Schistosomiasis in Mauritius

564 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE.

Vol. 47.

No. 6.

November, 1953.

SCHISTOSOMIASIS IN MAURITIUS BY

S. G. COWPER**

Pathologist, Colonial Medical Service (From the Central Health Laboratory, Mauritius) Schistosomiasis caused by Schistosoma haematobium occurs in a number of isolated loci outside Continental Africa and the Middle East, including several islands off the African Continent. I n the Mascarene Islands of the Indian Ocean schistosomiasis occurs only in Mauritius and Madagascar. It is unknown in R6union, Rodriguez and the Seychelles. T h e infection in Mauritius is confined to S. haematobh~m; no endemic case of S. mansoni infection being recorded. Mauritius would thus appear to be the only truly oceanic island in which schistosomiasis is endemic. T h e principal study of the disease in Mauritius made so far is that of ADAMS (1934, 1935) who incriminated Bulinus (Pyrgophysa) forskali Erhenberg, as the vector snail and believed it to be the only carrier. This snail has been under suspicion in many places but never incriminated with absolute certainty elsewhere. T h e present paper covers a study of schistosomiasis in Mauritius made during a period of a little over two years (January, 1951, to February, 1953). T h e work was designed to cover the following programme : (1) A general survey of the extent and distribution of schistosomiasis in the island with regard to any significant differences in seasonal, sex, age and racial incidence. (2) A reinvestigation of the r61e played by B. forskali and/or any other mollusc in the transmission of S. haematobium in the island. (3) Testing of the local value, for diagnosis, of the cercarial antigen skin test using antigen prepared in Southern Rhodesia. (4) The collection and summary of any data which might have value in connection with future research or control work and preparation of distribution maps. PART I--SuRvm~ T h e survey was planned along the following lines : (1) The record and classification of all specimens of urine found to contain S. haematobium ova received by the Laboratory Service as part of general routine work during a 2-year period. The Mauritius Laboratory Service comprises three laboratories, i.e., the Central Health Laboratory, Reduit, the Civil Hospital Branch Laboratory, Port Louis, and the Victoria Hospital Branch Laboratory, Quatre Bornes. (2) A survey of Mauritian school children with a view to establishing the extent and distribution of the infection among them. (3) The preparation of distribution maps. (4) Comparison of figures obtained with those of surveys made elsewhere and general commentary. * I am indebted to Dr. R. LAVOIPIERRE,Director of Medical Services, Mauritius, for permission to publish this paper and to Mr. C. OPPER, Director of Education, for authorizing the visits to schools. I further acknowledge the valuable assistance which I received from the technical staff of the Central Health Laboratory and from the school teaching staffs.

S. G. COWPER

565

(1) GENERAL SURVEY

T h e routine findings in the three laboratories over the 2-year period (lst January, 1951, to 31st December, 1952) gave the following figures : Central Laboratory, Reduit

222 positive urines received

Civil Hospital, Port Louis Victoria Hospital, Quatre Bornes

574 136

. . . . . .

932

. . . . . .

Total

. . . . . .

The sex of the patient was recorded in 821 cases as follows : Total infected males 5813 Total infected females 240f°ut" o f 821 Sex unrecorded 111 Total

932

This gives a sex ratio: 2.4 males to 1.0 female. The age of the patient was recorded in 649 cases and gave the following figures for age-groups : No. of positives in age-group 0- 5 years = 21 ( 3.2o,/0)] o 6-15 ,, 277 (42.7~o)[ .. .. .. .. .. .. .. .. .. .. 15-25 ,, 241 (37.2°,.o) i.out of 649 over 25 ,, 110 (16.9%)) age unrecorded = 283 Total

932

T h e locality from which the patient came, if stated, was recorded on a wall map by means of cotoured pins, and the number of cases from any one locality was recorded in 2-month periods. T o g e t h e r with the school survey, described later, this gave an indication of the distribution of the infection. It is recognized, however, that it is only an approximate distribution estimate, since in an island which is small (about 720 square miles), densely populated and with excellent road communications and bus services, the patient may not have been infected in the place from which the specimen was received. It was, however, possible to map the more heavily infected regions. In the case of the urban centres of Plaines Wilhems (Rose Hill, Beau Bassin, Quatre Bornes, Phoenix and Vacoas) it seems probable that while there is some local infection, especially in the Rose Hill area, in many cases the infection was acquired elsewhere, e.g., on holidays. In all, S . k a e m a t o b i u m was recorded from 67 different localities in Mauritius, either in the routine work of the three laboratories or in the school survey. It is unnecessary to record here the names of the localities in detail. Localities recorded during 1951 are listed (CowPER, 1952) in the annual report of the Mauritius Health Department for that year. But it might be mentioned here that the highest numbers of cases recorded in the three laboratories during the 2-year period, were recorded from the following 12 localities : - - P o r t Louis (382), Rose Hill (35), Argy (30), Quatre Bornes (22), Beau Bassin (19), Mahebourg (19), Carreau Accacias (19), Flacq (17), Long Mountain (15), Mapou (14), Pamplemousses (13), Vacoas (13). T h e figures in brackets are the n u m b e r of cases recorded in the course of ordinary routine work between January, 1951, and December, 1952, out of 745 cases where the locality was given, and do not incorporate the separate school findings (see later). In addition a number of cases were recorded in the Prison, Mental Hospital and Industrial School at Beau Bassin-institutions whose inmates are drawn from all parts of the island. T h e Mobile Dispensary also reported a high incidence at Carreau Accacias in the extreme south of the island. Port

566

SCHISTOSOMIASIS IN MAURITIUS

Louis itself is by far the most heavily infected region in Mauritius, and that part of the island which lies approximately north of the line Port Louis--Argy--Flacq is more heavily infected than the rest. There is another belt of infection in Grand Port and the south, and a considerable incidence in the towns of Plaines Wilhems. In general, the results of the school MAP. 1.

