Part V. Unstable highland Malaria—the entomological picture

537

PART V.

UNSTABLEHIGHLAND MALARIA-THE ENTOMOLOGICAL PICTURE BY W. PETERS AND S. H. CHRISTIAN

Malaria Section, Department of Public Health, Port Moresby, Territory of Papua and New Guinea

In their preliminary paper on malaria in the highlands of Papua and New Guinea PETERS et al. (1958) drew attention to several gaps in our knowledge of the bionomics of the anopheline vectors of highland malaria. It is of prime importance to ascertain the natural longevity of these species and seasonal variations that may occur in this. As in the first section of this paper attention is drawn constantly to the dynamic situation which exists in the highlands. METHODS

Daily collections of mosquito larvae have been made from a wide variety of breeding sites and samples from most of these have been bred out for confirmation of the species. Since November, 1957, a number of outlet window traps have been in operation attached to the sides of ordinary native houses scattered about the Minj area. A daily record is kept of the numbers and species of mosquitoes found in these traps in the morning and the number of human beings and animals sleeping in the trap houses. A sample of mosquitoes is dissected from each day's catch and records are kept of gland infections and ampulla sizes, the latter from females with stage III ovaries only. Over a period of 8 months female anophelines were kept alive in an attempt to assess the duration of the gonotrophic cycle. INCRIMINATION OF THE VECTOR SPECIES

The followinganophelinespecieshave been recorded fromthe Mini area : A. punctulatus D6nitz, A. punctulatus " intermediate forms " (? = A. koliensis Owen), A. farauti Laveran, A. annulipes Walker, A. bancrofti Giles, and A. papuensis Dobrotworsky (-- A. stigmaticus Walker of earlier writers). No attempt was made to classify separately the intermediate forms which were only present in small numbers.

The vast majority of the

A. punctulatus complex studied were A. farauti. A successful malaria vector must fulfil the following criteria : (i) It must show a significant degree of anthropophily (ii) It must have a natural life span adequate for the completion of the extrinsic cycle of the malaria parasites (iii) It must occur in adequate numbers (iv) It must favour the development of the malaria parasites.

While A. punctulatus, A. farauti, A. koliensis and A. bancrofti are known to transmit malaria at lower altitudes, A. annulipes has never been found infected in nature. Although larvae of this species are the most abundant in the Mini area (73.3 per cent. of 3,096 collections of anopheline larvae) only 0.12 per cent. (of 22,591 anophelines caught in window traps) of the house-visiting anophelines are A. annulipes, and it has not been taken biting man outdoors. A. farauti on the other hand represented 20.2 per cent. of larval collections

538

EPIDEMIOLOGY OF MALARIA I N NEW GUINEA

and 94 per cent. of adult collections in window traps. A. punctulatus represented 0.11 per cent. and A. bancrofti 5.8 per cent. of adult collections. On these grounds alone A. farauti appears as the only vector species of major importance. General experience has shown that A. punctulatus is usually rare in the Mini area, only appearing at times of exceptionally favourable climatic conditions. B R E E D I N G SITES

Of all A./arauti larval collections, 71.2 per cent. were made from purely transient sites such as those favoured by A. punctulatus, and 28.8 per cent. from more permanent waters. Over 90 per cent. of larval collections of the latter species were from transient sites. The permeable nature of the soil at Minj and the local topography do not favour the accumulation of water in transitory ground pools in normal years, but the rising water table level maintains adequate breeding sites of the type adaptable by A. farauti. When the daily rainfall pattern is particularly suitable, A. punctulatus may increase in abundance, but such years in addition are favourable to A. farauti which can also take advantage of such sites and the result is an exceptionally heavy population density of the latter species followed by a severe epidemic of malaria. FEEDING HABITS

