Anopheles gambiae complex and disease transmission in Africa

Anopheles arabiensis and An. funestus are equally important vectors of malaria in Matola coastal suburb of Maputo, southern Mozambique

Transmission characteristics of malaria were studied in Matola, a coastal suburb of Maputo, the capital City, in southern Mozambique, from November 1994 to April 1996. The local climate alternates between cool dry season (May-October) and hot rainy season (November-April) with mean annual rainfall 650-850 mm. Saltmarsh and freshwater pools provide mosquito breeding sites in Matola. Malaria prevalence reached approximately 60% among people living nearest to the main breeding sites of the vectors. Plasmodium falciparum caused 97% of malaria cases, others being P. malariae and P. ovale. Potential malaria vector mosquitoes (Diptera: Culicidae) collected at Matola during daytime indoor-resting (n = 1021) and on human bait at night (n = 5893) comprised 12% Anopheles coustani Laveran (93% biting outdoors), 46% An. funestus Giles (68% biting indoors) and 42% An. gambiae Giles sensu lato (60% biting outdoors). All 215 specimens of An. gambiae s.l. identified genetically were An. arabiensis Patton. Anopheles funestus populations remained stable throughout the year, whereas densities of the An. gambiae complex fluctuated considerably, with An. arabiensis peaking during the rainy season. No concomitant rise in malaria incidence was observed. Human landing indices of An. funestus and An. arabiensis averaged 1.8 and 3.8 per man-night, respectively. Overall Plasmodium sporozoite rates were 2.42+/-1.24% in 2181 An. funestus and 1.11+/-1.25% in 1689 An. arabiensis dissected and examined microscopically. Mean daily survival rates were 0.79 for both vector species. Estimated infective bites/person/year were 15 An. funestus and 12 An. arabiensis. Biting rates were greatest at 2100-24.00 hours for An. funestus (68% endophagic) and 21.00-03.00 hours for An. arabiensis (40% endophagic). The entomological inoculation rate (EIR) declined sharply over very short distances (50% per 90m) away from breeding-sites of the vectors. Consequently, P. falciparum prevalence among Matola residents was halved 350 m within the town. Implications for the protective effectiveness of a 'cordon sanitaire' by residual house-spraying and/or the use of insecticide-treated bednets are discussed.

Interspecific competition between sibling species larvae of Anopheles arabiensis and An. gambiae

Mosquito larvae of the sibling species Anopheles arabiensis Patton and An. gambiae Giles sensu stricto (Diptera: Culicidae) were investigated for interspecific competition. Single-species and mixed-species populations were reared at 27 degrees C from the first instar to pupation at different densities (100, 200 or 400 larvae/200 cm2 tray) with a constant amount of food, 0.2 mg/larva/day. Pupae obtained from mixed populations were identified to species using PCR. Both species had a 1:1 sex ratio at pupation. Development time to pupation averaged about one day less for An. arabiensis compared to An. gambiae, ranging from 0.93-1.49d for males and from 0.44-0.84 d for females in single populations. In mixed species populations the difference for males ranged from 0.99-1.58d and for females from 0.93-1.62d. Survival rates of An. gambiae s.s. were significantly higher than those of An. arabiensis in both the single-species and mixed-species populations. Mixed-species rearing did not have an effect on the survival of An. gambiae, whereas the mortality rate of An. arabiensis was significantly higher in mixed populations than when only this species was reared at the same densities, suggesting a competitive disadvantage for An. arabiensis in mixed populations. High proportions of larvae (4-35%) were lost during development; these losses could not be accounted for by corpses found in the rearing pans. The possibility of cannibalism was investigated by rearing each species separately in small containers (five per 50 ml), inspected every 6h, but no cannibalism was detected at any stage of development in either species. It was concluded that, under these experimental circumstances, interspecific competition between both species did occur but with a detrimental effect on An. arabiensis only. Relevance of these findings to the ecology of both species in the field is discussed briefly.

Variation in cuticular hydrocarbons among strains of the Anopheles gambiae sensu stricto by analysis of cuticular hydrocarbons using gas liquid chromatography of larvae

Cuticular hydrocarbons of larvae of individual strains of the Anopheles gambiae sensu stricto were investigated using gas liquid chromatography. Biomedical discriminant analysis involving multivariate statistics suggests that there was clear hydrocarbon difference between the Gambian(G3), the Nigerian (16CSS and, its malathion resistant substrain, REFMA) and the Tanzanian (KWA) strains. The high degree of segregation (95%) in hydrocarbons among the four strains investigated indicates that further analysis is needed to enable understanding of hydrocarbon variation in samples of An. gambiae especially from areas where these populations co-exist.

Earth observation, geographic information systems and Plasmodium falciparum malaria in sub-Saharan Africa

This review highlights the progress and current status of remote sensing (RS) and geographical information systems (GIS) as currently applied to the problem of Plasmodium falciparum malaria in sub-Saharan Africa (SSA). The burden of P. falciparum malaria in SSA is first summarized and then contrasted with the paucity of accurate and recent information on the nature and extent of the disease. This provides perspective on both the global importance of the pathogen and the potential for contribution of RS and GIS techniques. The ecology of P. falciparum malaria and its major anopheline vectors in SSA in then outlined, to provide the epidemiological background for considering disease transmission processes and their environmental correlates. Because RS and GIS are recent techniques in epidemiology, all mosquito-borne diseases are considered in this review in order to convey the range of ideas, insights and innovation provided. To conclude, the impact of these initial studies is assessed and suggestions provided on how these advances could be best used for malaria control in an appropriate and sustainable manner, with key areas for future research highlighted.