I

He,~,,v

Approximate Distribution of S. haematobiura in Mauritius

L,ghf Occoslo~ol Co~es P.obobly Ac~qo,ved

ELsewhere

None r ~ o ~ d e d

T~lc Mo~ A.[

Arg*¢

survey confirm this approximate distribution (see later). Infection is scanty or absent in the central uplands where the population is thinnest. The results are illustrated by Maps 1-4. The case of Port Louis city is considered separately. Graphs 1-4 illustrate the number of cases recorded for 1951-52 at 2-month intervals at the Reduit, Port Louis and Quatre Bornes laboratories independently and in aggregate total. There does not appear to be any obvious seasonal variation but there does appear to be some indication of a seasonal occurrence of the snail-transmission cycle (see Part II). (2) SURVEY OF SCHOOL CHILDREN

Between January, 1951, and February, 1953, 50 schools were visited. Ten of these were in Port Louis and 40 spread throughout all parts of the island. The results from the first 14 schools have already been recorded (COWPER,1952). T h e t e c h n i q u e u s e d in c o l l e c t i n g t h e u r i n e s p e c i m e n s w a s as f o l l o w s : t h e h e a d t e a c h e r was n o t i f i e d

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1951

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1951

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pF MA MJ JA SO ND1 1952

568

SCHISTOSOMIASIS IN MAURITIUS

of the proposed visit and requirements a few days in advance and was requested to give the children a period of play before the collection of specimens for it is well known that the number of schistosome ova passed after exercise is considerably greater than that passed while at rest. Usually specimens were collected from about 150 children in each school of which approximately one-half were boys and one-half girls. They were selected from different age-groups ranging between 5 and 15 years, 15 being the usual leaving age. With each child, the sex, age and religion were noted, the religions as a rough indication of race since it was desired to note any significant racial incidence. As there are Indian and Chinese Christians in Mauritius, this was not an exact differentiation but the assumption that the Hindus and Moslems were Indians, the Buddhists Chinese, and the Catholics "Creoles," is approximately correct. The Mauritian community comprises (in round figures) about 500,000 persons of whom 320,000 are Indians (Hindus, Moslems, and Tamils), about 10,000 Europeans (predominantly French), about 15,000 Chinese and about 155,000 Creoles of mixed African stock. The urine was collected from each child, under the supervision of a member of the school staff, in an 8-oz. medicine flat and brought to the laboratory by car. The last few drops were especially requested. Examination consisted of straightforward microscopy of the centrifuged deposit. Refinements, such as the miracidiaseope, (valuable for N. mansoni but hardly necessary for S. haematobium), were not used. The urine was marked as positive if a single ovum, dead or alive, or a hatched miracidium, was observed. Haematuria without ova, though highly suggestive, was considered a negative result. T h e detailed results of the s u r v e y of 50 schools are s u m m a r i z e d i n the following T a b l e . ']'ABLE

I.

No. of schools visited No. of children on combined school rolls (i.e. section of child population sampled) No. of children examined No. of boys examined No. of girls examined No. of positive urines recorded Percentage of urines positive No. of positive urines (males) No. of positive urines (females) Ratio of positive males : females No. of positive urines in age-group 0-5 years . . . . urines in age-group 6-10 years . . . . urines in age-group 11-15 years . . . . urines in age-group over 15 years . . . . children of Creole race (approximation) . . . . children of Indian race ,, ,,

,,

children of Chinese race

,,

50 22,948 7,318 (32%) 4,168 3,150 666 9.2% 437 229 1.9 to 1 29 (4%) 326 (49%) 291 (44%) 20 (3°0) 268 (40%) 386 Moslem 91 (14%) Hindu 295 (44%) 13

(2%)

T h e highest p e r c e n t a g e i n c i d e n c e recorded was in the following schools: Port Louis, t o w n a n d d i s t r i c t : - - P 6 r e Laval (63), E a s t e r n s u b u r b (29), N i c o l a y Road (25), Sacr6 C o e u r (15). O u t s i d e Port L o u i s : - - Q u e e n Victoria, Argy (59), Vall4e des Pr~tres (38), T r o i s B o u t i q u e s (Carreau Accacias) (45), B a m b o u s G o v e r n m e n t (29), P a m p l e m o u s s e s G o v e r n m e n t (17). (Percentages to nearest integer). T h e I n d u s t r i a l School Beau Bassin, where 27 per cent. were infected, is a centre for d e l i n q u e n t boys d r a w n f r o m all areas a n d has therefore no local geographical significance. T h e m a p s illustrate the area covered b y the School S u r v e y a n d the p e r c e n t a g e i n f e c t i o n rate, localities where B . f o r s h a l i was f o u n d , a n d estimated general d i s t r i b u t i o n of the disease. B . f o r s k a l i was f o u n d i n the n e i g h b o u r h o o d of the following schools : Pdre I,aval (Ste. Croix), Vall4e des Pr~tres, P a m p l e m o u s s e s , T r o i s B o u t i q u e s ( C a r r e a u Accacias), T e r r e Rouge, Rose Hill a n d B a m b o u s .

