger Province. R. Mando, east of Dawakin Bassa, boundary with Niger Province. R. Kaduna at Shakulape, Nakui, Kurmin Kaduna, Kaboresha, Ria, and Unguwan Barau south-west of Kaduna. R. Matsirga near Kafanchan, boundary with Plateau Province. Provincial summary : a small n u m b e r of dispersed foci, except in the south-west where foci become almost continuous with those in Minna Division and Abuja Emirate of Niger Province, This area is partly described under "Niger Province." (b)
N e g a t i v e areas.
A l t h o u g h S . d a m n o s u m is w i d e s p r e a d i n N o r t h e r n Nigeria, as t h e foregoing list of k n o w n foci will show, t h e r e are n e v e r t h e l e s s vast areas of the c o u n t r y i n w h i c h t h e fly either has n o t b e e n f o u n d or f r o m w h i c h it m a y be fairly safely a s s u m e d to b e absent. G e n e r a l l y these n e g a t i v e areas occur w h e r e t h e l a n d surface is relatively flat a n d of s e d i m e n t a r y f o r m ation ; i n such areas, often s a n d s t o n e plains, t h e rivers are as a rule s a n d y - or m u d d y b e d d e d a n d do n o t p r o v i d e s u i t a b l e c o n d i t i o n s for f l y - b r e e d i n g . T h e negative areas are those left u n s h a d e d o n M a p 1 ; t h e y are e n u m e r a t e d b e l o w i n b o t h geographical a n d a d m i n i s t r a t i v e terms. Negative areas : almost all the country lying to the north of the 11 ° N. parallel of latitude, except for parts of the Zamfara, Ka and Niger river valleys in the north-western part of the country ; the valley of the Niger downstream from a point a few miles below Jebba (Ilorin Province) ; the valley of the Benue River ; the Gongola plains of Bauchi Province. The latter negative area connects the negative area north of 11 ° with the negative area of the Benue valley, and separates widely the fly foci in the north-east of the country (northern Adamawa Province) from those on the Jos Plateau and the central regions of the country. In administrative terms the negative areas may be listed as follows : Adamawa Province : N u m a n Division and the parts of Muri and Adamawa Divisions lying along the Benue valley. Bauchi Province : Azare Division, Gombe Division except for Tangale-Waja, Bauchi Division except for parts of the south-west and n o r t h - w e s t - i.e. almost all the province. Benue Province : parts lying along the Benue valley and the Gboko plains of T i v Division. Bornu Province : almost the entire province, except Biu Division, and possibly the Gwoza hills of Dikwa Division. Ilorin Province : parts of all divisions, including Niger below Pategi. Kabba Province : most of the province except for small foci, mostly in Igala and Igbirra Divisions. Kano Province : all the province except the southern half of the T u d u n Wada District of Kano Division, and perhaps the Gwaram area. Katsina Province : the whole province, except that a small focus may exist somewhere in the vicinity of Mallumfashi, as some apparently endemic onchocerciasis has been found not far from Mallumfashi town. Niger Province : most of the Bida Division, southern Kontagora Division, parts of northern Kontagora Division, and the southern half of Abuja Emirate. Plateau Province : the bare open areas of the Plateau summit, Jos Division, north-east Pankshin Division, and probably the southernmost half of
R. W. CROSSKEY
385
Lowland Division. Sokoto Province : northern Sokoto Division, Argungu Division, and north-west Gwandu Division, i.e. most of the province except the Zamfara, Ka and Niger valleys. Zaria Province: the north-western parts of Zaria Province, and the central Zaria plain. DISCUSSION The survey has been sufficiently complete to permit some discussion of S. damnosum distribution in relation to some of the factors which are likely to influence it, and to the distribution of the disease to which it gives rise. S. damnosum is a specialized insect, and its pre-imaginal stages demand a very particular type of habitat. The distribution of this larval habitat is clearly the most important single factor determining the geographical areas occupied by the species. Certain environmental influences which might affect fly distribution are considered below, and maps are given showing the known fly foci in relation to these various factors.
Distribution and its limits in general. Northern Nigeria lies between 6 ° 30' N and 14 ° N and 3°E and 15 ° East (Map 1) ; the area is 281,000 square miles. S. damnosum is found to occur mainly between the southern boundary of the country (6 ° 30' - 9°N) and 11° North. In the north-west the range extends further than in the north-east, and reaches into the arid country north of the 12 ° parallel in Sokoto Province. Within this range the fly is not evenly distributed, but occurs only in particular areas ; these are delimited approximately as shown by the stippling on Map 1. The fly is spread over about one-third of Northern Nigeria, an area of some 90,000 square miles, but within this area are numerous localized regions in which it is not found ; an accurate estimate of the fly area cannot be made as the breeding distribution is linear along rivers, while adult distribution is far wider and is subject to immense seasonal fluctuation. Distribution into the far north appears to be limited by the absence of suitable perennial rivers, and probably also by the desiccating conditions of the prolonged dry season. This is discussed elsewhere for the family Simuliidae as a whole (CROSSKEY, in press). S. damnosum loci vary greatly in their extent and their degree of isolation ; they tend to become more sporadic in the far north towards the limits of the fly's range. M a n y of the foci of the central plateau area are of minor extent. Some well-isolated foci are, however, heavily infested and give rise to a heavy intensity of onchocerciasis, as occurs, for instance, in the Hawal valley on the border of Bornu and Adamawa Provinces. In two particular areas, the north-east of Niger Province (shown by the triangle on Map 1) and the Cameroons Trusteeship area of southern Adamawa Province, numerous infested rivers form a network and thereby give the fly an almost continuous distribution over a very large area. Distribution and physical features. (i) Geology. (Map 2). M u c h of the land surface of Northern Nigeria is formed of hard ancient pre-Cambrian rock ; along the broad low-lying valleys of the Niger and Benue and in the north-east and north-west of the country this is overlaid by Cretaceous or more recent sedimentary deposits. Where the pre-Cambrian forms the surface the rivers are often rocky bedded with rapids suitable for S. damnosum breeding, but in the sedimentary areas suitable rivers scarcely ever occur ; S. damnosum distribution is therefore closely correlated with the area of the pre-Cambrian rock (Map 2). This is well seen along the Niger River which between Yelwa (Sokoto Province) and Jebba (Ilorin Province), flows over pre.Cambrian formations, and exhibits numerous rocky stretches and sometimes well-developed
386
DISTRIBUTIONOF Simulium
damnosum
IN NORTHERNNIGERIA
rapids. Along this stretch from Yelwa to Jebba S. damnosum occurs at intervals, but is not found on the Niger further downstream where it flows over sedimentary deposits. The Jos (Bauchi) Plateau and the Biu Plateau of southern Bornu with its outliers in TangaleWaja (Bauchi Province) have been formed by the invasion of the old pre-Cambrian by more recent granites or by volcanic laval eruptions ; these intrusions have also resulted in rockybedded rivers similar to those of the pre-Cambrian and S. damnosum occurs in both the Jos Plateau and the Biu-Tangale-Waja areas. The Hawal River on the southern edge of the Biu Plateau has a much-fissured laval bed that provided a fly breeding ground for several miles.
MAP 2. - - Distribution of S. damnosum in relation to the geology of the country.
I
I CRETACEOUS, TERTIARY-PLEISTOCENE.
~
PLATEAU GRANITE.
~
VOLCANIC
ROCKS.
] i
Ioo~.
f
(ii) River-system (Map 1). Northern Nigeria is extremely well drained. The majority of the larger rivers rise in a radiating manner from the periphery of the central upland area of the country (the Jos Plateau and its westward extensions) ; in the main they fall to the Niger-Benue system, but a few rivers of the north-east drain to Lake Chad, an inland swamp at the north-eastern extremity of Nigeria. A further important series of rivers rises on the mountains of the northern Cameroons and drains north-westwards to the Benue River. Fly loci occur principally where the rivers leave the upland areas and traverse the escarpments to reach the low-lying plains. Hence loci tend to be more numerous on the periphery of hilly country, rather than in the centre ; it is particularly on the edges of the hill areas and on the steep escarpments that suitable rapids occur. In places, for instance central Abuja Emirate (Niger Province), the change from hill to plain is remarkably sudden and the character of the rivers alters from swift and rocky to placid and muddy very abruptly, the limits of the fly following this demarcation. I have found nothing to indicate thatpH in any way determines the S. damnosum breeding grounds. The p H of river water in Northern Nigeria is generally rather alkaline, except for the Niger which is about p H 7, neutrality. The waters become slightly less alkaline in the early rains when organic acids are leached from the soil.