Olfactory sensitivities of mosquitoes with different host preferences (Anopheles gambiae s.s., An. arabiensis, An. quadriannulatus, An. m. atroparvus) to synthetic host odours

Responses of antennal olfactory cells associated with sensilla trichodea were recorded in females of four Anopheles species (Diptera, Culicidae) with different host preferences: the anthropophilic An. gambiae s.s., the opportunistic An. arabiensis, and the zoophilic An. quadriannulatus and An. maculipennis atroparvus. Stimuli were vapours of synthetic host-odours: ethanoic, propanoic, butanoic, 3-methyl propanoic, 4-methyl butanoic acid, 1-octen-3-ol, and 3- and 4-methyl phenol. On stimulation with fatty acids and phenols either excitation or inhibition of spike activity was found, whereas responses to 1-octen-3-ol were invariably excitatory. The odour spectra of the cells could include activating as well as inhibiting substances. Differences in host preferences may be reflected in the numbers of olfactory cells responding to different odours and/or in the sensitivities of these cells. In An. gambiae more cells were excited by fatty acids than in An. arabiensis and An. m. atroparvus, whereas inhibition occurred more often in the latter two species. In addition, the fatty acid-excited cells in An. gambiae were more sensitive to these substances than in An. m. atroparvus and An. quadriannulatus. On the contrary, in the latter two species cells were more responsive to 1-octen-3-ol. In An. arabiensis, responses of stimulus-excited cells were intermediate between those in the anthropophilic and zoophilic species.

Sensitivities of antennal olfactory neurons of the malaria mosquito, Anopheles gambiae, to carboxylic acids

Single sensillum recordings on the antennae of female Anopheles gambiae s.s. mosquitoes revealed neurons sensitive to aliphatic carboxylic acids within (a) subtype(s) sensilla trichodea. The aliphatic acids, acetic acid, propionic acid, butyric acid, iso-butyric acid and iso-valeric acid evoked an inhibition reaction in one of the cell types recorded from. A different cell type was excited in response to the former aliphatic acids, but showed a broader range of sensitivity, as acids with a longer carbon chain length, like caproic acid, elicited excitations as well. In addition, occasionally 1-octen-3-ol elicited an excitation reaction. This article focuses on the carboxylic acid inhibited cell type and its temporal pattern of response to different doses of the compounds. Furthermore, in order to compare the stimulatory effectiveness of the compounds on a per molecule basis, corrections were made for differences in volatility by determining the absolute number of molecules in the stimulus puff.

Visual and olfactory responses of haematophagous Diptera to host stimuli

Key biotic and environmental constraints on the host-orientated behaviour of haematophagous Diptera are summarized. For each major group of biting Diptera, responses to host stimuli are reviewed, including activation and ranging behaviour, long-range and short-range olfactory responses and visual responses. Limitations to the comparison of results between groups of species, and the practical problems of experimental method and equipment are discussed.

Odor-mediated behavior of Afrotropical malaria mosquitoes

The African mosquito species Anopheles gambiae sensu lato s.l. and Anopheles funestus rank among the world's most efficient vectors of human malaria. Their unique bionomics, particularly their anthropophilic, endophagic and endophilic characters, guarantee a strong mosquito-host interaction, favorable to malaria transmission. Olfactory cues govern the various behaviors of female mosquitoes and here we review the role of semiochemicals in the life history of African malaria vectors. Recent evidence points towards the existence of human-specific kairomones affecting host-seeking A. gambiae s.l., and efforts are under way to identify the volatiles mediating this behavior. Based on examples from other Culicidae spp., it is argued that there is good reason to assume that mating, sugar feeding, and oviposition behavior in Afrotropical malaria vectors may also be mediated by semiochemicals. It is foreseen that increased knowledge of odor-mediated behaviors will be applied in the development of novel sampling techniques and possibly alternative methods of intervention to control malaria.

Cloning of inversion breakpoints in the Anopheles gambiae complex traces a transposable element at the inversion junction

Anopheles arabiensis, one of the two most potent malaria vectors of the gambiae complex, is characterized by the presence of chromosomal paracentric inversions. Elucidation of the nature and the dynamics of these inversions is of paramount importance for the understanding of the population genetics and evolutionary biology of this mosquito and of the impact on malaria epidemiology. We report here the cloning of the breakpoints of the naturally occurring polymorphic inversion 2Rd' of A. arabiensis. A cDNA clone that cytologically mapped on the proximal breakpoint was the starting material for the isolation of a cosmid clone that spanned the breakpoint. Analysis of the surrounding sequences demonstrated that adjacent to the distal breakpoint lies a repetitive element that exhibits distinct distribution in different A. arabiensis strains. Sequencing analysis of that area revealed elements characteristic of transposable element terminal repeats. We called this presumed transposable element Odysseus. The presence of Odysseus at the junction of the naturally occuring inversion 2Rd' suggests that the inversion may be the result of the transposable element's activity. Characteristics of Odysseus' terminal region as well as its cytological distribution in different strains may indicate a relatively recent activity of Odysseus.

