Anopheles gambiae complex and disease transmission in Africa

A field trial of microencapsulated deltamethrin, a synthetic pyrethroid, for malaria control

The synthetic pyrethroid deltamethrin was evaluated as a 15 mg/m2 residual application of a microencapsulated formulation for efficacy in malaria control in a 50 km2 area of north-eastern Zimbabwe. Results were compared with very large contiguous DDT sprayed (2g/m2) and unsprayed areas. A total of 3544 rooms were sprayed with deltamethrin. No significant side effects of the insecticide on spraymen were noted. Mosquito captures were poor and inconclusive due to drought conditions, but the malaria vectors Anopheles arabiensis and An. gambiae were identified in the area, the former being 14 times more abundant than the latter in bait collections from an unsprayed area. Monthly bioassays using laboratory-reared An. arabiensis showed a higher mortality of mosquitoes exposed on roofs for up to eight months after deltamethrin application and indicated a high mortality in deltamethrin sprayed houses than in DDT sprayed houses. Tests indicated that, after eight months, up to 50% of the deltamethrin remained on the roof and 25% on the wall. Malaria transmission, evaluated by blood slide surveys at the beginning and end of the transmission season, was high in the unsprayed area, slightly less in the adjacent and incompletely isolated deltamethrin sprayed area, and absent or very low in the DDT sprayed area. The results were considered to be favourable for the use of deltamethrin in malaria control by means of residual house-spraying with the non-irritant dosage of 15 mg/m2, but require evaluation in larger trials.

The behaviour of Homo sapiens, the forgotten factor in the transmission of tropical disease

The behaviour of the pathogens responsible for tropical disease and the behaviour of the hosts other than man are both studied in great detail, but the behaviour of man, the third component in these cycles of transmission, is for the most part totally and inexplicably disregarded. Even when the pathogens are actively brought to us through the agency of an arthropod host, we too often ease the passage of the vectors either by unthinkingly providing facilities for their breeding or by neglecting the simple steps that can be taken to prevent their feeding on us. The problem resolves itself into two parts, (i) the collection and collation of relevant data on human behaviour, and (ii) the taking of steps to change this behaviour. Part two has recently been greatly facilitated by the development of radio transmission via artificial satellite. While WHO is now making a start on both these aspects it is doing so at a relatively low level. Instead, the two phases of this new approach should be given top priority even if it means large scale reorganization of relevant university departments and even of WHO itself. We have, after all, had almost 100 years to try out the old methods and, as far as the Third World is concerned, they have for the most part failed.

Seasonal vector density and disease incidence patterns of malaria in an area of Zimbabwe

Anopheline mosquitoes were studied in an area of Zimbabwe to correlate their population densities with seasonality in malaria incidence. Four trapping methods were employed for adult mosquitoes: pit traps, outdoor and indoor man-baited nets and a sheep-baited net. Anopheles funestus exhibited two population peaks during the year, one in March and the other in September/October. The following over-all proportions of the An. gambiae complex were recorded: An. gambiae Giles (2.5%), An. arabiensis Patton (14.3%) and An. guadriannulatus Theobald (83.2%). Peak populations were observed in March, with most An. gambiae and An. arabiensis preferring outdoor man-baited nets. Approximately equal preference for outdoor nets and pit traps was observed for An. guadriannulatus, which was the only species present throughout the year. The implications of these findings for malaria transmission, which showed a peak in April, and the suitability of the sampling methods employed are discussed.

Biting activity and resting behaviour of Anopheles merus Donitz (Diptera: Culicidae) on the Kenya Coast

Behavioural studies on An. merus were conducted during a period of 11 months in Jimbo, a village on the Kenya Coast. The biting activity of An. merus both outdoors and indoors attained a peak between 24.00 and 01.00 hours. The mosquito showed a stronger tendency to bite outdoors than indoors and was markedly exophilic. Various outdoor resting sites were also identified.

The attractiveness of some birds and mammals for mosquitoes in The Gambia, West Africa

In separate experiments in a savanna area of The Gambia, mosquitoes attacking chickens, pigeons, ducks, goats, a man and a calf were compared in baited traps. Anopheles melas and Culex thalassius were common at all baits, but An. melas predominated only in the catches from mammals, where it formed 60-75% of the total catch. Aedes spp., mainly Ae. albocephalus and Ae. punctothoracis, and C. tritaeniorhynchus were a significant component in the mammal-baited catches but were poorly represented at avian baits. In contrast, C. invidiosus, C. neavei, C. tigripes and C. weschei made up a major part (20-32%) of the catches only in the bird-baited traps. Freshwater An. gambiae, probably An. gambiae s.s., were separated from An. melas on palpal index measurements. An. gambiae was taken in significantly larger numbers at man compared with the other baits where few were identified, demonstrating the extreme anthropophily of this species.