R.
W.
387
CROSSKEY
(iii) Altitude (Map 3). The greater part of Northern Nigeria lies between 1,000 and 2,500 feet above sea-level ; the centre of the country, formed by the Bauchi Plateau, is between 3,000 and 4,500 feet, with occasional hills reaching 5,000 feet. Otherwise high altitudes occur only in the extreme south-east, on the border with French Cameroons, where the Mambila Plateau attains a height of 5,000-6,000 feet. The Niger and Benue valleys are low lying, and vary between about 800 and 600 feet respectively, where these rivers enter Nigeria, and only 130 feet at their confluence at Lokoja ; the drop on these rivers averages only one foot per'mile or less.
• ,2~
) MAP 3. - - Distribution of S. darnnosum in relation to altitude in feet above sealevel.
[~
UNDER~ooo~,."'1 tooo-2ooo ~.
~
3ooo-5ooo~
2OOO-3OOOf,.
I
o v £ . sooo,~
,
,oo.,
u
It seems improbable that altitude in itself plays any part in determining S. damnosum distribution in the north of Nigeria, where very high altitudes do not exist ; foci occur at over 4,000 feet, while other foci are found in the Niger valley at only a few hundred feet. At Victoria in the southern Cameroons S. damnosum exists at sea-level. It is nevertheless found that the majority of foci are situated between 1,000 and 2,000 feet (Map 3) for it is usually between these heights that suitable stretches of river exist. A comparison of Maps 2 and 3 shows that most of the pre-Cambrian rock formations have an altitude of 1,000-2,000 feet, and, as already stressed above, the distribution of the foci appears to depend primarily upon the geology of the country. Distribution and climate.
The climate of Northern Nigeria exhibits marked extremes of rainfall, humidity, and temperature, depending upon latitude and season. The seasons are well defined, the rains lasting from May to October in most areas, and the dry season from November to April. In the far north the wet season is shorter, lasting from May or June to September, and the dry season correspondingly lengthened. Along the southern boundary of the country and in the southern Plateau area westwards to Abuja, the rains may begin in March, and not end until November, with an occasional rainstorm in the dry season months. The adult of S. damnosum occurs in the wet season (CRoSSKEY, 1955) when the humidity is high.
388
DISTRIBUTION OF
Simulium damnosum
IN NORTHERN" NIGERIA
(i) Rainfall and dry months (Map 4). Rainfall varies between 75 inches a year in the south-east to as little as 15 inches or less in the extreme north ; over most of the country it lies between 30 and 60 inches per annum. In very localized hilly country it may exceed the latter value, as at Abuja, Niger Province, where it reaches 65-70 inches, although at Yola, on the same parallel of latitude as Abuja (about 9 ° 15' N), the average annual rainfall is only 39 inches. The average rainfall at Zaria (11 ° 5' N, 2,100 feet above sea-level) is 44 inches, or much the same as at Lokoja (7 ° 45' N,137 feet above sea-level) where it is 46 inches ; the number of dry-season months (under one inch rainfall) is however six at Zaria, but three at Lokoja. Although the length of the dry season in two different areas may be the same, the rainfall may be different, while conversely two areas with a similar rainfall may have seasons of a very different length. In areas where the dry season is comparatively short the rivers
MAP 4. - - Distribution in relation to rainfall and to the length of the dry season ; the figures on the map represent the number of dry-season months with under one inch rainfall. The approximate houndaries between areas with dry seasons of different lengths t h eare u n shown b r olines. k e nby
'.'7~[[~:i:::>-> ~
/
•
(~_-~ • " e . . ~.........~
-
UNDER 30 INC KS.
I
I 30-45 'NCHES.
[
i
~
~
~
45- 60 INCHES.
~
60-7S '~C.ES . . . . . . . .
t
/
|
(though not the small streams) flow continually throughout the year, but in areas with a long dry season all except the very largest rivers dry up for a period of many weeks, or at any rate cease to flow until the arrival of the next rains. In Northern Nigeria S. damnosum foci appear always to be centred on a perenniallyflowing river of moderate or large size, although in the late dry season the flow may be so reduced as to be scarcely detectable ; in some cases flow may have ceased altogether in one part of a river's length, but may just continue as the merest trickle (sufficient to maintain the fly breeding) in another. As I have stated elsewhere (CROSSKEY, 1955), dry season survival appears to be assured by continuous breeding on a small scale in those rivers that just manage to maintain a flow, however reduced, throughout the drought period. Fly loci therefore occur mainly in areas where the dry season is not too severe, and where adequate rainfall keeps the rivers flowing throughout the year ; they are most prevalent where the dry season is of 5 months' duration or less, and the annual rainfall is more than 45 inches (Map 4). No foci are known from the far north where the dry season is 7 months long, or the annual rainfall under 30 inches.
R.
W.
CROSSKEY
389
(ii) Temperature and humidity. While temperature and humidity in microclimatic terms undoubtedly influence S. damnosum in individual loci, it is doubtful if their broad effects serve to control fly distribution in general, except perhaps in the most northerly regions. Minimum temperatures, sufficient to prevent the occurrence of S. damnosum, do not naturally occur, and temperatures are never excessive in the rains, the season at which adult Simulium are in evidence. The mean annual maximum temperature of the country is 90-95°F over a wide area, but lower than this in the southern areas, in the central upland region, and in Northern Adamawa where it varies between 75 ° and 90°F. Only the "artificial" Jega (Sokoto Province) focus described on page 384 occurs where the mean maximum temperature is in excess of 95°F. The hot desiccating conditions and very low humidities of the far north, north of 11 ° or 12 °, might operate to prevent S. damnosum from extending its range into these arid areas, but other factors - - the absence of rocky-bedded rivers with broken water or perennial flow, or the absence of grasses in the river-beds to provide holds for the immature s t a g e s - also make this country unsuitable for the fly.
•
•
..
:...
.
• :.~.:'....: ~:. ....... :......
~.". "....-.! ::).'.i. ) i i. ". MAP 5. - - D i s t r i b u t i o n
/ m
of
S. damnosum in r e l a t i o n to "
' ~:
-
0~
~
.\. . .
-
the vegetation zones.
• . '. . . o m . . ~ - .
.
:
~
MONTANE, DERIVED GUINEA
SAVANNAH. SAVANNAH.
[
1
SUDAN SAVANNAH. SAHEL SAVANNAH.
1
~ooM.
I ~.,~
Distribution and vegetation. (Map 5). Except for the rolling open grassland of the Mambila Plateau in southern Adamawa the north of Nigeria is almost entirely clothed with savannah bush, which typically consists of small much-branched trees providing only light shade and permitting the growth of a grass cover for the ground. The Derived Savannah of the southernmost areas (south-east Ilorin Province, Kabba Province and parts of southern Benue) is a more luxuriant woodland growth than that of the north, the Sudan and Sahel Savannahs, and is interspersed with numerous relict patches of high forest, characterized by true forest species. Small forest outliers and fringing forest along rivers and streams occur further north in the Guinea Savannah woodland, which occupies about half of Northern Nigeria, and lies between the Derived and the Sudan Savannah. In the Sudan Savannah, tree growth is sparser and grass shorter than in the Guinea Savannah, and the country becomes very arid in the dry season.