The first report of Anopheles farauti sensu stricto below the nineteenth parallel at Mackay, Queensland

Fifty-six adult female mosquitoes of the species Anopheles farauti sensu stricto--the arthropod vector for malaria--were collected from Mackay, central Queensland, at latitude 21 degrees 09'S, between August 1996 and August 1997. This is the southernmost record of this species, and may indicate that the malaria-receptive zone in Queensland needs to be expanded.

Mapping the ranges and relative abundance of the two principal African malaria vectors, Anopheles gambiae sensu stricto and An. arabiensis, using climate data

Members of the Anopheles gambiae complex are major malaria vectors in Africa. We tested the hypothesis that the range and relative abundance of the two major vectors in the complex, An. gambiae sensu stricto and An. arabiensis, could be defined by climate. Climate was characterized at mosquito survey sites by extracting data for each location from climate surfaces using a Geographical Information System. Annual precipitation, together with annual and wet season temperature, defined the ranges of both vectors and were used to map suitable climate zones. Using data from West Africa, we found that where the species were sympatric, An. gambiae s.s. predominated in saturated environments, and An. arabiensis was more common in sites subject to desiccation (r2 = 0.875, p < 0.001). We used the nonlinear equation that best described this relationship to map habitat suitability across Africa. This simple model predicted accurately the relative abundance of both vectors in Tanzania (rs = 0.745, p = 0.002), where species composition is highly variable. The combined maps of species' range and relative abundance showed very good agreement with published maps. This technique represents a new approach to mapping the distribution of malaria vectors over large areas and may facilitate species-specific vector control activities.

The Anopheles gambiae complex: a new species from Ethiopia

Historically, members of the Anopheles gambiae complex from Ethiopia have been identified chromosomally as either A. arabiensis or A. quadriannulatus. Recent collections from the Jimma area in Ethiopia, southwest of Addis Ababa, revealed 29 specimens of A. quadriannulatus based on the standard polymerase chain reaction (PCR) identification method. 'Wild' females were induced to lay eggs and the progeny reared as individual families. Resulting adults were cross-mated to a laboratory colony strain of A. quadriannulatus originating from the Kruger National Park, South Africa. Hybrid progeny were obtained only from the colony female x Ethiopian male cross. This cross produced a female/male sex ratio of 0.48. Male offspring were sterile and ovarian polytene chromosomes from hybrid females showed typical asynapsis as expected in interspecific crosses within the A. gambiae complex. The X chromosomes, although apparently having homosequential banding patterns, were usually totally asynapsed. All autosomes were homosequential. The lack of inversion heterozygotes, in both the wild and hybrid samples, may simply be a reflection of the small sample size. Until such time as the Ethiopian species can be formally described and assigned a scientific name, it is provisionally designated Anopheles quadriannulatus species B because of its close similarity to this species.

The role of four anopheline species (Diptera: Culicidae) in malaria transmission in coastal Tanzania

Malaria is holoendemic in coastal Tanzania with Anopheles funestus and members of the A. gambiae complex being mainly responsible for transmission. Over a 4 months' sampling period 2222 anopheline mosquitoes were collected using light-traps and indoor resting catches, of which 58.6% were A. gambiae, 7.6% A. arabiensis, 6.9% A. merus and 26.9% A. funestus. Plasmodium falciparum circumsporozoite antigen (CSA) rates were: A. funestus 6.05% (n = 479), A. gambiae 8.4% (n = 1042), A. arabiensis 7.3% (n = 136) and A. merus 9.8% (n = 122). The P. malariae CSA rate for all anophelines was 0.07% (n = 1862). Estimated sporozoite densities were less than 2000 for at least 50% of all the positive mosquitoes. Along the coast the abundance of A. merus (41.3%) and A. gambiae (46.1%) was similar, and their CSA rates were comparable (11.6% and 12.5%, respectively) and higher than those for A. arabiensis (7.7%) and A. funestus (4.6%). These results indicate that A. merus plays an unexpectedly important role in malaria transmission in coastal Tanzania.

Mosquito eye design: conical rhabdoms are matched to wide aperture lenses

Night-active mosquitoes have photopigment-containing rhabdoms that take the form of a wide hollow cone, with the distal apex at the focus of the ommatidial lens. It is shown geometrically, and with the aid of a gelatin model, that such a structure can trap light over an angle of up to 60 deg, roughly equal to the width of the cone of light provided by the lens. This gives a gain in light-gathering power of up to nine times that of the more usual cylinder-shaped rhabdom with an acceptance angle of less than 25 deg.

Host choice by indoor-resting Anopheles arabiensis in Ethiopia

The host preference of indoor resting Anopheles arabiensis has been determined using a direct enzyme-linked immunosorbent assay. A total of 611 specimens, 258 from human dwellings, 179 from mixed dwellings, and 174 from cattle sheds, was examined. The proportion of human blood meals identified was highest from mosquitoes caught in human dwellings (91.5%), followed by those from mixed dwellings (20.2%) and cattle sheds (3.5%) (P < 0.0001). The smaller proportion of human blood meals from mixed dwellings suggests that cattle may protect humans from A. arabiensis.

