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A study was undertaken to identify the major larval habitats of the Anopheles gambiae (Giles) complex in rural Gambia. Mosquito larvae and pupae were sampled along transects and in specific habitats in the central region of the country during the rainy seasons of 1996 and 1997. The sampling showed that the major breeding sites were located on the flooded alluvial soils bordering the river. The largest numbers of larvae were found during September, one month after the peak rains. Polymerase chain reaction analysis of specimens showed that Anopheles melas (Theobald) was the dominant species in the flooded areas (81.5%), followed by A. gambiae sensu stricto (Giles) (18.0%) and A. arabiensis (Patton) (0.5%). By sampling in specific habitats it was evident that A. arabiensis was mainly breeding in rain-fed rice fields along the edge of the alluvial soils. Anopheles melas and A. gambiae s.s. often coexisted but whereas A. melas were found in water with a salinity of up to 72% sea water (25.2 g NaCl l(-1)), A. gambiae s.s. only occurred in water with up to 30% sea water (10.5 g NaCl l(-1)). Anopheles melas larvae were found in association with plant communities dominated by sedges and grasses (Eleocharis sp., Paspalum sp., Sporobolus sp.) and sea-purslane Sesuvium portulacastrum (L.) and the presence of cattle hoof prints, whereas A. gambiae s.s. larvae mainly occurred in association with Paspalum sp. and Eleocharis sp. The study showed that even during the peak rainy season, breeding of the A. gambiae complex is almost entirely restricted to the extensive alluvial areas along the river.
After village huts in the Kisumu area of Kenya had been sprayed with a nominal 2 g/m2 of OMS-43 (fenitrothion) as part of an insecticidal evaluation programme 964 adults of the Anopheles gambiae complex collected from huts were identified cytogenetically as either An. gambiae (species A) or An. arabiensis (species B). Similarly, cytogenetic methods were used to identify 349 adults collected from granaries and artificial pit-shelters. In addition, 2203 larvae of the gambiae complex collected from different types of habitats were specifically identified cytogenetically. As in a previous survey made prior to insecticidal spraying, adults of An. arabiensis predominated in outdoor collections while An. gambiae was highly endophilic. However, it appeared that spraying with fenitrothion had resulted in an increase in both the degree of exophily in An. arabiensis and also its relative numbers in respect to An. gambiae, although the overall pooulation of both species was greatly reduced by spraying. Insecticidal spraying may have diverted both species from feeding on man to cattle. Except for one village, where larvae of An. arabiensis were commoner than expected in cattle hoof-prints, there was little difference between the selection of different types of larval habitats by the two sibling species. Chromosomal inversions were more common in adults of An. arabiensis than in An. gambiae.
Malaria was recognized as an important cause of death among early European visitors to The Gambia, but the infection was first studied systematically in the local population only in the 1950s. Studies undertaken in the village of Keneba at that time showed that nearly all children under the age of 5 years had parasitaemia throughout the year. More recent surveys in rural areas of The Gambia have shown much lower levels of parasitaemia, probably as a result of a decline in rainfall in The Gambia during the past 30 years and because of an increase in the availability of anti-malarial drugs. Nevertheless, community surveys and reviews of hospital statistics show that malaria is still one of the most important causes of death among Gambian children; about 1 in 25 rural Gambian children die from malaria before reaching the age of 5 years. Until recently, malaria control in The Gambia relied upon prompt treatment of clinical attacks, first with quinine and more recently with chloroquine, and upon some limited vector control in the capital, Banjul. However, during the past few years, it has been shown that mortality in rural children can be reduced substantially by means of chemoprophylaxis given by village health workers. Bed nets (mosquito nets) are used widely in The Gambia and epidemiological surveys have shown an association between the use of bed nets and protection against malaria. This observation led to a series of small scale intervention trials.(ABSTRACT TRUNCATED AT 250 WORDS)
Anopheles gambiae sensu lato Giles, 1902 and Anophelesfunestus Giles, 1900 are the main malaria vectors on the island of Bioko (Equatorial Guinea). This study was carried out to determine: a) members of the An. gambiae complex that may be present on the island of Bioko and, b) the sensitivity of An. gambiae sensu stricto to pyrethroids. The analysis by PCR detected the presence of An. gambiae s.s. as the major vector of the complex and the "forest chromosomal form" was demonstrated by cytogenetic analysis. The presence of Anopheles melas in the southwest, north and southeast of the island justifies its study as a vector. The molecular characterization of pyrethroid knockdown resistance (kdr) showed that the populations of An. gambiae s.s. were sensitive and no mutations were found. This fact justifies the implementation on a large scale of pyrethroid-impregnated bednets within the framework of the Malaria Control Program of Equatorial Guinea.