Anopheles gambiae and A. melas at Brefet, The Gambia, and their role in malaria transmission

Anopheles melas and A. gambiae were studied at the village of Brefet, The Gambia. The population density of A. gambiae varied according to the rainfall. However, this was not so with A. melas, the highest densities of which occurred after the cessation of the rains. The sporozoite rate averaged 3.5% in A. gambiae but only 0.35% in A. melas. Possible causes of these differences are discussed.

Identification of mosquitoes of Anopheles gambiae species complex A and B by analysis of cuticular components

Two important vectors of malaria in Africa, Anopheles gambiae and Anopheles arabiensis (Diptera: Culicidae), often occur sympatrically and cannot be distinguished morphologically. A chemical method was developed to identify individual laboratory-reared adult males or females of either species by extraction and analysis of cuticular components with gas chromatography. Statistically significant differences were seen between species when selected pairs of peaks were compared.

Intraspecific polymorphism of sex chromosome heterochromatin in two species of the Anopheles gambiae complex

The Hoechst 33258 banding pattern of the mitotic chromosomes of several laboratory and natural populations of the sibling species A. gambiae and A. arabiensis has been analyzed. A clear intraspecific polymorphism of sex chromosome heterochromatin has been observed. Nevertheless in each species heterochromatic variations fall within a characteristic species-specific pattern. Moreover, while laboratory polulations tend to be monomorphic for a given heterchromatic variant, natural populations exhibit a high degree of intrapopulation polymorphism. The possible role of sex chromosome heterochromatin in controlling fertility and mating behaviour of Anopheles mosquitoes is discussed.

Studies on filariasis transmission in Kwale, a Tanzanian coastal village, and the results of mosquito control measures

The main vectors of bancroftian filariasis in Kwale, Tanzania, were Anopheles gambiae complex and Culex p. quinquefasciatus. After vector control by the insecticides Dursban (chlorpyrifos) or Abate (temephos), the number of all man-biting mosquitoes was reduced by 94.8%, and that of house-resting mosquitoes by 95.3%. Anopheles gambiae complex were highly exophilic, possibly due to a high proportion of Anopheles merus in this village. The infection rate of A. gambiae complex rose significantly in the post-control samples (P less than 0.05), but nevertheless the overall reduction in filariasis transmission was 87.2%. The infection rate of A. gambiae complex caught off human bait was significantly higher than that of the spray-caught sample (P less than 0.05). It was suggested that the former sample contained a higher proportion of A. merus. This species may be a better vector of filariasis than freshwater forms.

Chromosomal differentiation and adaptation to human environments in the Anopheles gambiae complex

Speciation in the Anopheles gambiae complex is reviewed and discussed with emphasis on the patterns of chromosomal differentiation, particularly at the intraspecific level. The significance of inversion polymorphism in gambiae and arabiensis (the two species of greatest medical importance) is evaluated with reference to recent field investigations carried out in Nigeria. In both sibling species some of the inversions show clinical geographical changes in frequencies, with evident correlations with climatic conditions and vegetation zones. Microgeographical variations in species distribution and in intraspecific inversion frequencies are also present, which appear mostly related to man-made environmental contrasts. Parallel indoor-/outdoor collections of samples from polymorphic populations of arabiensis and gambiae show that adult mosquitoes carrying certain inversion karyotypes do not distribute at random in relation to the human environment, being significantly more frequent in outdoor than in indoor samples, or vice-versa. Optimal habitat choice appears to be involved in such variations of indoor resting behaviour, since the chromosomal types carried by less endophilic individuals are those more adapted to humid climates, i.e. those which tend to avoid the higher nocturnal saturation deficit of the indoor environment. This phenomenon, producing non-uniform exposure of the vector population to residual insecticides sprayed in houses, might explain the mediocrity of the results of malaria control projects based on house-spraying against endophilic vectors in the African savannas.

Fluorescence banding techniques in the identification of sibling species of the anopheles gambiae complex

The mitotic chromosomes of the sibling species A and B of the Anopheles gambiae complex were stained with Hoechst 33258 and examined by fluorescence microscopy. The autosomes fluoresce homogenously and similarly in both species while the sex chromosomes differ in the location and brightness of some heterochromatic blocks. These cytochemical differences allow the cytotaxonomic identification of these cryptic species.