Resting habits of Anopheles arabiensis in the Awash river valley of Ethiopia

The indoor- and outdoor-resting habits of Anopheles arabiensis were studied over 2 years in central Ethiopia. Pyrethrum-spray catches and outlet-window-trap collections were carried out in both DDT-sprayed village huts and unsprayed huts. Mosquitoes were also collected from pit shelters and natural outdoor-resting sites. Females were marked and released in order to investigate their daytime-resting sites in sprayed and unsprayed huts. The An. arabiensis population in the study area is partially (37.5%) exophilic, while those females resting indoors tend to avoid DDT-sprayed surfaces and thus their contact with the insecticide is minimized.

Comparative fecundity and associated factors for two sibling species of the Anopheles gambiae complex occurring sympatrically in The Gambia

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.

Identification of electrophysiologically-active compounds for the malaria mosquito, Anopheles gambiae, in human sweat extracts

Human sweat samples were chemically fractionated into acid and non-acid components. The most abundant volatile compounds present in the fractions were identified by linked gas chromatography mass spectrometry. The acid fractions were found to be composed of a range of twenty aliphatic and three aromatic carboxylic acids ranging, on average, from 0.02 to 20 micrograms per ml of sweat sampled. Non-acid fractions were found to contain: 6-methyl-5-hepten-2-one, 1-octen-3-ol, decanal, benzyl alcohol, dimethylsulphone, phenylethanol, phenol and 4-methylphenol, collectively amounting to 0.1 and 3 micrograms per ml of sweat. The major component of sweat was found to be L-lactic acid which constituted from 1 to 5 mg/ml. Using the intact antennae of the anthropophilic malaria vector mosquito Anopheles gambiae Giles, the peripheral olfactory activities of compounds identified in the sweat fractions were investigated by electroantennography (EAG). Short-chain saturated carboxylic acids, methanoic, ethanoic, propanoic, butanoic, pentanoic and hexanoic acids were found to elicit significantly larger EAG responses than longer chain saturated carboxylic acids from female An.gambiae. For a given dose the largest amplitude EAG response was elicited by methanoic acid. Pentanoic acid elicited larger EAG responses than either butanoic or hexanoic acids. Two non-acidic compounds, 1-octen-3-ol and 4-methylphenol, were found to elicit significant dose-dependent EAG responses from female An.gambiae. 1-Octen-3-ol elicited larger EAG responses than 4-methylphenol for a given dose, but both compounds elicited smaller EAG responses than the same dose of C1-C6 straight-chain aliphatic carboxylic acids. The possible behavioural significance of the EAG-active compounds identified in human sweat samples is discussed.

Density, survival and dispersal of Anopheles gambiae complex mosquitoes in a west African Sudan savanna village

To obtain information on adult populations of Afrotropical malaria vector mosquitoes, mark-release-recapture experiments were performed with Anopheles females collected from indoor resting-sites in a savanna area near Ouagadougou, Burkina Faso, during September 1991 and 1992. Results were used to estimate the absolute population densities, daily survival rates, and dispersal parameters of malaria vectors in that area. In 1991 a total of 7260 female Anopheles were marked and released, of which 106 were recaptured in the release village and 6 in the neighbouring villages, a total recapture rate of 1.5%. The following year 13,854 female Anopheles were released and 116 recaptured in Goundri and 8 in the neighbouring villages, a total recapture rate of 0.9%. Recaptures were found in three of eight villages near Goundri. Nearly all of the recaptured mosquitoes were An gambiae s.l. Of these, molecular determination revealed that An.gambiae s.s. and An.arabiensis were present in a ratio of approximately 2:3. Two simple random models of dispersal were simulated and the parameters of the models determined by searching for the least-squared fit between simulated and observed distributions. The mean distance moved by individual mosquitoes, estimated in this way, ranged 350-650 m day-1, depending on the model and the year considered. Population densities were estimated using the Lincoln Index, Fisher-Ford and Jolly's methods. The estimates of population size had high standard errors and were not particularly consistent A "consensus' value of 150,000-350,000 mosquitoes is believed to apply for the An.gambiae s.l. female population. Survival was estimated to be 80-88% per day.

Geographical perspectives on bednet use and malaria transmission in The Gambia, West Africa