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COWPER

569

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570

SCHISTOSOMIASISIN MAURITIUS (3) INCIDENCE IN THE ARMY

At the beginning of this enquiry a record of cases of schistosomiasis occurring in the Army Garrison was planned. It was found, however, that the garrison is composed almost entirely of the King's African Rifles, recruited in Africa, and Mauritius pioneers many of whom do service in Egypt and the Canal Zone. These were therefore rejected as irrelevant to a survey of Mauritius. But it should be noted that these African soldiers together with Mauritian pioneers returning from the Middle East may bring schistosomiasis with them and thus reinforce the local strains from time to time. It is very unlikely that S. mansoni (which is found sometimes in the African soldiers) could become established. ADAMS(1934) mentions Gyrauhls mauritianus (Morel) but during this survey no planorbis-like snails were ever seen. If they occur, they must be rare. MAP 4.

Schools in Port-Louis.

i EASTERN S U B U R B ' 2 9 SAC~E C C E U I ~ D i E J E ~ - ~ 5 NICOLAY ROAD-26

+

+

+

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FIGUPtES--

PERCENTAGE

INFECTION

laATE

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NEAREST

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INTEGEA

G:RLS-I

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LAVAL S C H O O L - 63 WAS MAPAREA TO NO~T~I

OUTSIOE

(4) THE SPECIAL CASEOF PORT LOUIS Map 4 illustrates the findings in the I0 schools visited in the Port Louis Area (P6re Laval School lies just outside the map area). Far more cases are recorded from Port Louis (adults and children) than from any other locality, and the northern half of the city is a major focus for this predominantly rural disease. A significant difference is to be noted between the infection rate in the schools in the northern and southern half of the city, although two of the former are situated in the rural belt outside the built-up area. Fiveschools in the northern half gave the following infection rates (per cent. to nearest integer, from a sample of about 150 children):--P6re Laval (Ste. Croix) 63, Eastern Suburb 29, Sacre Coeur 15, Nicolay Road 26, Central Boys 13. Average for five schools: 29 per cent. The five schools in the southern half gave the following figures:--Loretto Convent 2, De La Salle ii, Central Girls i, Bon Secours 4, Western Suburb I. Average for five schools: 4 per cent. Grand Rivi6re Nord Ouest just outside the urban area in the south showed 8 per cent. but Va]l~e des Pr6tres just outside in the

s. G. COWPER

571

north 38 per cent. T h e r e seems little doubt that this high incidence in northern Port Louis is associated with the River Latanier and P+re Laval stream (Ste. Croix) as foci of infection, and that this river and the irrigation channels it feeds constitute a main cause of the belt of high endemicity which extends from (approximately) Bambous in the south to Pamplemousses in the north. T h e lower reaches of the Latanier, however, seem to have few snails and no B. forskali was found there. This may be due to effluents from factories and to laundry work with soap in the river. Neither was B.forskag found high up on the flank of the mountains in the upper reaches though it was found in some numbers in the middle (Vall6e des Pr~tres) stretch and in various streams and irrigation channels between Port Louis and Pamplemousses. COMMENTARY ON THE ABOVE SURVEY

From the above investigation it is clear that schistosomiasis due to S. haematobium is widespread throughout large areas of Mauritius, particularly in the Port Louis area, in the northern third of the island and in a belt in Grand Port--Savanne, the southern provinces. T h e infection rate in children and adolescents is higher than in older subjects and the number of males infected is about twice the number of females. This difference is almost certainly occupational. These proportions approximate to those found in other parts of the world. BUCKLEY in Northern Rhodesia (1946) found that the percentage infection rate varied from 0 to more than 60 per cent. and was higher in children than in adults (children 45.9 per cent. out of 533, adults 20.6 per cent. out of 577) but he found no significant difference between infection rate in males (35.3 per cent.) and females (30.2 per cent.). BLACKIEin Southern Rhodesia (1932) found a general infection rate of 2 0 . 0 - 2 6 . 9 per cent. in snail infected areas. T h e sex ratio he found to vary in different localities and considered it entirely occupational. WATSON in Iraq (1949) found the ratio of infected males and females approximately 2 to 1, as in Mauritius. He found the highest incidence in early adult life but cases were rare in those under 3 or above 50 years. Rare in infants, the disease tends to die out in middle age. He found the average overall incidence to be approximately 27 per cent. in males and 14 per cent. in females. In Mauritius the highest overall incidence was in the age-group 6-25 years, and in children the incidence in the 6-11 and 11-15 years age-groups was almost the same (Table I). It was much lower in children below 5 years; it was also lower so far as routine specimens sent to the laboratory indicated in adults over 25 years. T h e r e is a natural tendency for the schistosomiasis infection rate to increase in relation to an increasing population and to be spread by irrigation (e.g., Egypt, Iraq, the Gezira region of the Sudan). T h e population of Mauritius is increasing rapidly and it is reasonable to surmise that a slow increase in schistosomiasis is to be expected. Although the overall infection percentage among 7,318 children examined was only 9.2, this cannot be taken as implying that the problem is not serious a one in the field of child health since the survey was deliberately planned to cover areas of low or of no infection as well as of heavy infection. In the heavily infected areas percentages of 63 and 58 were discovered in schools. In 10 schools no infection was found, while infection rates above 10 were recorded in 15 out of 50 schools, in eight of these the percentage was above 25 but under 50 and in two it was over 50. In assessing the problem it should be borne in mind that the cases diagnosed in routine work at the laboratory represent only a very small n u m b e r compared with those which are never diagnosed nor treated.