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.
Polytene chromosome studies on the member species of the Anopheles gambiae complex in The Gambia and surrounding areas in Senegal led to comparative observations on bionomics of sympatric populations of An. melas and An. gambiae. Moreover, inversion polymorphisms have been analyzed in An. melas and their possible relationships with behavioural variations in endophily and anthropophily have been considered. An. melas shows a remarkably short dispersal from typical larval breeding places associated with mangrove swamps and it is definitely more zoophilic and exophilic than An. gambiae. Only a very small fraction of An. melas biting outdoor on animals rests indoors and consequently the human blood index is largely overestimated if based on the examination of house samples alone. Differences in the frequencies of 2Rn inversion karyotypes of An. melas were observed between parallel samples obtained from animal shelters and houses, from night catches on man outdoor and from night catches on main indoor and on animal outdoor. Further differences were shown by blood meal identification between human and animal fed subsamples from the same house resting samples. Non-uniform feeding and/or resting behaviour between carriers of alternative 2Rn inversion karyotypes is postulated to explain these data.
In Mali the Anopheles gambiae complex consists of An. arabiensis and Mopti, Savanna and Bamako chromosomal forms of An. gambiae s.s. Previous chromosomal data suggests a complete reproductive isolation among these forms. Sequence analysis of rDNA regions led to the characterization of two molecular forms of An. gambiae, named M-form and S-form, which in Mali correspond to Mopti and to Savanna/Bamako, respectively, while it has failed so far to show any molecular difference between Savanna and Bamako. The population structure of An. gambiae s.l. was analysed in three villages in the Bamako and Sikasso areas of Mali and the frequency of pyrethroid resistance of the knock-down resistance (kdr) type was calculated. The results show that the kdr allele is associated only with the Savanna form populations and absent in sympatric and synchronous populations of Bamako, Mopti and An. arabiensis. This is the first molecular indication of barriers to gene flow between the Bamako and Savanna chromosomal forms. Moreover, analyses of specimens collected in the Bamako area in 1987 show that the kdr allele was already present in the Savanna population at that time, and that the frequency of this allele has gradually increased since then.
Two cloned DNA sequences, lambda C10 and lambda G12, have been isolated from a female Anopheles gambiae sensu stricto genomic DNA library in lambda EMBL4. The lambda C10 clone hybridized with equal intensity to all five of the six species in the An. gambiae Giles complex tested and was therefore suitable for use as a complex-specific clone. The lambda G12 clone was selected for its ability to distinguish the two major vectors of malaria within the complex, An. gambiae s.s. and An. arabiensis. Use of libraries consisting of only female DNA prevented the isolation of male-specific sequences. Southern blot analysis of the cloned DNA permitted the development of smaller Alu I subclones suitable for sequencing that still retained the original specificities and sensitivities of lambda C10 and lambda G12. Each clone was found to possess a series of repeated sequences in direct tandem array of 92-94 and 68 bases, respectively. A comparison of a number of copies of each of the repetitive sequences within the Alu I subclones enabled the definition of consensus sequences for the repetitive elements in lambda C10 and lambda G12. Based on these consensus sequences, two oligonucleotides of 21 and 23 bases designated pAngsl and pAngss were derived from lambda C10 and lambda G12, respectively. When tested as probes against DNA dot-blots and squash-blots of mosquito specimens, each oligonucleotide retained the same species specificity as the original clones from which they were derived. The nonradioactive, alkaline phosphatase-labeled pAngsl was able to detect as little as 1 ng of target genomic DNA by chemiluminescent detection in a 5-hr autoradiographic exposure. The pAngss probe could detect 5-10 ng of genomic DNA in similar assays. The new probes exhibit great potential for use in An. gambiae complex species identification because they provide both a means of distinguishing the two major vectors of malaria within the complex and of assessing the quality of squashed mosquito samples by providing a means of standardizing hybridization results.
Following the detection of two cases of leishmaniasis in The Gambia, the possible vectors and animal reservoirs were studied. A total of 5, 158 phlebotomine sandflies, in 20 species and subspecies were captured, including 98 males and 61 females of Phlebotomus duboscqi. This species is a vector of cutaneous leishmaniasis in Senegal and must be suspected as a vector in The Gambia; it was captured close to the dwellings of both patients and from rodent burrows, including those of Mastomys erythroleucus, a known reservoir of cutaneous leishmaniasis in Senegal. We report the first finding in The Gambia of visceral leishmaniasis in a dog captured near the house of the patient with visceral leishmaniasis. This strongly suggests that dogs could be a reservoir of this infection in this area. The vector was not determined.