Insecticide-impregnated bednets are now widely accepted as an important tool in reducing malaria-related deaths in children in Africa. Defining the circumstances in which net treatment programmes are likely to be effective is essential to a rational development of this control strategy. In The Gambia a National Impregnated Bednet Programme was introduced into the primary health care system in 1992. Prior to its introduction baseline epidemiological and entomological studies were conducted throughout the country. These studies showed that in areas where mosquito biting nuisance was high, people protected themselves with bednets and that where mosquito densities (and therefore bednet usage) was low malaria prevalence rates were relatively high. Since the national programme is designed to assist only those people who already own a bednet (by providing the insecticide) an understanding of the factors which determine bednet ownership is needed to help evaluate the programme's effectiveness and provide guidelines for increasing bednet usage. Village scale bednet usage rates and malaria prevalence rates obtained from the baseline survey were correlated with certain geographical variables: dominant ethnic group, area, habitat, distance from the River Gambia and distance from the 'bluffline' (the interface between the sandstone soils and alluvial soils which border the river system). In a multiple regression analysis, bednet usage was independently associated with area (P < 0.001), ethnic group (P = 0.010), habitat (P = 0.006) and distance from the river (P = 0.013). A negative association of bednet usage with malaria prevalence persisted after allowing for the other variables. Malaria prevalence was not independently associated with area, ethnic group, habitat or distance from the river. Our analysis showed that the impregnated bednet programme is likely to be most effective in villages which are sited near to or on the alluvial soils in the middle and lower river zones. These villages, which were originally settled for easy access to the river (for transport) and its swampy margins (for rice production) are within the flight distance of mosquitoes that have their breeding sites on the poorly drained alluvial soils. Variation in malaria prevalence rates (after bednet usage has been taken into account) may be related to factors such as poverty and access to health care, and/or to localized differences in the ecology of The Gambia, which determine the duration and intensity of transmission. If the National Bednet Programme is to be effective throughout The Gambia it is vital to develop promotional activities which will encourage bednet usage in areas where nuisance biting by mosquitoes is low.

Mosquito responses to carbon dioxide in a west African Sudan savanna village

Mosquito responses to carbon dioxide were investigated in Noungou village, 30 km northeast of Ouagadougou in the Sudan savanna belt of Burkina Faso, West Africa. Species of primary interest were the main malaria vectors Anopheles gambiae s.s. and An.arabiensis, sibling species belonging to the An.gambiae complex. Data for An.funestus, An.pharoensis, Culex quinquefasciatus and Mansonia uniformis were also analysed. Carbon dioxide was used at concentrations of 0.04-0.6% (cf. 0.03% ambient concentration) for attracting mosquitoes to odour-baited entry traps (OBETs). The "attractiveness' of whole human odour was also compared with CO2 emitted at a rate equivalent to that released by the human bait. In a direct choice test with two OBETs placed side-by-side, the number of An.gambiae s.l. entering the trap with human odour was double the number trapped with CO2 alone (at the human equivalent rate), but there was no significant difference between OBETs for the other species of mosquitoes. When OBETs were positioned 20 m apart, again CO2 alone attracted half as many An.gambiae s.l. and only 40% An.funestus, 65% Ma.uniformis but twice as many An.pharoensis compared to the number trapped with human odour. The dose-response for all mosquito species was essentially similar: a linear increase in catch with increasing dose on a log-log scale. The slopes of the dose-response curves were not significantly different between species, although there were significant differences in the relative numbers caught. If the dose-response data are considered in relation to a standard human bait collection (HBC), however, the behaviour of each species was quite different. At one extreme, even the highest dose of CO2 did not catch more An.gambiae s.l. than one HBC. At the other extreme, the three highest doses of CO2 caught significantly more Ma.uniformis than did one HBC. An.pharoensis and Cx quinquefasciatus showed a threshold response to CO2, responding only at doses above that normally released by one man. An.funestus did not respond to CO2 alone at any dose in sufficient numbers to assess the dose response. Within the An.gambiae complex, An.arabiensis "chose' the CO2-baited trap with a higher probability than An.gambiae s.s. Also An.arabiensis, the less anthropophilic of the two species, was more abundant in CO2-baited OBETs than in human bait collections.

Evolution of the mitochondrial DNA control region in the Anopheles gambiae complex

We have sequenced the AT-rich control region of the mitochondrial DNA (mtDNA) of six species in the Afrotropical Anopheles gambiae complex and the closely related A. christyi. Contrary to expectations, the AT-rich region in this group is evolving rather slowly, more slowly than the third position of mtDNA protein-coding genes. Despite being relatively conserved between species, we detected intraspecific and intra-individual (heteroplasmy) variation in this region. Phylogenetically, we found we could place the rare endemic A. bwambae as a sister taxon to A. melas, the same evolutionary position as indicated by chromosomal inversions. The outgroup, A. christyi, gave evidence of the root of the tree. In comparing the molecular trees with that deduced by chromosomal inversions, they are completely congruent with the exception of the placement of A. arabiensis. The anomalous position of this species can be explained by introgression with A. gambiae. From the phylogenetic position, we could infer mtDNA gene flow from A. gambiae to A. arabiensis.