The prevailing mean temperatures at Mini result in a prolongation of the aquatic stages at Minj, and this in turn results in the production of adult mosquitoes which are considerably larger than their coastal counterparts (LEE, 1946). This is a significant feature since the the larger the blood meal that the female can imbibe, the greater are its chances of ingesting gametocytes from the host's blood. This is particularly important when the gametoeyte density is low. Precipitin tests were performed on 633 blood meals collected from A. farauti caught in window traps in October, 1958. Of the 616 blood spots which gave clear reactions, 71.3 per cent. were derived from man, 28.1 per cent. from pig, and 0.7 per cent. from dog hosts. The average population of the houses from which these specimens were collected was, men and women 2.2, children 0.4, pigs 7.5, and dogs 0.2 per house. Thus there was adequate opportunity for the mosquitoes to feed on a variety of hosts. Clearly there is a distinct preference for human blood, although the fact that the Minj indigenes smear themselves with pig grease as a protection against the cold may have some bearing on the mosquito's selection of a host by his skin smell. There is no evidence so far to show whether A. farauti feeds indoors or outdoors by preference. It certainly does feed indoors in spite of the high concentration of smoke which is a constant feature of the interior of Minj houses. At Minj A. farauti shows a distinct preference for feeding in the early hours of the evening, 50 per cent. feeding before 10 p.m., and only 10 per cent. after 2 a.m. In the laboratory this species will rarely take a blood meal after 8 p.m. HOUSE-RESTING HABITS

BLACK (1955) recorded that A. farauti could be taken resting in native houses in the Minj area in the daytime and that the majority of these were engorged females. They were always found resting on walls below 7 inches from the ground in the darkest parts of the

W. PETERS AND S. H. CHRISTIAN

539

houses or occasionally on various domestic objects such as yams, string bags etc. This observation was confirmed by SPENCER and SPENCER (1955). In the present series of observations A. farauti was collected regularly resting in houses, but no attempt was made to ascertain the house-resting densities. DURATION OF THE GONOTROPHIC CYCLE

The only species studied at Minj from this viewpoint is A. farauti. It is expected that the gonotrophic cycle would be longer at high altitudes than in coastal areas. GILLIES (1953) showed that in A. funestus the duration of the gonotrophic cycle was 2 days above 78°F., 2 to 3 days at 76.5 -78 ° F., and 3 days below 76.5 ° F. Temperature is the controlling factor. The mean temperature at Minj is 69 ° F. (20.6 ° C.). The commonest interval between egg layings appeared from our observations to be 3 days, oviposition occuring on the fourth day. NATURAL LONGEVITY

It is of particular interest to ascertain the natural longevity (p) of a malaria vector, and there are several modes of approach of which observation of the natural daily mortality in caged specimens is the simplest. Of the 30 female A. farauti, held for study of the gonotrophic cycle, 50 per cent. survived 16 days and the natural daily mortality followed a fairly straight line. Two specimens lived for 47 and 49 days respectively. Applying the formula of MACDONALD(1957) which states that the probability of a mosquito surviving through n days is pn, it is seen that p16 = 0.5 from which we calculate that p ----0.96. This is probably somewhat higher than would be found in nature and is in any case based on small numbers. The most direct methods for the measurement of longevity are those based on an examination of the ampullae of the oviducts as described by DAVIDSON(1955), or on the study of the ovarioles as described by LEWIS (1958). Freshly caught A. farauti with ovaries at stage III were examined by Davidson~s technique and a monthly figure for the proportion parous (P) was obtained. The value of P varied from month to month (Fig. 3), and the general trend roughly followed that for the sporozoite rate and population density. As would be expected, the sudden influx of young mosquitoes in December, 1958, was reflected in a sharp drop in P for that month. From Figure 3 it does appear that there is a slight over-all rise in P in the wet season and a relative fall in the dry season. The significance of this will be discussed later. A l t h o u g h ovariole studies b y L e w i s ' s m e t h o d have only b e e n m a d e for 2 m o n t h s , t h e figure o b t a i n e d for P o n parallel series agrees w i t h t h a t o b t a i n e d b y D a v i d s o n ' s m e t h o d . A l t o g e t h e r 3,584 dissections were m a d e b y D a v i d s o n ' s m e t h o d a n d values of P r a n g e d f r o m 0 . 6 5 in D e c e m b e r , 1958, to 0 . 9 3 in July, 1959. DAVlOSON (1955) s h o w e d t h a t w h e n t h e g o n o t r o p h i c cycle is n days, p ~ ~ / ~ - . A s s u m i n g t h a t n for -/l. farauti at M i n j is 4 days, we can calculate t h a t t h e m e a n value of P - 0. 8431 a n d h e n c e o f p = 0.96.