390
DISTRIBUTION OF
Simuliurn damnosum
I N NORTHERN NIGERIA
The boundary between the Guinea and Sudan Savannahs shows a remarkable conformity with that between areas with 30-45 inches and areas with 45-60 inches annual rainfall (cf. Maps 4 and 5). Those parts of the Guinea Savannah where stands of heavy forest ("kurmis") exist are associated with high rainfall ; for example, northern Abuja Emirate which is situated near the centre of the Guinea Savannah zone but has a heavy local rainfall of 65-70 inches a year. S. damnosum loci exist principally in the Guinea Savannah (Map 5) ; in this zone the breeding rivers generally have a lightly or moderately developed forest fringe of larger trees than in the surrounding bush, and it is found that such trees asAdinamicrocephala ("kadanyarraft"), Vitex doniana ("dinya"), and Khaya senegalensis ("madachi") are common on the banks of infested rivers. Isoberliniasp.("doka")andButyrospermumparkii("kadanya")arethemost commonly represented woodland trees in the fly loci. A small sallow, Salix ledermannii ("baruwana"), is very often found growing in the bed of the breeding rivers in boththe Guinea and the Sudan Savannah. Only a few fly loci are known from the Sudan Savannah area, for it is in this zone that the rivers largely dry up in the dry season, and are sandy bedded ; rainfall is low, and the dry season very protracted and desiccating. Some loci, however, exist in the Zamfara and Ka valleys of Sokoto Province. In the southern parts of the country, south of the Niger and Benue, many foci are found in the area of the Derived Savannah. This zone has resulted from the invasion of the original high forest by the savannah, so that it now presents an intermixture of savannah flora with remnants of the old forest. The forest persists mainly where surface water is abundant, and consequently several breeding rivers of the Derived Savannah zone, especially in Kabba Province, are heavily shaded with oil palm (Elaeis guineensis) and other forest trees. Similar conditions occur in Abuja area of Niger Province, where outliers of the forest block exist ; here also the oil palm is found, sometime s forming heavy kurmis along the river banks. Vegetation near the breeding rivers is preferred to complete openness, although loci exist on the Jos Plateau at over 4,000 feet where there is very little vegetation ; these bare areas of the Plateau are, however, believed to be largely the result of denudation by man, although the presence of some high altitude plants suggests that the semi-montane appearance found now is partly natural. The loci of the Plateau might once have been more flourishing than they are today, for considering the apparent suitability of the rivers and streams for S. damnosum, the infestations are comparatively minor and produce only light to moderate intensities of onchocerciasis. The loci of the Sudan Savannah also occur in rather bare regions, and are only light infestations, though there is nothing to suggest the vegetation of these loci has ever been much different from its present day appearance.
Distribution in relation to population density (Map 6). For an African territory, Northern Nigeria is very densely populated ; the population (1952 census) is 17,000,000 and the mean density 60 persons to the square mile. Population is, however, not evenly distributed but tends to be concentrated in the far north (Kano, Katsina, and Sokoto Provinces), and along the southern boundary ; density on the Jos Plateau is also comparatively high, 60-150 to the square mile. Over at least half the country, population density is much lower than in these areas, and numbers only 15-60 inhabitants per square mile ; while southern Adamawa, Borgu Division of Ilorin Province, and parts of northern Niger Province are almost uninhabited.
R. W.
CROSSKEY
391
T h e majority of S. damnosum foci exist in areas where population density is low, 0-60 persons per square mile, as may be seen from Map 6 ; a n u m b e r of loci occur where the density is higher (60-150), but hardly any foci are known from areas of dense population. S. damnosum and onchocerciasis is therefore very much a rural problem, and scarcely ever an urban one. T h e large and important native towns - - such as Kano, Sokoto, Maiduguri, Zaria, Bauchi and Y o l a - lie well outside the range of fly distribution, but S. damnosum does occur at or in the vicinity of some large towns which have sprung up since European colonization ; these include Kaduna the capital, Lokoja and Minna. (As early as 1908, PARSONS had observed onchocerciasis in Lokoja). Fly breeding-grounds occur within 20 miles of Jos, the commercial tin-mining centre, and near some small native townships including Yelwa (Sokoto Province) and Abuja.
MAP 6.--Distribution of S. damnosum in relation to population density, persons per square mile. The map is approximate only and is based on the 1931 census with the density figures increased by 50%, as the population increased approximately by this amount between 1931 and the latest (1952) census; an even increase of population over the country has been assumed. UNDER
I
I ,s-6o.
15
~
60-lSO.
~
OVE. ,so.
L
ioo M.
i
S. damnosum breeding areas are mainly associated with a hilly terrain and rocky, perennial rivers. This type of country, as in southern Adamawa, the Kaduna River valley from Kaduna to Zungeru and the Abuja Emirate of Niger Province, is usually only lightly settled (0-25 persons to the square mile), in spite of the abundance of surface water. T h e rocky nature of the land often limits the area available for farming, and this perhaps serves to determine the population that the land can carry. On the other hand, the fertile plains of the far north are heavily settled, although almost all water has to be obtained from wells ; in parts of Kano Province, which becomes very arid in the dry season, the inhabitants n u m b e r 300 and more per square mile. T h e truly riverine areas with perennially-flowing rivers occur principally between 8 ° and 11 ° N, and within these latitudes most of the S. damnosum loci occur ; the population density is low. T h e area north of 11 °, where the fly is absent, supports a high population. As a generalization it may be said that S. damnosum is. associated in Northern Nigeria with areas of low or moderate population density. S. damnosum IN RELATION TO
ONCHOCERCIASIS
T h e localities where onchocerciasis is endemic in Nigeria are listed by BUDDEN (1956).
392
D I S T R I B U T I O N OF
Simulium damnosum
IN
NORTHERN
NIGERIA
Although as yet not all the known onchocerciasis areas have been investigated for S. damnosum and similarly not all the known fly areas have been surveyed for onchocerciasis, it appears from the evidence so far available that onchocerciasis does not occur in the absence of S. damnosum, and that the presence of the fly almost invariably, if not always, gives rise to the disease.* Nothing comparable to the findings of LEWIS (1953) in the Sudan has been noticed in N i g e r i a ; he found that in the northern Sudan onchocerciasis was absent, although S. damnosum occurs, but onchocerciasis was prevalent in parts of the southern Sudan, and he concluded that the longevity of the fly in the northern arid areas was reduced so that the development of Onchocerca to the infective stage could not take place. I n Nigeria the most northerly fly area yet discovered lies on the Zamfara River in Sokoto Province at 12 ° 1 0 ' 12 ° 18' N. Here, in spite of the prolonged and severe dry season, moderate intensities of onchocerciasis are found in certain villages of Anka and Bukwium Districts, and blindness results.
SUMMARY (1) This paper gives the results of a survey for the foci of Simulium damnosum in N o r t h e r n Nigeria, accompanied by. distribution maps. (2) A table of local native names for the fly is included. (3) Those negative areas which are, or m a y be assumed to be, free from S. damnosum are listed. (4) T h e distribution of the fly in relation to various environmental factors is discussed. Distribution is fixed primarily b y the geology of the country, which determines the areas of suitable breeding rivers. Altitude, vegetation and climate appear to play less part in determining distribution, except that climatic factors m a y operate to prevent the fly f r o m establishing itself in the far north. T h e absence of the fly from the far north is more certainly attributable to the absence of suitable breeding grounds. (5) S. damnosum is associated with areas of low to moderate population density ; all fly foci that have so far been surveyed for onchocerciasis have shown the presence of the disease. I n N o r t h e r n Nigeria onchocerciasis occurs even in the most northerly fly foci. REFERENCES BUODEN, F. H. (1955). Brit. J. Ophthal., 39, 321. - (1956). Trans. R. Soc. trop. Med. Hyg., 50, 366. CROSSKEY, R. W. (1954). W. Afr. reed. J., 3, 75. - (1955). Ann. trop. med. Parasit., 49, 142. FREEMAN, P. & DE MEILLON, B. (1953). Nimuliidae of the Ethiopian Region. (Nat. Hist.). LEwis, D. J. (1953). Bull. ent. Res., 43, 597. PARSONS, A. (1908). Parasitology, 1, 359. SIMPSON, J. J. (1912). Bull. ent. Res., 2, 301.
London : Brit. Mus.
* Simulium bovis de Meillon bites man avidly in certain localized areas of Northern Nigeria and in parts of Abuja Emirate, where S. damnosum also occurs, it is suspected of acting as a subsidiary vector of human onchocerciasis. That it may act as a vector is inferred from the fact that in the village of Izom a standardized infection rate with onchocerciasis of 759 inhabitants per 1,000 over 10 years of age occurs (BUDDEN, 1955, Brit. J. Ophthal., 39, 324) although the S. damnosum population is extremely small ; on the other hand the S. bovis population is very high, and biting rates up to 222 flies per boy-hour have been recorded. Furthermore, dissections of S. boris have shown the presence in the thorax of worms morphologically indistinguishable from O. volvulus, although as yet this does not constitute evidence that S. bovis is a natural vector. I am indebted to Mr. P. Freeman for confirming the identity of S. boris.