Anopheles arabiensis and An. gambiae chromosomal inversion polymorphism, feeding and resting behaviour in relation to insecticide house-spraying in Tanzania

Differential responses of the mosquitoes Anopheles arabiensis and An. gambiae sensu stricto to house-spraying with DDT or lambda-cyhalothrin were evaluated in relation to chromosomal inversion polymorphism, feeding and resting behaviour of these malaria vectors in Tanzania. Blood-fed mosquitoes from pit traps outdoors, exit traps on windows and indoor-resting catches were identified cytogenetically and the chromosomal inversion frequencies compared between samples and species. Their outdoor-resting behaviour was assessed by a mark-release-recapture experiment and by determining the proportion of freshly blood-fed individuals in exit traps. The source of bloodmeals was analysed by an ELISA method. Endophagic females of An. arabiensis were more likely than those of An.gambiae to exit from a house on the night of blood-feeding. Only in one out of three villages was there evidence that chromosomally distinct individuals within a species had different preferences for resting sites. There were indications, but not conclusive evidence, that mosquitoes caught indoors or outdoors had a tendency to return to the same type of resting site. In villages sprayed with either insecticide, the mean age of the vector populations was greatly reduced, compared with those in the unsprayed villages. An.arabiensis females exited from DDT sprayed houses after blood-feeding, whereas with lambda-cyhalothrin those exiting were mostly unfed and there was a decline in the human blood index. The excitorepellency of DDT was perceived as a disadvantage, whereas lambda-cyhalothrin apparently had more impact on malaria transmission by An.arabiensis.

Comparison of sampling anopheline mosquitoes by light-trap and human-bait collections indoors at Bagamoyo, Tanzania

The mosquito sampling efficiency of CDC miniature light-traps, relative to night-biting collections, was evaluated indoors at two sites in coastal Tanzania. We found that the total number of anophelines captured overnight by light-traps (hung beside a bednet in use) was 1.23 times the number of anophelines captured by human-bait collections. This relationship was not affected significantly by changes in the mosquito density, order of trapping method, date of sampling, or number of household occupants. Malaria sporozoite rates were twice as high among mosquitoes captured by light-trap as compared to those captured by night-biting collection. This was attributed to the tendency of light-traps to capture a larger proportion of gravid mosquitoes, which also had high sporozoite rates. The differences in sporozoites rates according to abdominal stage indicates that unfed mosquitoes captured by light-traps may define more precisely the human-biting activity and sporozoite rates as seen by night-biting collections. Our study shows that light-traps, when used in combination with night-biting collections, can be an effective and sensitive means for measuring human-biting activity and the sporozoite rate.

Activation of Anopheles gambiae mosquitoes by carbon dioxide and human breath

Female Anopheles gambiae Giles mosquitoes were observed individually in a cage within a wind tunnel and their responses to pulses of carbon dioxide recorded on video tape. The range of concentrations tested revealed an 'activation' threshold concentration of carbon dioxide in the region of 0.01% above background. At this concentration, approximately 60% of the mosquitoes took off and flew upwind. Pulses of human breath, diluted with wind tunnel air to reproduce equivalent concentrations of carbon dioxide, elicited similar levels of response and the same 'activation' threshold concentration. These findings are discussed in relation to the activation of host-seeking mosquitoes.

Malaria infection potential of anopheline mosquitoes sampled by light trapping indoors in coastal Tanzanian villages

Anopheline mosquito populations were studied during 1992 in seven villages south of Bagamoyo, coastal Tanzania, prior to malaria control intervention using insecticide treated bednets. To collect mosquitoes, CDC light traps were used in ten houses per village fortnightly for 12 months. Anopheles females were identified and checked by ELISA for the presence of malaria sporozoite antigen and source of bloodmeal. An.funestus peaked in June-July after the long rains. Three members of the An.gambiae complex had different seasonality: An.arabiensis, An.gambiae and small numbers of An.merus were collected. In most villages transmission was extremely high and perennial with the entomological inoculation rate reaching three to eleven infective bites per person per night in July and persisting at around 0.1 and 1 for most of the remainder of the year. Sporozoite infection rates within the An.gambiae complex ranged from 2% to 25%, with the peaks in January and July following the two rainy periods. An.funestus showed a similar pattern. The light traps were reliable, simple to operate, and proved to be satisfactory to study the mosquito vector population.

Distribution of genetic diversity in relation to chromosomal inversions in the malaria mosquito Anopheles gambiae

The epidemiology of malaria in Africa is complicated by the fact that its principal vector, the mosquito Anopheles gambiae, constitutes a complex of six sibling species. Each species is characterized by a unique array of paracentric inversions, as deduced by karyotypic analysis. In addition, most of the species carry a number of polymorphic inversions. In order to develop an understanding of the evolutionary histories of different parts of the genome, we compared the genetic variation of areas inside and outside inversions in two distinct inversion karyotypes of A. gambiae. Thirty-five cDNA clones were mapped on the five arms of the A. gambiae chromosomes with divisional probes. Sixteen of these clones, localized both inside and outside inversions of chromosome 2, were used as probes in order to determine the nucleotide diversity of different parts of the genome in the two inversion karyotypes. We observed that the sequence diversity inside the inversion is more than three-fold lower than in areas outside the inversion and that the degree of divergence increases gradually at loci at increasing distance from the inversion. To interpret the data we present a selectionist and a stochastic model, both of which point to a relatively recent origin of the studied inversion and may suggest differences between the evolutionary history of inversions in Anopheles and Drosophila species.

Selection of biting sites on man by two malaria mosquito species

While searching for blood, female mosquitoes pass through a behavioural process involving responses to visual, physical and chemical properties of the host. Temperature and humidity are thought to dominate mosquito orientation near the host. We observed that biting of two malaria mosquito species, i.e. Anopheles atroparvus (van Thiel) and Anopheles gambiae s.s. (Giles) preferentially occurs on different body regions of a naked motionless human host. Their preference for the head and foot regions respectively correlated with particular combinations of skin temperature and eccrine sweat gland density. Subsequent modification of the host's odour profile by removing exhaled breath and washing feet results in significant changes of these preferences.