Population dynamics of the Anopheles gambiae complex of malaria vector mosquitoes were studied in four small hamlets in The Gambia. Bednets were used to reduce man/vector contact in two of the hamlets. High densities of An. gambiae, sensu lato, were present for only 3-8 weeks during the rainy season, depending on the position of the hamlet within the study area. The proportions of blood-fed mosquitoes caught indoors (83.0%) and existing from houses (11.6%) were lower in hamlets where bednets were used than in hamlets without (96.5% and 33.1% respectively). Fewer of the blood-fed mosquitoes had fed on man in houses where people slept under bednets (68.2%) than in those without (81.5%). However, the average number of infective bites received by children was still greater than one a year in hamlets where bednets were used. Consequently bednets are considered unlikely to be an effective malaria control measure so long as they are untreated with insecticide.
Malaria vector mosquitoes belonging to the Anopheles gambiae complex were studied in four hamlets in The Gambia. All inhabitants were given bednets treated either with a placebo (milk) in two hamlets or with the pyrethroid insecticide permethrin (500 mg/m2) in two other hamlets. Malaria transmission occurred mainly during a few weeks of the rainy season, in September and October 1987. The indoor resting densities of mosquitoes in permethrin-treated hamlets were reduced, and we estimated over 90% reduction in biting on man by An. gambiae Giles sensu stricto in these hamlets. No mosquitoes were found under permethrin-treated bednets compared with eighty-one recovered from placebo-treated bednets. Mosquitoes exited more readily from rooms where permethrin-treated bednets were used than from rooms with placebo-treated nets. The annual mean probability that a child would receive an infective bite was estimated to be 0.09 in hamlets with insecticide-treated bednets, compared with 1.9 where placebo-treated bednets were used. Permethrin-treated bednets are therefore recommended as a means of effectively reducing the risk of exposure to malaria transmission, particularly in areas of low seasonal transmission.
Movement of mosquitoes belonging to the Anopheles gambiae complex (mixed wild populations of An.arabiensis, An.gambiae and An.melas) between three neighbouring rural villages in The Gambia was investigated by mark-release-recapture. A total of 12,872 mosquitoes were collected in bednets, marked with a magenta fluorescent powder and released over a 15-day period in one of the villages. A further 15,507 mosquitoes were collected in exit traps, marked with a yellow powder and released over the same period. Mosquitoes were captured daily in all three villages using pyrethrum spray catches, as well as bednet and exit trap catches. The catching period extended for 6 days after the last day of release. Of the mosquitoes released, 372 (1.3%) were recaptured 2-21 days later. Of these recaptures, 272 were caught in the release village, and 98 were caught in other villages situated 1-1.4 km away. The 'movement index' between villages was calculated as 17.2% (12.2-22.4% confidence limits) for mosquitoes released after feeding and 20.1% (14.7-25.3%) for those released unfed. These results suggest that movement of mosquitoes between neighbouring villages in The Gambia seriously affects the entomological evaluation of pyrethroid-impregnated bednet programmes in areas where treated and untreated villages are interspersed.
The structure of the rDNA repeating unit of Anopheles gambiae (Diptera: Culicidae) was determined by restriction endonuclease mapping and hybridization analyses on four independent clones obtained from a genomic library of a colony (G3) from the Gambia (West Africa). rDNA gene coding sequences are conserved, but much intragenomic and intraspecific (geographic) variation occurs in the intergenic spacer. Hybridization of subclones from spacer and coding sequences to genomic DNA that was isolated from single mosquitoes from laboratory colonies of four other A. gambiae complex species reveals conservation of coding sequences but concerted evolution in the intergenic spacers.
In The Gambia, the use of permethrin-treated bed nets has led to a reduction in morbidity and mortality from malaria in children. However, no clear evidence has been found for a 'mass-killing effect' on the mosquito vectors as a result of this intervention. Two further entomological studies to investigate this phenomenon have been carried out. In one study, 20 villages were paired so that bed nets in one member of each pair were treated with permethrin. In the other, a cross-over design was used in which treated and untreated bed nets were exchanged between 2 villages. Longevity, biting rate and resting density of the malaria vector population and sporozoite rates were assessed in both studies. Malaria vectors were equally abundant and long-lived, and as likely to be infective, in villages with treated bed nets as in those with untreated nets. However, a clear reduction in the density of the indoor-resting population of mosquitoes in rooms with treated bed nets was found, probably reflecting the excito-repellency of the insecticide. This study confirmed that, in The Gambia, the protection against death and morbidity from malaria seen in children using treated bed nets must be due primarily to personal protection rather than to a 'mass-killing effect' on the mosquito vector population at a village level.