THE DISTRIBUTION OF S I M U L I U M D A M N O S U M THEOBALD IN NORTHERN NIGERIA BY R. W. CROSSKEY*
Entomologist, Medical Department, Northern Nigeria. Simulium damnosum Theobald was unknown from Northern Nigeria until 1944, when Dr. T. A. M. NASH collected some~adult female material from the K u d a r u area, on the border of eastern Zaria Province, and forwarded specimens to the British M u s e u m (Natural History) for confirmatory identification. SIMPSON(1912) had earlier collected some man-biting Simuliidae at four localities in the north of Nigeria, from two areas of Niger Province, from the upper Niger River near Bussa, and from Lokoja ; Simpson's records almost certainly relate to S. damnosum, as this is the only truly anthropophilie species of Simulium in Nigeria, and has now been collected from each of the above localities. I have lately published a few locality records of the fly together with some discussion of its bionomics (CROSSKEY, 1954, 1955). Knowledge of S. damnosum and its r61e in the transmission of onchocerciasis in Northern Nigeria was until recently extremely meagre, and the historical outline above gives all that was known until 1951. It should be noted, however, that a n u m b e r of literature records had appeared of S. damnosum from Southern Nigeria and the British Cameroons ; complete references are given by FREEMAN and DE MEILLON (1953). Since 1951, onchocerciasis surveys of much of Northern Nigeria have shown that the distribution of the disease is far wider than was previously supposed, and that several highly endemic loci exist in scattered areas of the country. T h e disease and its occular effects are discussed by BUDDEN (1955, 1956). It was thought that investigation of the entomological aspect of the disease would contribute to a proper understanding of the onchocerciasis problem in Northern Nigeria, and the mapping of the distribution of the vector was considered to be an essential preliminary for onchocerciasis control. For this purpose a survey of the geographical distribution of S. damnosum was carried out, and this paper seeks to present the information obtained. * I am greatly indebted to the late Professor P. A. Buxton for his helpful advice ; to my colleague Dr. F. H. Budden for providing information on onchocerciasis distribution and for criticizing the manuscript of this paper ; to my wife for identifying pupal material in the course of the survey ; and to Mallam Tsofo Makarfi, African assistant, for collecting material, sometimes under difficult circumstances. The permission of the Director of Medical Services, Northern Nigeria, to publish this paper is gratefully acknowledged.
380
D I S T R I B U T I O N OF
Simulium damnosum
IN
NORTHERN
NIGERIA
Table of local names for S. damnosum.
Language Hausa
Name
District or village areas in which used
Province
Kuskuru
Kudaru and Saminaka
Zaria
Kurukuru
Zungeru area
Niger
Kurukundi
Izom, Bwari
Niger
Kurukundu
Kagara, Kusheriki, Tegina
Niger
Serti Zema
Adamawa Niger/Sokoto
Shakwibi*
Abuja, Izom, Madalla
Niger
Cikwibi*
Abuja
Niger
Fulani
Mukwai
Serti, Beli
Adamawa
Nupe
Makurgi
Lafiagi, Ebba, Kutiwenji
Niger
Jukun
Yandori
Takum
Benue
Tiv
Kyowa
Katsina Ala
Benue
~J
Abunbu
Manor
Benue
Bumpwi
Abuja Uke
Niger Benue
Bumpu
Lafiagi, and Kataeregi
Niger
Bopi, Bopwi
Izom, Gwachipe
Niger
Chamba
Dori
Kogin Baba
Adamawa
Gflra
Ita
Ogugu
Kabba
Bangawa
Luntulumu
Zema
Niger/Sokoto
Koro
Akurundi
Abuja Emirate
Niger
Bassa
Angabu
Gwagwalida
Niger
Gwandara
Akirindi
Abuja Emirate
Niger
Filfiluwa
Gwari
. These two names, meaning waist-drinker in view of the biting habits of the fly.
and
waist-biter,
are singularly inappropriate
T h e S. damnosum survey was c o m m e n c e d in 1952 and completed in 1955. Each province of N o r t h e r n Nigeria was toured, but more emphasis was laid u p o n those areas in which topographical conditions m a d e the presence of the fly likely, and on those areas where the occurrence of onchocerciasis was already established. T h e area of N o r t h e r n Nigeria is 281,000 square miles, and the survey covered approximately three-quarters of the country ;
tl.
~V.
CROSSKEY
381
a general description of the country in relation to its Simuliid fauna has been given elsewhere (CROSSKEY, in press). LOCAL KNOWLEDGE AND NAMES OF S .
damnosum
Little notice of S. damnosum is taken by the native inhabitants of Northern Nigeria, even in areas where it is particularly prevalent. But nowhere is the fly a sufficient scourge to make farming or fishing irksome activities. In my experience the fly is not associated by the natives with onchocerciasis, and local knowledge extends little beyond an association of the fly with the "damina" or wet season. Nevertheless most tribes living in S. damnosum foci give the insect a vernacular name, though as a rule such names are only of extremely localized application ; even in Hausa, the lingua franca of the north, no universal name exists in spite of the wide distribution of the fly. I
*ol
~ol
~ol
JoOl
,2oi
,4ol
,o°t
,2°k
,4. t
,~o
MAP ] . - - D i s t r i b u t i o n of S. damnosum in relation to t h e river system and the positions of t h e chief towns. The stippling represents t h e general p a t t e r n of fly distribution, and the uns h a d e d p a r t s of t h e m a p r e p r e s e n t t h e negative areas. T h e area enclosed w i t h i n t h e triangle is discussed in t h e text a n d r e p r e s e n t s a heavily infested area of N i g e r P r o v i n c e w h e r e fly foci are n u m e r o u s a n d f o r m a n e t w o r k a l o n g t h e river system. 4q
601
DISTRIBUTION OF 8 .
e°]
~e
damnosum
S. damnosum, by virtue of the unusual and specialized requirements of the immature stages, shows a sporadic distribution, each focus being generally distinct and centred upon a suitable river or river-system. Only in a few areas, notably north-eastern Niger Province and southern Adamawa Province (in the Cameroons Trusteeship Territory), is the fly more or less continuous over hundreds of square miles. On Map 1 each S. damnosum focus is shown by a solid black circle, and the over-all picture of fly distribution is indicated by the stippled areas of the map. There are many areas within the stippled parts of Map 1 in which 8. damnosum either does not occur or has not been found ; nevertheless the stippled areas with their boundary lines represent broadly the pattern of fly distribution in Northern Nigeria. The loci discovered in the course of the survey are listed below province by province ; the name of the infested river is followed by its situation within the province, and where known by the approximate limits of the infested reaches.
382
DISTRIBUTION OF S i m u l i u m d a m n o s u m IN NORTHERN NIGERIA
(a) List of foci. ADAMAWA PROVINCE R. Hawal from Garkida to Kwafatu, border with Biu Division of Bornu Province. R. Song, Song town, Adamawa Division. R. Fan-Manga 30 miles south-west of Jalingo, Muri Division. R. Kam at crossing of Yola-Wukari road, Muri Division. R. Taraba near Bell, Muri Division. R. Taraba near Kogin Baba and Garbabi, Cameroons Trusteeship Territory. R. Kogin Baba, Kogin B a b a , Cameroons Trusteeship Territory. R. Serti and R. Jim, near Serti, Cameroons Trusteeship Territory. P r o v i n c i a l s u m m a r y : the fly occurs both north and south of the Benue River, with heavy infestations in the south of the province (Cameroons Trusteeship Territory), and in hilly country in the north of Adamawa Division. No loci are known from N u m a n Division. BAUCHI PROVINCE R. Kano, boundary of Bauchi and Kano Provinces. R. Bunga, nine miles above Bunga town, Bauchi Division. R. Balanga, 13 miles east of Tula, Tangale-Waja, Gombe Division. P r o v i n c i a l s u m m a r y : a very lightly infested province, with only three widely-scattered loci known. BENUE PROVINCE R. Katsina Ala, Guguwu rapids, eight miles above Katsina Ala town, Tiv Division. R. Ayia, near Vande-Kya, T i v Division. R. Sonkwala, Manor, T i v Division. Streams on the border of T i v Division with Ogoja Province (Eastern Nigeria). R. Gamana near Fikyu, 16 miles south of Takum, Wukari Division. R. Uke near Uke village, 13 miles west of Keffi, Nassarawa Division. P r o v i n c i a l s u m m a r y : loci mostly in the hilly south-eastern parts of the province, on the rivers which drain off the Adamawa Massif into the plain of the Benue River. A few very minor loci elsewhere. BORNU PROVINCE R. Hawal from Yelwa to Jelaviruwa and Lokwoja, Biu Division, boundary with Adamawa Province. Provincial summary : only the single focus above is so far known from Bornu Province, but the Hawal River is very intensely infested for 15 miles downstream from the bridge carrying the BiuYola road, and a heavy intensity of onchocerciasis occurs. ILORIN PROVINCE R. Niger from Bussa to Kurwasa, including the Bussa rapids, Borgu Division. R. Niger above and for two miles below Jebba, Ilorin Division. R. Oshin, mile 17 Ilorin-Jebba road, Ilorin Division. R. Egwa ( = R . Oro), south of Lafiagi town, Lafiagi-Pategi Division. P r o v i n c i a l s u m m a r y : heavy fly infestations occur on the Niger River in the vicinity of Bussa and Jebba ; there is little doubt that similar infestations exist on the rapids between these two points, but these were not reached in the course of the survey. A single pupa was collected from the Niger near Pategi, but it was found on trailing grass that had almost certainly been swept down from somewhere upstream, probably Jebba. Below Jebba the Niger runs over sedimentary deposits and nowhere provides conditions suitable for fly-breeding. Foci other than those on the Niger are of minor extent. It is probable that undiscovered loci exist on the large rivers of the remote parts of northern and western Borgu Division, towards the border with French Dahomey ; this country was not surveyed, partly because of its inaccessibility, but also because it is so sparsely inhabited as to be of little practical importance. KABBA PROVINCE R. Oyi east of Egbe, Kabba Division. R. Osara between Okene and Lokoja, Igbirra Division. R. Mimi, four miles west of Lokoja town, Igbirra Division. R. Obelle near Ogugu, R. Owe, and R. Ajonye near Adoru, Igala Division. R. Okulu near Allomo, Igala Division. R. Ofu near Ojifu, Idah-Ejule road, Igala Division. P r o v i n c i a l s u m m a r y : minor and dispersed loci on small, heavily shaded, rivers exist in Kabba Province both east and west of the Niger. A moderate to heavy breeding infestation exists along the Mimi River to within two miles of the provincial headquarters at Lokoja, though no breeding appears to occur in the Niger Rivet:. KANO PROVINCE R. Galma valley in the vicinity of Doguwan Giginia, Dadin Kowa, and Unguwan Mallam-Ali,
R.