572

SCHISTOSOMIASIS IN

MAURITIUS

As regards the racial incidence there is nothing to suggest any special racial susceptibility or immunity. No cases were recorded in Europeans during the enquiry and only a few in Chinese. T h i s is almost certainly occupational since these races are seldom exposed to infection. T h e predominance of cases among children of Indian race (Hindu and T a m i l 44 per cent., Moslem 14 per cent.), and among Creoles (40 per cent.), is to be expected since these make up the majority of the agricultural labourer population. Especially in the heavily infected north of the island the labourers employed on the sugar estates are predominantly Hindus. Although schistosomiasis is predominantly an occupational disease of agricultural workers, the case of Port Louis city shows that a high incidence can exist in an urban area given a focus of infection. T h e northern half of the city, adjacent to the Latanier river, has a m u c h higher incidence than the South (see figures in text and map), and this belt of high endemicity extends north to Pamplemousses. B. forskali has been found throughout this area. T h e Latanier river and P6re Laval (Ste. Croix) would be a suitable locality to initiate any control measures which might be carried out in the future. T h e suggestion has been made that an animal reservoir host might exist in Mauritius. CAMERON (1928) has described natural infection of the monkey Cercopithecus sabeus with Schistosoma mansoni in St. Kitts (West Indies), an island not dissimilar to Mauritius. CRAIG and FAUST (1945) state that S. haematobium has been found occurring naturally in the monkey C. torquatus atys but do not say where. T h e m a m m a l s occurring in Mauritius (apart from domestic stock) are the monkey, deer, wild pig, mongoose, hare, tenrec (Centetes), rat, a mouse, a shrew and flying fox. T h e monkey is by far the most likely as a reservoir host. It is well known that both wild and domestic animals act as reservoirs for Asiatic schistosomiasis (S. japonicum). It has not been possible to carry out an investigation so far, but examination of wild monkeys r~ight be worth undertaking. While the figures do not indicate any seasonal variations in numbers of cases diagnosed there does seem to be some evidence that the snail-transmission cycle occurs chiefly during the hot rainy season ( D e c e m b e r to April). PART I I THE R6LE OF Bulinus (Pyrgophysa)forskali EHRENBERGIN THE TRANSMISSIONOF S. haematobium IN MAURITIUS T h e r61e of this mollusc in the transmission of S. haematobium has long been debated and it has been under suspicion in a n u m b e r of countries including Egypt, Uganda, Kenya, Arabia, N o r t h e r n Rhodesia, Nyasaland and Portuguese Guinea. As far back as 1915 LEIPER recorded it as one of the Egyptian molluscs which showed attraction for schistosome miracidia, while MANSON-BAHR and FAIRLEY (1920) mention its occurrence in the endemic area of Egypt. None of these considered it a proved vector. PINTO (quoted in Trop. Dis. Bull., 1950) considers B. forskali the vector in Portuguese Guinea but seems to base this conclusion on epidemiological evidence alone without experimental proof. SCHWETZ(1951), suspecting B.forskali in Uganda, examined 2,000 specimens without finding cercariae. He comments on the seasonal fluctuation of the B. forskali population which virtually disappears in the dry season. MANSON-BAHR(1948) lists B. forskali as a vector in Kenya but does not give a reference to the record. BUCKLE',"(1946) obtained no conclusive results in experiments with B. forskali in Northern Rhodesia. In a personal communication (1951) the late Dr. ABDULAZIM (of Cairo) stated that he found B. forskali in Saudi Arabia in a spot near Mecca where 88 per cent. of the population had urinary schistosomiasis and also that BARLOWtried to infect B.forskali in Egypt with negative results. DYE (1924) lists B.forskali (under the name of Isidora forskali) as indigenous in an area of northern Nyasaland where he made a study of schistosomiasis, but he does not incriminate it as a vector.