It was noted above that highland A. farauti are larger than coastal specimens owing to the prolonged duration of the aquatic stages. The evidence of MACKERRASand ROBERTS(1947) shows quite clearly that A. punctulatus lives longer at lower temperatures. It is most likely that _//. farauti survives longer at lower temperatures, as does A. punctulatus. This increaso in longevity combined with its greater size increases the danger of .//. farauti as a vector. D

540

EPIDEMIOLOGY OF MALARIA IN NEW GUINEA THE SPOROZOITE RATE

Of 11,745 A . farauti dissected at Mini, 0.54 per cent. had sporozoites in the salivary glands. None of the 190 A . bancrofti dissected was infected. There are old records of sporozoite infections in A . punctulatus at Mini, but no accurate data were kept of infection rates at that time. The mean monthly sporozoite rate (SR) varies considerably from zero to over 2.2 per cent. The highest rates recorded were 2.2 per cent. ± 1.3 (in 136 dissections, February, 1956) and 1.4 per cent + 0.5 (in 570 dissections, June 1959) for A. farauti. During the dry season the small numbers available for dissection make negative findings of less statistical value, but an SR in the region of 0.1 or 0.2 per cent. seems the most probable. Monthly variations in the SR are shown in Figure 3. The main contributing factor to the SR is the natural longevity of the vector although other factors such as the gametocyte reservoir are of importance. These will be discussed later. THE EXTRINSIC CYCLE

The only direct evidence of the extrinsic cycle obtained at Minj was that produced by the delayed sporozoite rate method. After 16 days the SR rose to 6.3 per cent. in 48 specimens. Indirect evidence comes from experimental work elsewhere. MACDONALD(1957) has summarized the available information on the duration of the extrinsic cycle of P. faldparum and P. vivax with varying temperatures, and from his graph we can interpolate the values of 16 and 21 days for the extrinsic cycles of these species at 20.6 ° C. MACKERRAS and ERCOLE (1948) found that P. malariae required about 28 days in A. punctulatus, while P. vivax required 15 to 16 days, and P. faMparum 18 to 20 days, in concurrent batches of mosquitoes. The mean temperature for the duration of the extrinsic cycle in their experiments was 22.8 ° C. INTERRUPTED FEEDING AND HOUSE INFECTIONS

It was noted earlier in this paper that under laboratory conditions A . farauti rapidly becomes conditioned to being offered a blood meal every night. BLACK(1955) noted the close association between A . farauti resting in the vegetation outside the walls of native houses with the sources of their food inside the houses. If interrupted feeding is a common feature in nature, the chances of a single infected anopheline infecting several inhabitants of the household is a potentially serious matter. VlSWANATI-IAN(1959) has taken this into account and introduced the symbol " c " to represent the number of persons bitten by an infected mosquito in one night. In his example which is related to the frequency of house visits for surveillance purposes, he assumes that " b " (the proportion of anophelines with sporozoites that are actually infective, i.e. a figure less than 1.0) and " c " cancel each other out. The factor " b " however tends towards unity as the effects of " immunity" become less. In the case of highland malaria " c " may therefore be of real significancesince while the quantum of premunity is low, gametocyte rates and densities are high. POPULATION DENSITY VARIATIONS

Daily collections have been made since December, 1957, of mosquitoes captured in window traps of which between 20 and 30 have been employed. The mean number of inhabitants per night forming the bait for these traps was 85 persons plus pigs and dogs, so that an adequate sample of the population density variations of A . farauti was obtained over quite a large area. It is not known what proportion of A . farauti visiting the houses

W.

541

PETERS A N D S. H . C H R I S T I A N

throughout the night are found next morning in the traps. A factor of 0.07 has been selected for this area (i.e. 1 in 14 are found in the traps) based on experience at Maprik (PETERS and STANDFAST, 1960) and this is borne out by the few all-night catches made in trap houses. T h e r e is a considerable seasonal variation in the monthly population density of A. farauti. T h e data summarized in Table VI are illustrated in Figure 3 from which a single annual peak of density is seen which slightly precedes the peak of new infections in man. Even if the arbitrary figure of 0.07 is incorrect, the order of magnitude of the seasonal change is quite clear, the peak density being about 80 times the minimum. TABLEVI.