W.
CROSSKEY
3.83
east of Kudaru, on the boundary of T u d u n Wada District and Zaria Province. R. Kano east of Ririwei and near Zenabi, boundary with Bauchi Province. Provincial s u m m a r y : two moderate infestations exist in the southern half of the T u d u n Wada District, on the Kano and Galma rivers and the watershed between. KATSINA PROVINCE NO known loci. NIGER PROVINCE R. Ka at Zema, 28 miles north of Zuru, Kontagora Division, border with Sokoto Province. R. Sakaba at Gulbin Boka, Kontagora Division. R. Malendo near the crossing of the Yelwa-Kontagora road, Kontagora Division, border with Sokoto Province, arid R. Malendo 16 miles north of Kontagora town. R. Kontagora 13 miles west of Kontagora town. R. Niger opposite Garafini and near the confluence of the Kontagora River, Kontagora Division. R. Mariga on the Kontagora-Tegina road, Kontagora Division. R. Kaduna above Zungeru and for five miles south of Zungeru, Kontagora Division. R. Suka, Minna town, and R. Chanchaga seven miles south of Minna, Minna Division. R. Mando, four miles west of Kusheriki ; R. Durumi, five miles east of Katako and Tegina, Minna Division. R . K a d u n a at Maikkakaki, Gunugo, Manta, Layi, Guwa, Shiroro, Galadima Kogo and Madna, Minna Division. R. Sarldn Pawa near Sarkin Pawa railway station and at the confluence with R. Kaduna, Minna Division. R. Dinya near Guni, Minna Division. R. Eba near Kutiwenji, Bida Division. R. Chanchaga near Lafiagi and R. Jatau at Ebba, Bida Division. R. Niger east bank above Jebba town, Bida Division. R. Gurara at and near Izom, including Gurara Falls, Abuja Emirate. R. Iku in and near Abuja town, and R. Tapa from Daku to confluence with R. Gurara, Abuja Emirate. R. Usuman and tributaries from Ushapa to Gwagwalida, Abuja Emirate. Provincial s u m m a r y : Niger ProVince is more highly infested than the other provinces of Northern Nigeria, although infection rates with onchocerciasis are often lower than elsewhere, for instance Adamawa Province. T h e Kaduna River valley of Minna Division and the northern half of the Abuja Emirate are more or less infested throughout, but the loci in northern Kontagora Division and in Bida Division are more isolated. Bida is much less infested than the other divisions of the province, owing to the much flatter nature of the country. T h e Kaduna River is infested almost continuously from Kaduna, in Zaria Province, to Zungeru, a distance of approximately 140 miles ; other infested rivers, the Sarkin Pawa, Dinya, Gurara, Chanchaga, and the rivers of northern Abuja Emirate, either drain to the Kaduna River or to the Niger, and together with the Kaduna River may be regarded as forming one very large fly area of several thousand square miles. T h e form of this fly area is triangular (see Map 1) and bounded approximately by the lines Kaduna-Abuja : Abuja-Minna-Zungeru : Zungeru-Kaduna. A few other minor fly loci exist near the periphery of this area. T h e north-eastern part of the fly area lies in Zaria Province, with the apex of the triangle at Kaduna ; the Kaduna River follows the north-western border of the area. T h e area as a whole appears to be the most concentrated region of S . damnosum in the country, although similar conditions may exist in the remote parts of southern A d a m a w a Province which at present are not quite so well known. PLATEAU PROVINCE. R. Mafara near Zagun, about 20 miles west of Jos, Jos Division. R. Garle, Dawaki, east of Pankshin, Pankshin Division. R. Dundai, Panyam, Pankshin Division. R. Zandur and R. Takkos, Gindiri, Pankshin Division. R. Sherigia, west of Wamba, Southern Division. River at mile 54 WambaKeffi road, three miles west of Wamba town, Southern Division. R. Mada, mile 32 Wamba-Kefiq road, Southern Division. R. Mayel, mile 70 Jos-Wamba road, Southern Division. R. Assob near Assob Falls, plateau escarpment, Jos Division. R. Kogum near Jagindi, Jema'a Division. R. Matsirga two miles north of Kafanchan town, Jema'a Division. Provincial s u m m a r y : much of Plateau Province is formed by an upland massif lying about 4,000 feet above sea-level, the Bauchi ( = J o s ) Plateau. Many of the great rivers of Northern Nigeria, for instance the Kaduna and the Gongola, rise on the Plateau and flow either north-eastwards to Lake Chad, south-eastwards to the Benue River, or south-westwards to the Niger. These rivers begin life as small streams running down the Plateau escarpments, and it is on these escarpments that most of the fly loci occur: T h e loci are generally of small extent, and of very minor importance, and give rise only to light or moderate intensities of onchocerciasis. SOKOTO PROVINCE R. Ka, at and near Dan Gulbi, Sokoto Division. R. Ka south of Bukwium, Sokoto Division, boundary with Niger Province. R. Zamfara near Takare, between Anka and Bukwium, Sokoto
384
DISTRIBUTION OF Simulium damnosum IN NORTHERN NIGERIA
Division. R. Zamfara at Jega (motor-road ford), Gwandu Division. R. Niger north and south of Yelwa, Gwandu Division. R. Malendo at Makris, east of Bin Yauri, Gwandu Division, boundary with Niger Province. Provincial summary - scattered minor foci occur in the Ka and Zamfara valleys, with more extensive areas on the Niger and its tributary the Malendo. T h e focus at Jega on the Zamfara River appears to be entirely " man-made." T o enable motor vehicles using the Sokoto-Yelwa road to cross the Zamfara, boulders have been laid in the river bed ; these boulders constrict the water through two channels, and thereby considerably accelerate the flow. Silt accumulating in the crevices between the boulders has enabled grass to gain a hold in a river-bed that is otherwise entirely of sand. T h u s swift, broken water and vegetation ideal for the breeding of S. damnosum have been artificially provided, and the fly has gained a hold. Breeding is confined to the Zamfara ford, and removal of the boulders would probably eradicate this minute focus. At present the focus is of no importance, as no onchocerciasis has been found in Jega town ; this could perhaps be due to the focus being newly established, or because no Onchocerca-carrying persons have resided in Jega. Furthermore, the fly may in this area be a nonman-biter, or it may not live sufficiently long as the adult to act as the vector, in this latitude (12 ° 18' N). The fact that onchocerciasis in moderate intensities, though producing ocular effects, occurs on the same latitude in the Zamfara valley 80 miles east of Jega makes the latter two possibilities unlikely. ZARIA PROVINCE
R. Galma, in the Kudaru area of Lere District. R. Kaduna, Kaduna township area. R. Sarkin Pawa, Sarkin Pawa town, boundary with Niger Province. R. Mando, east of Dawakin Bassa, boundary with Niger Province. R. Kaduna at Shakulape, Nakui, Kurmin Kaduna, Kaboresha, Ria, and Unguwan Barau south-west of Kaduna. R. Matsirga near Kafanchan, boundary with Plateau Province. Provincial summary : a small n u m b e r of dispersed foci, except in the south-west where foci become almost continuous with those in Minna Division and Abuja Emirate of Niger Province, This area is partly described under "Niger Province." (b)
N e g a t i v e areas.