S. G. COWPER

573

Also in Nyasaland, RANSFORD(1948), in a study of schistosomiasis in the Kotakota district, records B. forshali as occuring in very large numbers. He found no trematodes in many dissections, but considered B. forshali a possible " emergency " host. With regard to Mauritius, Balfour K:RK (1931) states that at that time no local mollusc had been incriminated in spite of the examination of thousands of specimens. MOMPLI~ (1928) made a study of cercariae in Mauritian fresh water gasteropods but failed to find those of Schistosoma. He does not mention B. forshali. ADAMS,however, definitely incriminated B. forskali as the principal, and probably only, vector in the island. In two papers (1934, 1935) he describes the transmission of the local strain of S. haematobium through B. forskali in the laboratory and in a concrete drain outside the building. He successfully infected mice by exposure to large numbers of cercariae shed by B. forskali and recovered adult worms from the liver, mesenteries and, particularly, the lungs, as well as viable ova. He failed to infect mice by exposure to smaller numbers of cercariae and does not record natural infection of wild B. forshali with Schistosoma cereariae. ADAMSalso comments on the small size of B. forskali and surmises that assuming the number of cercariae a snail can produce must bear a direct relation to its bulk, the heavy incidence of schistosomiasis in Mauritius must be accounted for by the large numbers and wide distribution of this species of mollusc which he found to be c o m m o n in most parts of the island.

Description of the Snail Family, Bulinidae. Subfamily, Bulininae. Genus, Bulinus Miill., 1781. Section Pyrgophysa Crosse, 1879. Bulinus (Pyrgophysa)forskali Erhenberg: The species, presumably introduced into Mauritius from Africa, has an extensive synonymy but CONOLLY(1939) thinks there is probably only one, or at most two species, with considerable variations in size and shape of shell. The empty shell is very pale brownish-white while in the living snail it appears shiny olive-black. It is narrow and elongate with a sinistral spiral of two to five whorls and there is no operculum. Long axis measurements in six random specimens were 8, 7, 5, 5, 4, 4, ram. (to nearest nun.). The local boys recognize a large and a small type--" un gros forskali " and " un petit forskali." J. GAUD (personal communication, 1952) considers the average size of Mauritian specimens to be notably smaller than African specimens and considers that they might constitute a subspecies or even species. None of the specimens studied bred in captivity; but they were kept and infected successfully in small glass museum jars containing a little clean sand and pond weed (Elodea), without artificial aeration, as well as in a concrete drain out of doors (Adams' method). They were fed on young unboiled lettuce or local spinach leaves. In the wild state they are bottom dwellers found largely on or under stones, less frequently in weeds, in clear, running torrential streams, irrigation channels or roadside ditches, often lying in the perforations of the local pumice-like volcanic boulders. Widespread throughout the island at the time when ADAMS worked (1933-36) their numbers have greatly declined and B. forskali is now very local (see p. 000).

Experiments with Snails Between 30 and 40 exposure experiments were carried out in the course of the investigation. All the experiments with snails other than B. forskali i.e., Limnea mauritiana, Physa borbonica, Melania tuberculata, and Melania scabra, the four commonest and most widespread freshwater gasteropods, were negative, but on occasion L. mauritiana and Ph. borbonica showed attraction for miracidia. It is unnecessary to detail these negative experiments but one is of some interest. At Argy, where the incidence of schistosomiasis in the children examined in the Queen Victoria Estate School was 58.2 per cent. with many cases of severe haematuria, the little river Cognard flows close by the school and is used by the children for washing, playing and rinsing clothes.

574

SCHISTOSOMIASIS IN MAURITIUS

I t w o u l d b e s u r p r i s i n g if it w e r e n o t a s o u r c e o f infection. H o w e v e r , n o B. forskali were f o u n d in t h e c o u r s e o f t h r e e s e p a r a t e searches along different s t r e t c h e s b y f o u r different searchers. A c o l l e c t i o n of m i x e d snails (M. tuberculata, M . scabra, L. mauritiana, a n d p r o b a b l y s o m e Ph. borbon~ca w h i c h is v e r y s i m i l a r to L. mauritiana in a p p e a r a n c e ) was exp o s e d to v e r y n u m e r o u s freshly h a t c h e d m i r a c i d i a . T h e m i r a c i d i a c a m e f r o m u r i n e o f c h i l d r e n in this school a n d t h e snails f r o m the a d j a c e n t s t r e t c h of river. N o cercariae w e r e p r o d u c e d u p to 55 d a y s after exposure. T h e critical e x p e r i m e n t s w i t h B. forskali are s h o w n in T a b l e I I . I n general, e x p o s u r e was c a r r i e d o u t in ½ to 1 i n c h of w a t e r in a cigarette t i n o v e r n i g h t b u t s o m e snails w e r e given a b r i e f p r e l i m i n a r y e x p o s u r e to a h i g h c o n c e n t r a t i o n o f m i r a c i d i a in a w a t c h glass. TABLE II. Serial o.

Mollusc

Exposure experiments with B. forskali.

Parasite

Date and period of exposure

Observations and results

17

5 B. forskali

S. h. miracidia +++

15.XI.51 Short exposure in watch glass then overnight in cigarette tin in½-1 inch water.

No cercariae shed by l l . X I I . 5 1 ; 2 crushed--no trematodes. Last remaining snail crushed on 15.XII.51 showed typical active schistosome cercariae after 30 days. No cercariae actively shed.