The response of Anopheles gambiae complex mosquitoes to men sleeping under insecticide-impregnated or untreated bednets in six verandah trap huts was studied during the dry season in The Gambia. With this type of hut it was possible to collect live and dead indoor-resting mosquitoes and estimate the number of wild mosquitoes which entered, bloodfed on man, and exited each night. Bednets were treated with emulsions targetted to leave deposits of 25 mg/m2 lambda-cyhalothrin, or 5, 50 or 500 mg/m2 permethrin, diluted from emulsifiable concentrates (EC), or a blank formulation similar to the EC except that the permethrin was omitted; the sixth net was left untreated. Nets and sleepers were rotated between huts on different nights, the design being based on a series of Latin squares and conducted double-blind. Permethrin-impregnated bednets deterred mosquitoes from entering the huts. The degree of deterrency was proportional to the dosage of permethrin. This effect was also caused by the blank formulation and therefore attributed to other components of the formulation, rather than to the permethrin itself. The net impregnated with 500 mg permethrin per square metre gave the best individual protection, reducing mosquito bloodfeeding by 91% compared with untreated nets. However, lambda-cyhalothrin was proportionately more insecticidal than permethrin at doses of equivalent deterrency. At this stage of research, it remains conjectural whether chemical deterrency or killing of malaria vectors is better for community protection.
For two sibling species of mosquitoes belonging to the Anopheles gambiae complex of malaria vectors, the effects of body size (wing length) and bloodmeal size (haematin excretion) on fecundity of wild females were investigated in The Gambia, West Africa. Freshly blood-fed individuals from sympatric populations of An.arabiensis and An.gambiae sensu stricto were sampled by collection at 07.00-09.00 hours from within bednets during July/August 1993, at the beginning of the rainy season. The possible confounding effect of infection with Plasmodium parasites was removed by eliminating infected mosquitoes from the study samples. An.arabiensis females comprised 75% of the An.gambiae sensu lato population and were significantly larger (greater mean wing length) than those of An.gambiae s.s. mosquitoes. Mean egg production per female (for the subsequent gonotrophic cycle, excluding pre-gravids) for the two species was not significantly different, though the relationship between wing length and egg production showed An.gambiae s.s. to be more fecund than the An.arabiensis of the same size. Pre-gravid An.gambiae s.s had consumed significantly smaller bloodmeals than gravid females but the mean wing length of these two gonotrophic categories was not significantly different. In contrast, An.arabiensis pre-gravids were smaller and had consumed smaller bloodmeals than the gravids.
Permethrin-impregnated bednets protect children against malaria in The Gambia, where Anopheles gambiae complex mosquitoes are the main vectors of malaria. However, no effect has been found on mosquito density, parous rates or sporozoite rates in An.gambiae sensu lato populations; only a reduction in the numbers of mosquitoes resting indoors in rooms with treated bednets. A possible explanation for this paradox is that exposure to treated bednets leads to changed vector behaviour such as a shift in biting time, a diversion to biting outdoors instead of indoors, to biting animals instead of humans, or to increased duration of the gonotrophic cycle. To investigate these possibilities, we observed the biting and existing behaviour of An.gambiae in ten pairs of villages, in half of which the residents used permethrin-treated bednets. The possible influence of treated bednets on the gonotrophic cycle length was evaluated by mark-release-recapture experiment. No significant difference was found between villages with treated and untreated bednets in the indoor/outdoor ratio of human biting, in mean biting times or in human blood indices of An.gambiae females found resting indoors in the mornings. The proportions of unfed, fed or gravid An.gambiae females collected in exit traps, and the number of females exiting showed no significant differences between rooms with treated and untreated bednets. Indications for a gonotrophic cycle length of 2 days were found. No evidence for any change in duration of the gonotrophic cycle in relation to exposure to treated bednets was found, although the number of recaptures was low in the villages with treated bednets. Since equal numbers of infective An.gambiae were found in villages with treated or untreated bednets, and no changes in mosquito behaviour were detected, we cannot account for how children are protected against malaria by treated bednets. One possibility is that mosquitoes divert to bite other hosts, including adults.