A l t h o u g h S . d a m n o s u m is w i d e s p r e a d i n N o r t h e r n Nigeria, as t h e foregoing list of k n o w n foci will show, t h e r e are n e v e r t h e l e s s vast areas of the c o u n t r y i n w h i c h t h e fly either has n o t b e e n f o u n d or f r o m w h i c h it m a y be fairly safely a s s u m e d to b e absent. G e n e r a l l y these n e g a t i v e areas occur w h e r e t h e l a n d surface is relatively flat a n d of s e d i m e n t a r y f o r m ation ; i n such areas, often s a n d s t o n e plains, t h e rivers are as a rule s a n d y - or m u d d y b e d d e d a n d do n o t p r o v i d e s u i t a b l e c o n d i t i o n s for f l y - b r e e d i n g . T h e negative areas are those left u n s h a d e d o n M a p 1 ; t h e y are e n u m e r a t e d b e l o w i n b o t h geographical a n d a d m i n i s t r a t i v e terms. Negative areas : almost all the country lying to the north of the 11 ° N. parallel of latitude, except for parts of the Zamfara, Ka and Niger river valleys in the north-western part of the country ; the valley of the Niger downstream from a point a few miles below Jebba (Ilorin Province) ; the valley of the Benue River ; the Gongola plains of Bauchi Province. The latter negative area connects the negative area north of 11 ° with the negative area of the Benue valley, and separates widely the fly foci in the north-east of the country (northern Adamawa Province) from those on the Jos Plateau and the central regions of the country. In administrative terms the negative areas may be listed as follows : Adamawa Province : N u m a n Division and the parts of Muri and Adamawa Divisions lying along the Benue valley. Bauchi Province : Azare Division, Gombe Division except for Tangale-Waja, Bauchi Division except for parts of the south-west and n o r t h - w e s t - i.e. almost all the province. Benue Province : parts lying along the Benue valley and the Gboko plains of T i v Division. Bornu Province : almost the entire province, except Biu Division, and possibly the Gwoza hills of Dikwa Division. Ilorin Province : parts of all divisions, including Niger below Pategi. Kabba Province : most of the province except for small foci, mostly in Igala and Igbirra Divisions. Kano Province : all the province except the southern half of the T u d u n Wada District of Kano Division, and perhaps the Gwaram area. Katsina Province : the whole province, except that a small focus may exist somewhere in the vicinity of Mallumfashi, as some apparently endemic onchocerciasis has been found not far from Mallumfashi town. Niger Province : most of the Bida Division, southern Kontagora Division, parts of northern Kontagora Division, and the southern half of Abuja Emirate. Plateau Province : the bare open areas of the Plateau summit, Jos Division, north-east Pankshin Division, and probably the southernmost half of
R. W. CROSSKEY
385
Lowland Division. Sokoto Province : northern Sokoto Division, Argungu Division, and north-west Gwandu Division, i.e. most of the province except the Zamfara, Ka and Niger valleys. Zaria Province: the north-western parts of Zaria Province, and the central Zaria plain. DISCUSSION The survey has been sufficiently complete to permit some discussion of S. damnosum distribution in relation to some of the factors which are likely to influence it, and to the distribution of the disease to which it gives rise. S. damnosum is a specialized insect, and its pre-imaginal stages demand a very particular type of habitat. The distribution of this larval habitat is clearly the most important single factor determining the geographical areas occupied by the species. Certain environmental influences which might affect fly distribution are considered below, and maps are given showing the known fly foci in relation to these various factors.
Distribution and its limits in general. Northern Nigeria lies between 6 ° 30' N and 14 ° N and 3°E and 15 ° East (Map 1) ; the area is 281,000 square miles. S. damnosum is found to occur mainly between the southern boundary of the country (6 ° 30' - 9°N) and 11° North. In the north-west the range extends further than in the north-east, and reaches into the arid country north of the 12 ° parallel in Sokoto Province. Within this range the fly is not evenly distributed, but occurs only in particular areas ; these are delimited approximately as shown by the stippling on Map 1. The fly is spread over about one-third of Northern Nigeria, an area of some 90,000 square miles, but within this area are numerous localized regions in which it is not found ; an accurate estimate of the fly area cannot be made as the breeding distribution is linear along rivers, while adult distribution is far wider and is subject to immense seasonal fluctuation. Distribution into the far north appears to be limited by the absence of suitable perennial rivers, and probably also by the desiccating conditions of the prolonged dry season. This is discussed elsewhere for the family Simuliidae as a whole (CROSSKEY, in press). S. damnosum loci vary greatly in their extent and their degree of isolation ; they tend to become more sporadic in the far north towards the limits of the fly's range. M a n y of the foci of the central plateau area are of minor extent. Some well-isolated foci are, however, heavily infested and give rise to a heavy intensity of onchocerciasis, as occurs, for instance, in the Hawal valley on the border of Bornu and Adamawa Provinces. In two particular areas, the north-east of Niger Province (shown by the triangle on Map 1) and the Cameroons Trusteeship area of southern Adamawa Province, numerous infested rivers form a network and thereby give the fly an almost continuous distribution over a very large area. Distribution and physical features. (i) Geology. (Map 2). M u c h of the land surface of Northern Nigeria is formed of hard ancient pre-Cambrian rock ; along the broad low-lying valleys of the Niger and Benue and in the north-east and north-west of the country this is overlaid by Cretaceous or more recent sedimentary deposits. Where the pre-Cambrian forms the surface the rivers are often rocky bedded with rapids suitable for S. damnosum breeding, but in the sedimentary areas suitable rivers scarcely ever occur ; S. damnosum distribution is therefore closely correlated with the area of the pre-Cambrian rock (Map 2). This is well seen along the Niger River which between Yelwa (Sokoto Province) and Jebba (Ilorin Province), flows over pre.Cambrian formations, and exhibits numerous rocky stretches and sometimes well-developed
386
DISTRIBUTIONOF Simulium
damnosum
IN NORTHERNNIGERIA
rapids. Along this stretch from Yelwa to Jebba S. damnosum occurs at intervals, but is not found on the Niger further downstream where it flows over sedimentary deposits. The Jos (Bauchi) Plateau and the Biu Plateau of southern Bornu with its outliers in TangaleWaja (Bauchi Province) have been formed by the invasion of the old pre-Cambrian by more recent granites or by volcanic laval eruptions ; these intrusions have also resulted in rockybedded rivers similar to those of the pre-Cambrian and S. damnosum occurs in both the Jos Plateau and the Biu-Tangale-Waja areas. The Hawal River on the southern edge of the Biu Plateau has a much-fissured laval bed that provided a fly breeding ground for several miles.
MAP 2. - - Distribution of S. damnosum in relation to the geology of the country.
I
I CRETACEOUS, TERTIARY-PLEISTOCENE.
~
PLATEAU GRANITE.
~
VOLCANIC
ROCKS.
] i
Ioo~.
f
(ii) River-system (Map 1). Northern Nigeria is extremely well drained. The majority of the larger rivers rise in a radiating manner from the periphery of the central upland area of the country (the Jos Plateau and its westward extensions) ; in the main they fall to the Niger-Benue system, but a few rivers of the north-east drain to Lake Chad, an inland swamp at the north-eastern extremity of Nigeria. A further important series of rivers rises on the mountains of the northern Cameroons and drains north-westwards to the Benue River. Fly loci occur principally where the rivers leave the upland areas and traverse the escarpments to reach the low-lying plains. Hence loci tend to be more numerous on the periphery of hilly country, rather than in the centre ; it is particularly on the edges of the hill areas and on the steep escarpments that suitable rapids occur. In places, for instance central Abuja Emirate (Niger Province), the change from hill to plain is remarkably sudden and the character of the rivers alters from swift and rocky to placid and muddy very abruptly, the limits of the fly following this demarcation. I have found nothing to indicate thatpH in any way determines the S. damnosum breeding grounds. The p H of river water in Northern Nigeria is generally rather alkaline, except for the Niger which is about p H 7, neutrality. The waters become slightly less alkaline in the early rains when organic acids are leached from the soil.