18

B. forskali kept in concrete drain

S. h. ova in urine added to drain

Ova added to drain from 5.XI.51 onwards

Snails removed from drain on 12.XII.51 and exposed to sun, 15.XII.51 crushed preparation showed typical active schistosome cercariae. 22.XII.51, last specimen crushed cercariae + + ; 2 snails found infected.

24

8 B. forskali

S . h. miracidia

7.III.52 2 exposed in watch glass, 6 in cigarette tin overnight

9.IV.52. All dead but one and no trematodes found. Last snail shed typical schistosome cercariae on II.IV.52 after 35 days, died after shedding about 8-9 days. Mice successfully infected. (See later).

10 B. forskali +

S . h. miracidia

7.XI.52 Overnight in cigarette tin. More eggs added to jar later

12.XII.52. Cercariae shed after 35 days. 2 dissected showed living cercariae. 15.XII.52. 5 snails have shed cercariae, others dead. One snail still shedding up to 10.II.53 i.e., shed continuously but in small numbers for 53 days. Seven out of 10 snails infected.

31

3 more next day

++

++

T h e i n c u b a t i o n p e r i o d a p p e a r s to b e a b o u t 30-35 days (ADAMS r e c o r d e d 28-33 days). M i c e w e r e t h e n e x p o s e d b y s e m i - i m m e r s i o n to cercariae s h e d b y a snail in e x p e r i m e n t

S. G. COWPER

575

No. 24 (Table II). T h e method used for enumeration of cercariae and exposure was that described in detail by AZIM and COWPER (1950) as used in Egypt. Clean laboratory-bred white mice were used. Out of four mice exposed one was lost: results are shown in Table III. TABLE III.

Experiments with mice.

No.

Animal

1

1 White mouse

Exposure period about 40 minutes for each application of cercariae. M e t h o d of exposure

Date of exposure

No. of cercariae

12.IV.52 16.IV.52

160170}=330

13.IV.52 15.IV.52

80(=190 110f

....

Lost.

17.IV.52

300

....

Autopsy, 23.VII.52: 1 coupled pair and a number of fragments from liver and mesentery after 97 days. Pigment + + in liver.

18.IV.52

230-250

Semi-immersion

Results A u t o p s y , 17.VI.52 : 1 c o u p l e d p a i r of w o r m s a n d 2 f r a g m e n t s recovered after 66 days.

Autopsy, 30.VII.52: 1 coupled pair f r o m m e s e n teries a n d 2 pairs f r o m liver after 103 days.

It was found that by centrifuging the water in which the mouse had been semi-immersed and examining the deposit the shed tails of cercariae could be observed in numbers, thus indicating that penetration had, indeed, occurred. T h e worms were recovered from the mesenteries and liver, but not from the lungs where ADAMS found them most numerous. Stained and mounted, these worms showed the morphology of male and female S. haematobium though the sex organs were not yet fully developed. However, paraffin sections of liver showed both worms in section and ova surrounded by characteristic pseudotubercles, so presumably some sexually mature worms were missed. T h e first autopsy showed that coupling was completed by 66 days after exposure but the proportion of immature worms even after 100 days suggests that full development is very slow. In Egypt AZlM and COWPER (1950) failed to infect mice with S. haematobium though cercariae penetrated the skin of newborn mice. T h e y were successful with S. mansoni. T h e production of cercariae by infected B. forskali always occurred between December and April which corresponds with the season of maximum temperatures and rainfall. It seems possible that transmission occurs only or mainly during this period. These experiments confirm ADAMS' work and demonstrate that B. forskali is in fact a potential vector. Up to now there is no record of natural infection found in wild B. forskali and no trematodes were found in 250 that I examined. COMMENTARY ON THE ABOVE

In a period of about 2 years' work the writer, assisted by local personnel well acquainted