Origin of blood meals in indoor and outdoor resting malaria vectors in western Kenya

Blood meals were obtained from indoor and outdoor resting malaria vectors in three villages of western Kenya and tested by sandwich ELISA to determine host preferences and their human blood index (HBI). Anopheles gambiae s.s. collected indoors at Kisian village had a HBI of 0.97 while that of Anopheles arabiensis collected at Ahero was 0.23. However, the HBI of A. arabiensis varied depending on the availability of outdoor resting shelters. Most female A. arabiensis (98.9%) collected outdoors in granaries at Ahero had fed on cattle. Indoor-collected female Anopheles funestus had mainly fed on people (93.0%), but taken at least some of their blood (20.2%) from cattle. Although small numbers of A. arabiensis fed on sheep or goats and birds, none of the female A. gambiae s.s. and A. funestus tested had fed on these hosts. The absence of human-fed A. arabiensis in outdoor shelters indicated that exiting after feeding, a behaviour pattern that mitigates indoor insecticidal spraying, is not prevalent in this species in western Kenya.

Molecular phylogeny of the Anopheles gambiae complex suggests genetic introgression between principal malaria vectors

The six Afrotropical species of mosquitoes comprising the Anopheles gambiae complex include the most efficient vectors of malaria in the world as well as a nonvector species. The accepted interpretation of evolutionary relationships among these species is based on chromosomal inversions and suggests that the two principal vectors, A. gambiae and Anopheles arabiensis, are on distant branches of the phylogenetic tree. However, DNA sequence data indicate that these two species are sister taxa and suggest gene flow between them. These results have important implications for malaria control strategies involving the replacement of vector with nonvector populations.

DNA-based methods for the identification of insect vectors

Many insect vectors are members of complexes composed of morphologically identical sibling species. The identification of individual species, a requirement of epidemiological studies and control programmes, has traditionally relied upon techniques such as chromosomal analysis or isoenzyme typing. Owing to the limitations of these techniques, the last few years have seen many developments in DNA-based technologies for identification. DNA-based protocols have advantages over the other techniques utilized, in that they may identify all insect stages of both sexes using alcohol-preserved, dried, fresh or frozen specimens. The methods ultimately rely upon either DNA probe hybridization or the polymerase chain reaction (PCR). This review describes a number of approaches taken towards the development of these techniques. The aim of these approaches, whether directed or random, is to produce a methodology that is cheap, accurate and easy to use. In this review, the DNA-based techniques developed for the identification of Anopheles gambiae complex mosquitoes are used to illustrate the power of these methods, although, as the review demonstrates, the technology is directly applicable to many other mosquito or insect vectors. In addition, the methods discussed may be utilized for generating additional epidemiological data, such as identification of parasites within the vector or origin of the bloodmeal. A comprehensive survey of the probe systems available for the identification of insect vectors and the disease-causing organisms they transmit to the human population is therefore included. Given further advances in this technology, it may be anticipated that DNA-based approaches to identification may eventually supersede more traditional methodologies in the fields of tropical medicine and parasitology.

Field testing an enzyme-linked synthetic oligonucleotide probe for identification of Anopheles gambiae s.s. and An. arabiensis

Tests were carried out in Kenya to determine whether the enzyme-linked synthetic oligonucleotide probe (pAna 1) developed for identifying species of the Anopheles gambiae complex could be used under field conditions. The An. arabiensis male-specific pAna 1 probe was able to identify all male larval instars and adult males. However, the non-radioactive assay was not sufficiently sensitive to identify male sperm DNA in all the mated female An. arabiensis. Although the ratio of An. arabiensis to An. gambiae s.s. identified with pAna 1 in males during the dry season was in agreement with the ratio in half-gravid females identified cytogenetically, the ratios were different during the wet season. This study demonstrates that the enzyme-linked DNA probe assay is applicable under field conditions.

Laboratory colonization of Anopheles quadriannulatus from sympatry with other sibling species of the Anopheles gambiae complex in Zimbabwe

A laboratory colony of the mosquito Anopheles quadriannulatus was established from a wild population occurring sympatrically with An.arabiensis in Zimbabwe. These sibling species are members of the An.gambiae Giles complex and were distinguished primarily by means of their specific polytene chromosome banding patterns. By using an ox-baited trap, we sampled selectively for the more zoophilic An.quadriannulatus. It was confirmed that An.quadriannulatus has the diagnostic slow allozyme of aspartate aminotransferase (AAT95/95). In a mixed population under laboratory conditions, An.arabiensis displaced An.quadriannulatus within eight generations, without introgression. Colonization of An.quadriannulatus was facilitated by pooling the progeny from wild-caught mothers of confirmed identity and by using a specially adapted cage to promote mating.