R.
W.
387
CROSSKEY
(iii) Altitude (Map 3). The greater part of Northern Nigeria lies between 1,000 and 2,500 feet above sea-level ; the centre of the country, formed by the Bauchi Plateau, is between 3,000 and 4,500 feet, with occasional hills reaching 5,000 feet. Otherwise high altitudes occur only in the extreme south-east, on the border with French Cameroons, where the Mambila Plateau attains a height of 5,000-6,000 feet. The Niger and Benue valleys are low lying, and vary between about 800 and 600 feet respectively, where these rivers enter Nigeria, and only 130 feet at their confluence at Lokoja ; the drop on these rivers averages only one foot per'mile or less.
• ,2~
) MAP 3. - - Distribution of S. darnnosum in relation to altitude in feet above sealevel.
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UNDER~ooo~,."'1 tooo-2ooo ~.
~
3ooo-5ooo~
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I
o v £ . sooo,~
,
,oo.,
u
It seems improbable that altitude in itself plays any part in determining S. damnosum distribution in the north of Nigeria, where very high altitudes do not exist ; foci occur at over 4,000 feet, while other foci are found in the Niger valley at only a few hundred feet. At Victoria in the southern Cameroons S. damnosum exists at sea-level. It is nevertheless found that the majority of foci are situated between 1,000 and 2,000 feet (Map 3) for it is usually between these heights that suitable stretches of river exist. A comparison of Maps 2 and 3 shows that most of the pre-Cambrian rock formations have an altitude of 1,000-2,000 feet, and, as already stressed above, the distribution of the foci appears to depend primarily upon the geology of the country. Distribution and climate.
The climate of Northern Nigeria exhibits marked extremes of rainfall, humidity, and temperature, depending upon latitude and season. The seasons are well defined, the rains lasting from May to October in most areas, and the dry season from November to April. In the far north the wet season is shorter, lasting from May or June to September, and the dry season correspondingly lengthened. Along the southern boundary of the country and in the southern Plateau area westwards to Abuja, the rains may begin in March, and not end until November, with an occasional rainstorm in the dry season months. The adult of S. damnosum occurs in the wet season (CRoSSKEY, 1955) when the humidity is high.
388
DISTRIBUTION OF
Simulium damnosum
IN NORTHERN" NIGERIA
(i) Rainfall and dry months (Map 4). Rainfall varies between 75 inches a year in the south-east to as little as 15 inches or less in the extreme north ; over most of the country it lies between 30 and 60 inches per annum. In very localized hilly country it may exceed the latter value, as at Abuja, Niger Province, where it reaches 65-70 inches, although at Yola, on the same parallel of latitude as Abuja (about 9 ° 15' N), the average annual rainfall is only 39 inches. The average rainfall at Zaria (11 ° 5' N, 2,100 feet above sea-level) is 44 inches, or much the same as at Lokoja (7 ° 45' N,137 feet above sea-level) where it is 46 inches ; the number of dry-season months (under one inch rainfall) is however six at Zaria, but three at Lokoja. Although the length of the dry season in two different areas may be the same, the rainfall may be different, while conversely two areas with a similar rainfall may have seasons of a very different length. In areas where the dry season is comparatively short the rivers
MAP 4. - - Distribution in relation to rainfall and to the length of the dry season ; the figures on the map represent the number of dry-season months with under one inch rainfall. The approximate houndaries between areas with dry seasons of different lengths t h eare u n shown b r olines. k e nby
'.'7~[[~:i:::>-> ~
/
•
(~_-~ • " e . . ~.........~
-
UNDER 30 INC KS.
I
I 30-45 'NCHES.
[
i
~
~
~
45- 60 INCHES.
~
60-7S '~C.ES . . . . . . . .
t
/
|
(though not the small streams) flow continually throughout the year, but in areas with a long dry season all except the very largest rivers dry up for a period of many weeks, or at any rate cease to flow until the arrival of the next rains. In Northern Nigeria S. damnosum foci appear always to be centred on a perenniallyflowing river of moderate or large size, although in the late dry season the flow may be so reduced as to be scarcely detectable ; in some cases flow may have ceased altogether in one part of a river's length, but may just continue as the merest trickle (sufficient to maintain the fly breeding) in another. As I have stated elsewhere (CROSSKEY, 1955), dry season survival appears to be assured by continuous breeding on a small scale in those rivers that just manage to maintain a flow, however reduced, throughout the drought period. Fly loci therefore occur mainly in areas where the dry season is not too severe, and where adequate rainfall keeps the rivers flowing throughout the year ; they are most prevalent where the dry season is of 5 months' duration or less, and the annual rainfall is more than 45 inches (Map 4). No foci are known from the far north where the dry season is 7 months long, or the annual rainfall under 30 inches.
R.
W.
CROSSKEY
389
(ii) Temperature and humidity. While temperature and humidity in microclimatic terms undoubtedly influence S. damnosum in individual loci, it is doubtful if their broad effects serve to control fly distribution in general, except perhaps in the most northerly regions. Minimum temperatures, sufficient to prevent the occurrence of S. damnosum, do not naturally occur, and temperatures are never excessive in the rains, the season at which adult Simulium are in evidence. The mean annual maximum temperature of the country is 90-95°F over a wide area, but lower than this in the southern areas, in the central upland region, and in Northern Adamawa where it varies between 75 ° and 90°F. Only the "artificial" Jega (Sokoto Province) focus described on page 384 occurs where the mean maximum temperature is in excess of 95°F. The hot desiccating conditions and very low humidities of the far north, north of 11 ° or 12 °, might operate to prevent S. damnosum from extending its range into these arid areas, but other factors - - the absence of rocky-bedded rivers with broken water or perennial flow, or the absence of grasses in the river-beds to provide holds for the immature s t a g e s - also make this country unsuitable for the fly.
•
•
..
:...
.
• :.~.:'....: ~:. ....... :......
~.". "....-.! ::).'.i. ) i i. ". MAP 5. - - D i s t r i b u t i o n
/ m
of
S. damnosum in r e l a t i o n to "
' ~:
-
0~
~
.\. . .
-
the vegetation zones.
• . '. . . o m . . ~ - .
.
:
~
MONTANE, DERIVED GUINEA
SAVANNAH. SAVANNAH.
[
1
SUDAN SAVANNAH. SAHEL SAVANNAH.
1
~ooM.
I ~.,~
Distribution and vegetation. (Map 5). Except for the rolling open grassland of the Mambila Plateau in southern Adamawa the north of Nigeria is almost entirely clothed with savannah bush, which typically consists of small much-branched trees providing only light shade and permitting the growth of a grass cover for the ground. The Derived Savannah of the southernmost areas (south-east Ilorin Province, Kabba Province and parts of southern Benue) is a more luxuriant woodland growth than that of the north, the Sudan and Sahel Savannahs, and is interspersed with numerous relict patches of high forest, characterized by true forest species. Small forest outliers and fringing forest along rivers and streams occur further north in the Guinea Savannah woodland, which occupies about half of Northern Nigeria, and lies between the Derived and the Sudan Savannah. In the Sudan Savannah, tree growth is sparser and grass shorter than in the Guinea Savannah, and the country becomes very arid in the dry season.