576

SCHISTOSOMIASIS IN

MAURITIUS

with this snail, found it only in the following localities: Carreau Accacias in the south, Rose Hill in Plaines Wilhems, Bambous and in various localities north of Port Louis, e.g., Vall6e des Pr~tres, P~re Laval (Ste. Croix), Terre Rouge and Pamplemousses. Only at Carreau Accacias and St. Croix were these snails numerous. There the incidence of infection in the neighbouring school and village is high and at Carreau Accacias inhabitants use the B. forskali infected stream for washing clothes. In Rose Hill it was found in an irrigation canal, also used for washing clothes, but it disappeared when this canal was cemented. All these streams dry up in the dry season and no living snails could then be found under stones. They reappear with the storm water after the rains begin and the streams are in spate. SCHWETZ (1931) comments on a similar seasonal incidence in Uganda. It is hard to believe that such a delicate little snail can survive prolonged dessication in the Mauritius climate or can penetrate deeply to a subterranean water table. A more plausible theory is that some snails follow the receding waters back, or survive in the upland stretches where the high rainfall (150-170 inches a year at Curepipe) maintains the streams throughout the year, and then return with the spate. But I failed to find B. forskali in an upper reach of the River Latanier at a time when snails were still present in residual pools lower down. Further research on the bionomic and seasonal incidence of B. forskali is desirable. The decline in numbers of B. forskali, already mentioned, has to be reconciled with the widespread incidence of schistosomiasis. The population of Mauritius is increasing rapidly and there seems to be some evidence that schistosomiasis is increasing with it. Assuming a relationship between size of snail and number of cercariae shed, one cannot but wonder whether this small, seasonal and local species can be wholly responsible for such a high infection rate and wide incidence. One snail under observation, however, continued shedding in small numbers for at least 53 days, so that, once infected, a snail may produce a few cercariae over a long period and this might in part compensate for the small size and scanty numbers. According to local suggestions, the reduction in numbers may be associated with inundations caused by the 1945 cyclone and with the drainage works of the Malaria Eradication Scheme. It seems highly probable that a local mollusc, through which the local strain can be passaged successfully in the laboratory, is also a vector in the field. The fact remains that the epidemiological picture does not exactly tally with the experimental results. The existence of localities where a high incidence of infection occurs, but B. forskali has not yet been found, have to be explained. Summing up, the experimental evidence support ADAMS' thesis that B. forskali is the vector of S. haematobium in Mauritius but the epidemiological evidence is conflicting and as already stated, natural infection has not yet been found. It would seem that in general there is a close host-parasite specificity between any local strain of Schistosoma and its snail vector, and that normally only one mollusc transmits any one species in a particular area. This has been previously discussed (COWPER, 1947) and is the subject of a very detailed experimental study by FILES (1951). But until further work has been carried out the possibility of a second vector in Mauritius cannot be absolutely excluded, though contrary to general experience. For practical purposes B. forskali should be accepted as the presumptive carrier and any control measures undertaken should be primarily directed against this species. Mauritius appears to be the only country where B. forskali has been definitely incrimin-

S. G. COWPER

577

ated. In a personal communication GAUD states, " Maurice est le seul endroit du monde ak une pyrgophyse ait (t( reconnue vectrice avec certitude." He also states that in Madagascar the established vector is Bulinus (Isidora) madagascariensis Angas, 1877, and that while a Bulinus (Pyrgophysa) namely, B. (P) mariei Crosse, 1879, is known in Madagascar it has not been incriminated as a vector. Neither schistosomiasis nor B. forskali occurs in the sister islands of R~union and Rodriguez nor in the Seychelles. It would be instructive to expose known snail vectors from Africa to miracidia from the Mauritian strain. Only a single such experiment was possible during this investigation. Twenty-two Bulinus truncatus from Egypt were exposed to miracidia and showed attraction. Up to 45 days after exposure no cercariae were shed and crushed snails showed no developmental forms. Little can be deduced from this one experiment. Map 3 correlates localities where B. forskali was found with the general distribution of S. haematobium. PART I l l CERCARIAL A N T I G E N SKIN TEST

At the beginning of the survey it was planned to combine urine examination in the schools with skin tests using the cercarial antigen which has given good results in Southern Rhodesia. Accordingly a supply was obtained from Salisbury (S.R.). ALVES and BLAIR (1946), considered the test to be reliable and superior to urine microscopy and stated that no negative skin reactors were found to be passing ova. In a later paper, however, (1947) they found that, after the issue of a quarter of a million doses to medical practitioners in Southern Rhodesia, the test had not always proved uniformly satisfactory, although they themselves had found it a perfect negative screen. T h e whole subject of skin tests is summarized by LURIE and de MEILLON (1951). In the present survey the skin test was first tried in the Queen Victoria Estate school, Argy where 58.2 per cent. of 150 children were found infected. Fifty children, including known positives and negatives after urine microscopy, were tested. Of these, 30 were girls and 20 boys, and they were of Indian and Creole race, ages varying from 5 to 14 years. A comparison of the skin test results with the urine findings gave the following data: TABLE I V .

U+

U+

U -

U -

U+

U -

U&ST

U & ST

ST +

ST-

ST-

ST +

ST ±

ST ±

Similar

Different

5

21

29

13 U = Urine.

ST = Skin test.

-4- = Equivocal results difficult to read with conviction.

T h u s the urine findings and skin test agreed in less than half the tests. These results seemed to indicate that the test was unreliable. Accordingly, skin testing was suspended in the schools while 19 out of 20 persons of the laboratory staff were tested. T h e y included known positives. T h e instructions sent from Rhodesia were carefully followed and each test was observed for over 15-20 minutes by more than one observer. T h e r e was no question of going away and returning later to examine the arms.

578

SCHISTOSOMIASIS IN MAURITIUS

T h e results were as follows : TABLE V.

No.

ST

Urine

No.

ST.

+ +

+

11 12 13 14 15 16 17

± + + ?