Mean density of A. farauti per man per night. Month M

1~ ay ]Jun____~e July

Aug.

Sept.

Oct.

Nov.

6.86

0.84

0.70

0.28

0.98

1.68

3.64 11.76

20.58 14.98 17.50 22.26 12.88

4.34

2.52

3.10

Mar.

A lpril

Jan.

Fq !b.

3.78

8.54 13.58 18.2

Dec.

1957 1958 1959

T h e relation between these variations and the malaria reproduction rate in the community will be analysed below, but it is interesting at this point to note the theoretical changes that can take place in the inoculation rate. MACDONALD(1957) states that when " h " is the inoculation rate, " m " is the man-biting density, " a " is the number of men bitten by one mosquito in one night and " s " is the SR expressed as a proportion, h = mabs h If " a " is constant, a- = m b s Substituting the data for " m " and " s " already referred to and assuming that " b " is 1.0 in the dry season and 0.8 in the epidemic season, we can calculate that the inoculation rate is increased 900 times in the epidemic season as compared with its dry season level. This indeed is a highly unstable dynamic situation. DISCUSSION As MACKeR~S (1947) pointed out, the essential characters of a malaria vector species are : (i) Susceptibility to infection ; (ii) Abundance ; (iii) Association with man ; (iv) Avidity for h u m a n blood ; (v) A long life span. It has been shown that A. farauti in the Mini area fulfils these criteria. T o g e t h e r with the great seasonal increases in population density of this species it is possible that the genesis of malaria epidemics is aided by slight seasonal changes in its natural longevity. A temporary increase in the numbers of A. punctulatus in exceptionally favourable years no doubt makes a significant contribution to the quantum of transmission at such times. METSEL~R (1959) suggested that the exophilic habits and short average duration of life of the vector anophelines might be an impediment to the spread of malaria to altitudes above about 5,000 feet. W e do not know yet enough about the exophilic tendencies of A. farauti at Mini, but there is abundant evidence to show that it freely enters houses to feed and is markedly anthropophilic. As to its longevity, all the evidence indicates that this species lives longer at the reduced temperature of the highlands than it does in coastal areas.

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EPIDEMIOLOGY OF MALARIA I N NEW GUINEA

While the mean temperature at Mini is low (20.6 ° C.) P. vivax at least can undergo development in the mosquito at even lower temperatures such as those found at 6,000 feet. SUMMARY

The followingentomologicalfeatures of malaria in the Minj area have been established. I) The main characteristic is the variability of population density, sporozoite rate, and natural longevity with thc season. The peak man-biting density of .4. faraut{, the main vector, is 80 times its dry season level. Sporozoiterates vary from zero to about 2 per cent. The natural longevity ranges betwcen about 0.88 and 0.96 (i.e. there is a daily mortality of under 10 per cent. of the population). 2) A. punctulatus occurs seasonally, its distribution being limited by the pattern of rainfall and nature of the soil. A. bancrofti and A. annulipes play no known part in malaria transmission here. 3) About 70 per cent. of all A. farauti entering houses feed on man. This species freely enters and feeds in houses, resting on the walls after feeding. 4) The gonotrophic cycle of A. farauti is probably 4 days at a mean temperature of 20.6 ° C. The extrinsic cycle of P. falciparum is 21 days, P. vivax 16 days, and P. malariae 28 days at this temperature. 5) Interrupted feeding may be a factor in house infections since A. farauti commonly takes daily interrupted feeds under laboratory conditions.

PART VI.

UNSTABLE HIGHLAND MALARIA--ANALYSIS OF DATA AND POSSIBILITIES FOR ERADICATION OF MALARIA BY W. PETERS

Malaria Section, Department of Public Health, Port Moresby, Territory of Papua and New Guinea

As in the first paper of this series (PETERSand STANDFAST,1960), the mathematical presentation of data makes no pretensions to exactness, but is designed to illustrate the order of magnitude of certain situations and events in such a manner that they can be related to each other and to the pattern of malaria in other localities. The main features of the clinical and entomological data described in the first sections of this paper are summarized in Table VII. Since it is impractical to list all possible variations at different seasons, we have selected figures that seem to give a reasonable representation of the situation at seasons of maximum and minimum transmission. In this Table and the following pages only A. farauti is considered in detail.