390
DISTRIBUTION OF
Simuliurn damnosum
I N NORTHERN NIGERIA
The boundary between the Guinea and Sudan Savannahs shows a remarkable conformity with that between areas with 30-45 inches and areas with 45-60 inches annual rainfall (cf. Maps 4 and 5). Those parts of the Guinea Savannah where stands of heavy forest ("kurmis") exist are associated with high rainfall ; for example, northern Abuja Emirate which is situated near the centre of the Guinea Savannah zone but has a heavy local rainfall of 65-70 inches a year. S. damnosum loci exist principally in the Guinea Savannah (Map 5) ; in this zone the breeding rivers generally have a lightly or moderately developed forest fringe of larger trees than in the surrounding bush, and it is found that such trees asAdinamicrocephala ("kadanyarraft"), Vitex doniana ("dinya"), and Khaya senegalensis ("madachi") are common on the banks of infested rivers. Isoberliniasp.("doka")andButyrospermumparkii("kadanya")arethemost commonly represented woodland trees in the fly loci. A small sallow, Salix ledermannii ("baruwana"), is very often found growing in the bed of the breeding rivers in boththe Guinea and the Sudan Savannah. Only a few fly loci are known from the Sudan Savannah area, for it is in this zone that the rivers largely dry up in the dry season, and are sandy bedded ; rainfall is low, and the dry season very protracted and desiccating. Some loci, however, exist in the Zamfara and Ka valleys of Sokoto Province. In the southern parts of the country, south of the Niger and Benue, many foci are found in the area of the Derived Savannah. This zone has resulted from the invasion of the original high forest by the savannah, so that it now presents an intermixture of savannah flora with remnants of the old forest. The forest persists mainly where surface water is abundant, and consequently several breeding rivers of the Derived Savannah zone, especially in Kabba Province, are heavily shaded with oil palm (Elaeis guineensis) and other forest trees. Similar conditions occur in Abuja area of Niger Province, where outliers of the forest block exist ; here also the oil palm is found, sometime s forming heavy kurmis along the river banks. Vegetation near the breeding rivers is preferred to complete openness, although loci exist on the Jos Plateau at over 4,000 feet where there is very little vegetation ; these bare areas of the Plateau are, however, believed to be largely the result of denudation by man, although the presence of some high altitude plants suggests that the semi-montane appearance found now is partly natural. The loci of the Plateau might once have been more flourishing than they are today, for considering the apparent suitability of the rivers and streams for S. damnosum, the infestations are comparatively minor and produce only light to moderate intensities of onchocerciasis. The loci of the Sudan Savannah also occur in rather bare regions, and are only light infestations, though there is nothing to suggest the vegetation of these loci has ever been much different from its present day appearance.
Distribution in relation to population density (Map 6). For an African territory, Northern Nigeria is very densely populated ; the population (1952 census) is 17,000,000 and the mean density 60 persons to the square mile. Population is, however, not evenly distributed but tends to be concentrated in the far north (Kano, Katsina, and Sokoto Provinces), and along the southern boundary ; density on the Jos Plateau is also comparatively high, 60-150 to the square mile. Over at least half the country, population density is much lower than in these areas, and numbers only 15-60 inhabitants per square mile ; while southern Adamawa, Borgu Division of Ilorin Province, and parts of northern Niger Province are almost uninhabited.
R. W.
CROSSKEY
391
T h e majority of S. damnosum foci exist in areas where population density is low, 0-60 persons per square mile, as may be seen from Map 6 ; a n u m b e r of loci occur where the density is higher (60-150), but hardly any foci are known from areas of dense population. S. damnosum and onchocerciasis is therefore very much a rural problem, and scarcely ever an urban one. T h e large and important native towns - - such as Kano, Sokoto, Maiduguri, Zaria, Bauchi and Y o l a - lie well outside the range of fly distribution, but S. damnosum does occur at or in the vicinity of some large towns which have sprung up since European colonization ; these include Kaduna the capital, Lokoja and Minna. (As early as 1908, PARSONS had observed onchocerciasis in Lokoja). Fly breeding-grounds occur within 20 miles of Jos, the commercial tin-mining centre, and near some small native townships including Yelwa (Sokoto Province) and Abuja.
MAP 6.--Distribution of S. damnosum in relation to population density, persons per square mile. The map is approximate only and is based on the 1931 census with the density figures increased by 50%, as the population increased approximately by this amount between 1931 and the latest (1952) census; an even increase of population over the country has been assumed. UNDER
I
I ,s-6o.
15
~
60-lSO.
~
OVE. ,so.
L
ioo M.
i
S. damnosum breeding areas are mainly associated with a hilly terrain and rocky, perennial rivers. This type of country, as in southern Adamawa, the Kaduna River valley from Kaduna to Zungeru and the Abuja Emirate of Niger Province, is usually only lightly settled (0-25 persons to the square mile), in spite of the abundance of surface water. T h e rocky nature of the land often limits the area available for farming, and this perhaps serves to determine the population that the land can carry. On the other hand, the fertile plains of the far north are heavily settled, although almost all water has to be obtained from wells ; in parts of Kano Province, which becomes very arid in the dry season, the inhabitants n u m b e r 300 and more per square mile. T h e truly riverine areas with perennially-flowing rivers occur principally between 8 ° and 11 ° N, and within these latitudes most of the S. damnosum loci occur ; the population density is low. T h e area north of 11 °, where the fly is absent, supports a high population. As a generalization it may be said that S. damnosum is. associated in Northern Nigeria with areas of low or moderate population density. S. damnosum IN RELATION TO
ONCHOCERCIASIS
T h e localities where onchocerciasis is endemic in Nigeria are listed by BUDDEN (1956).
392
D I S T R I B U T I O N OF
Simulium damnosum
IN
NORTHERN
NIGERIA
Although as yet not all the known onchocerciasis areas have been investigated for S. damnosum and similarly not all the known fly areas have been surveyed for onchocerciasis, it appears from the evidence so far available that onchocerciasis does not occur in the absence of S. damnosum, and that the presence of the fly almost invariably, if not always, gives rise to the disease.* Nothing comparable to the findings of LEWIS (1953) in the Sudan has been noticed in N i g e r i a ; he found that in the northern Sudan onchocerciasis was absent, although S. damnosum occurs, but onchocerciasis was prevalent in parts of the southern Sudan, and he concluded that the longevity of the fly in the northern arid areas was reduced so that the development of Onchocerca to the infective stage could not take place. I n Nigeria the most northerly fly area yet discovered lies on the Zamfara River in Sokoto Province at 12 ° 1 0 ' 12 ° 18' N. Here, in spite of the prolonged and severe dry season, moderate intensities of onchocerciasis are found in certain villages of Anka and Bukwium Districts, and blindness results.
SUMMARY (1) This paper gives the results of a survey for the foci of Simulium damnosum in N o r t h e r n Nigeria, accompanied by. distribution maps. (2) A table of local native names for the fly is included. (3) Those negative areas which are, or m a y be assumed to be, free from S. damnosum are listed. (4) T h e distribution of the fly in relation to various environmental factors is discussed. Distribution is fixed primarily b y the geology of the country, which determines the areas of suitable breeding rivers. Altitude, vegetation and climate appear to play less part in determining distribution, except that climatic factors m a y operate to prevent the fly f r o m establishing itself in the far north. T h e absence of the fly from the far north is more certainly attributable to the absence of suitable breeding grounds. (5) S. damnosum is associated with areas of low to moderate population density ; all fly foci that have so far been surveyed for onchocerciasis have shown the presence of the disease. I n N o r t h e r n Nigeria onchocerciasis occurs even in the most northerly fly foci. REFERENCES BUODEN, F. H. (1955). Brit. J. Ophthal., 39, 321. - (1956). Trans. R. Soc. trop. Med. Hyg., 50, 366. CROSSKEY, R. W. (1954). W. Afr. reed. J., 3, 75. - (1955). Ann. trop. med. Parasit., 49, 142. FREEMAN, P. & DE MEILLON, B. (1953). Nimuliidae of the Ethiopian Region. (Nat. Hist.). LEwis, D. J. (1953). Bull. ent. Res., 43, 597. PARSONS, A. (1908). Parasitology, 1, 359. SIMPSON, J. J. (1912). Bull. ent. Res., 2, 301.
London : Brit. Mus.
* Simulium bovis de Meillon bites man avidly in certain localized areas of Northern Nigeria and in parts of Abuja Emirate, where S. damnosum also occurs, it is suspected of acting as a subsidiary vector of human onchocerciasis. That it may act as a vector is inferred from the fact that in the village of Izom a standardized infection rate with onchocerciasis of 759 inhabitants per 1,000 over 10 years of age occurs (BUDDEN, 1955, Brit. J. Ophthal., 39, 324) although the S. damnosum population is extremely small ; on the other hand the S. bovis population is very high, and biting rates up to 222 flies per boy-hour have been recorded. Furthermore, dissections of S. boris have shown the presence in the thorax of worms morphologically indistinguishable from O. volvulus, although as yet this does not constitute evidence that S. bovis is a natural vector. I am indebted to Mr. P. Freeman for confirming the identity of S. boris.