~z 5: + +

18

19 20

9

10

++

Urine

± (or weak +) + + (weak)

ST ~ Skin test; ± = Equivocal reaction. Nos. refer to members of laboratory staff. In one case the skin test was not performed. Thus in 19 out of 20 test subjects, the urine findings and skin test agreed in seven cases and differed in 12. One man who w a s a chronic egg-passer gave a negative skin test ( N o . 10). In both the above tests the n u m b e r of equivocal reactions, difficult to read, was high and it was decided to abandon the test

altogether and to rely on microscopy of urine. T h e test appears to have yielded good results in the hands of Rhodesian experts and it seems just possible that the test is reliable only when the antigen is prepared from cercariae of the same strain of schistosome as that infecting the tested patient. I n this case there were more false positives than false negatives but both occurred. SUMMARY

(1) I n the course of a survey of schistosomiasis (S. haematobium only) in Mauritius, this infection was found to be widespread throughout large areas of the island. (2) T h e most heavily infected regions are the Port Louis-Pamplemousses area, an area round Argy and an area in the south centering round Carreau Accacias ( G r a n d Port). T h e part of the island lying north of the Port Louis-Argy-Flacq line is more heavily infected than the rest but there is also a good deal of infection in the built-up areas of Plaines Wilhems with a possible source of infection in Rose Hill. (3) T h e city of Port Louis is dealt with in some detail. T h e infection rate in the northern half of the city is significantly higher than in the southern half. It is believed that the River Latanier and various streams and irrigation channels in the Ste. Croix (P~re Laval) area are a principal source of infection. (4) I n a survey of 50 schools extending throughout the island an infection rate above 50 per cent. was found in two schools, above 25 per cent. in eight and above 10 per cent. in 15 schools. Full details are presented in a table. (5) N o significant seasonal incidence in cases recorded in the laboratory routine w a s noted, but there is some evidence that transmission corresponds with the hot wet season ( D e c e m b e r to April). T h e sex incidence showed that about twice as m a n y males as females are infected (all ages). Age-groups indicated that the disease is u n c o m m o n in infants below 5 years and in the middle-aged, while incidence is highest in school children, adolescents

s. ¢. COWPER

579

and young adults. T h e r e is no indication of any racial i m m u n i t y or susceptibility, the predominance of infection among Indians and Creoles being certainly occupational. (6) In a series of experiments the local strain of S. haematobium was passed through Bulinus (Pyrgophysa)forskali Erhenberg, cercariae were shed, mice infected, and adult worms and ova recovered. T h i s confirms the previous experimental work of ADAMS (1934-35). A description of this snail is given. All experiments with other molluscs were negative. (7) T h e epidemiological evidence in favour of B. forskali as the only natural vector is considered and found to be conflicting. While a second vector cannot be absolutely ruled out, B. forskali should certainly be considered the presumptive vector. No natural wild infection has yet been discovered. Part of the literature dealing with B. forskali as a suspect is summarized. T h i s species has not been definitely incriminated outside Mauritius so far as the writer can ascertain. (8) An attempt to use the cercarial antigen skin test for screening and diagnosis is described. Reasons are given for rejecting this test as locally unreliable. REFERENCES

ADAMS, A. R. D. (1934). Ann. trop. Med. Parasit., 28, 195. (1935). Ibid., 29, 255. ALV-~S,W. & BLAIR, D. (1946). Lancet, 2, 556. - &- (1947). S. Afr. reed. J., 21, 352. AzIM, M. A. (1951). Personal communication. & BAaLOW, C. H. (1945). First Annual Report, Bilharzia Snail Destruction Section, for 1942. 2, Cairo: Ministry of Public Health. & COWPER, S. G. (1950). Brit. J. exp. Path., 31, 577. BLACKIE,W. K. (1932). Mere. Lond. Sch. Hyg. trop. Med., No. 5, 17. BUCXLEY, J. J. C. (1946). .7. Helminth., 21, 111. CAMERON, T. W. M. (1928). Ibid., 6, 219. CONNOLLY,M. (1939). Ann. S. Aft. Mus., 23, 507. COWPER, S. G. (1947). Ann. trop. Med. Parasit., 41, 173. - (1952). Annual Report, Medical and Health Dept., Mauritius, for 1951. Appendix II, 106. CRAIG, C. F. & FAUST, E. C. (1945). Clinical Parasitology, 415. London: Henry Kimpton. DYE, W. HooD (1924). ft. R. Army reed. Cps., 43, 161. FILES, V. S. (1951). Parasitology, 41, 264. GAUD, J. (1952). Personal communication. KIRK, J. BALFOUR(1931). Public Health Practice in the Tropics, p. 254. London : Churchill. LEIPER, R. T. (1915). ft. R. Army reed. Cps., 26, 38. - (1928)..7. Helminth., 6, 117. LURIE, H. I. & DE MEILLON, B. (1951). S. Aft. reed..7., 25, 321. MANSON-BAHR, P. (1948). Manson's Tropical Diseases. London: Cassell. - & FAIRLEY, N. H. (1920). Parasitology, 12, 33. MOMPLE, F. J. (1928). Colony of Mauritius. Annual Medical and Sanitary Report for 1927, p. 24. PINTO, A. R. (1949). Ann. Inst. Med. trop. Lisbon, 6, 75. (Summary in T.D.B., 1950, 47, 10). RA.NSFORD, O. N. (1948). Trans. R. Soc. trop. Med. Hyg., 41, 617. SCHWETZ, J. (1951). Ibid., 44, 501 and 515. WATSON, J. M. & AL HAMAMI, A. (1949). .7. roy. Fac. Med. Baghdad, 13, 49. -

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