Allozyme analysis reveals six species within the Anopheles punctulatus complex of mosquitoes in Papua New Guinea

Among samples collected from nineteen localities in Papua New Guinea, we have identified six species within the Anopheles punctulatus complex of mosquitoes, by means of cellulose acetate allozyme electrophoresis. An.punctulatus Dönitz sensu stricto was collected from seven villages in the Madang area and from Buksak, Sausi Mission and an area 18 km SW of Tari; An.koliensis Owen from eight villages in the Madang area, from Popondetta and Brown River near Karema; and An.farauti No. 1 from ten coastal areas including Madang, Lorengau, Popondetta, Port Moresby, Rabaul and Wewak. Three newly recognized species, reported here for the first time, are designated as An.farauti No. 4 from Gonoa and Hudini, Madang area; An.farauti No. 5 from Ketarabo near Goroka; and An.farauti No. 6 from Hiwanda near Tari. Three other known members of the complex, An.clowi Rozeboom & Knight, An.farauti No. 2 (Bryan, 1973) and An.farauti No. 3 (Mahon & Meithke, 1982) were not detected in Papua New Guinea. Problems arising with morphological characters for the identification of species in this group are discussed.

Identification of filarial larvae in vectors by DNA hybridization

Infectivity rates of insect vectors are the best criteria by which to assess the transmission of filarial parasites and the efficacy of filariasis control programs. Currently available DNA probes can be used to estimate the proportion of vectors containing larvae of a given filarial species but provide no information on three other important variables in the transmission dynamics of filarial nematodes: the developmental stage, the location and the actual number of larvae that are present in the vector, all of which can be reliably determined by microscopy. However, species identification is often difficult and sometimes impossible by conventional microscopy, which requires morphologically intact specimens. DNA is tough and DNA probing can identify worms that are dead and that have lost morphologic integrity; it also permits multiple analyses of the same specimen with different probes and has the potential for simultaneous processing of very large numbers of samples. Here, Senaroth Dissonoyake and Willy Piessens outline the route to the development of species- and life cycle stage-specific DNA/RNA probing reagents, and simple, reliable and quantitative technologies that can supplement and ultimately replace microscopic dissection.

The susceptibility of five laboratory colonies of mosquitoes to the human nematode Wuchereria bancrofti (Cobbold)

Laboratory colonies of Anopheles gambiae, An. arabiensis, An. merus, An. quadriannulatus and Aedes aegypti formosus were artificially fed on blood containing microfilariae of Wuchereria bancrofti. The anopheline colonies all supported parasite development to the infective stage, with An. quadriannulatus being the most heavily infected. The parasite did not develop at all in the Ae. aegypti formosus colony.

Isolation of leishmanial parasites from a wild caught Anopheles gambiae mosquito in Kenya

A total of 232 mosquitoes were collected and dissected for leishmanial parasites in the Baringo District, Kenya. Anopheles gambiae sensu lato comprised 90.9% of the sample. One female A. gambiae was found to be infected with leishmanial promastigotes. The parasites when injected into Balb C mice caused skin lesions characterized by heavy amastigote infections. The average size of the parasite was: body length, 11.7 +/- 0.19 microns; width, 1.3 +/- 0.04 microns; flagellum length, 15.5 +/- 0.28 microns.

Application of a biochemical key to study transmission of malaria and Bancroftian filariasis in sibling species of the Anopheles gambiae complex in north-eastern Tanzania

A biochemical key was applied in order to study transmission of malaria and Bancroftian filariasis in Anopheles gambiae sensu stricto, An. arabiensis and An. merus in different localities in north-eastern Tanzania. The technique was found to be a useful additional taxonomic tool for field entomologists. Significant differences between species in the rate of infection with Bancroftian filariasis were obtained between An. gambiae s.s. and An. funestus (P less than 0.005) and between An. funestus and An. arabiensis (P less than 0.0001). There were also significant differences between most of the investigated localities in the rate of filarial infection. However, there were no significant differences between the three species or between localities with respect to malaria sporozoite rates. Possible reasons for the observed variation between species and localities with respect to vectorial activity for Bancroftian filariasis are discussed.

Microgeographic variation in rDNA intergenic spacers of Anopheles gambiae in western Kenya

The genetic population structure of Anopheles gambiae (Diptera: Culicidae) in western Kenya was investigated by hybridizing a rapidly evolving rDNA intergenic spacer sequence to restriction endonuclease digests of genomic DNA extracted from single mosquitoes from seven localities. Significantly different distributions of restriction fragment arrays were obtained from field sites less than 10 km apart, which suggests restricted gene flow and a subdivided population structure. Eight of twenty-one possible comparisons between pairs of populations yielded significant differences. An eastern Kenya coastal population did not share its restriction fragment arrays with any of the western populations, suggesting that isolation by distance can be complete on a relatively small geographic scale (700 km).

The Anopheles culicifacies complex and control of malaria

Of the 50 or so species and varieties of anopheline mosquito in India, about 20 are implicated as vectors of human malaria. Of these, perhaps the most important and widespread is Anopheles culicifacies s.l. For the first 10 years of widespread DDT spraying, An. culicifacies remained susceptible to this insecticide - indeed, some thought it lacked resistance genes altogether. By 1960 however, resistance to DDT was apparent. DDT is a cheap insecticide, and its use is still favoured for mosquito control wherever it remains effective. But by the end of the late 1970s it appeared that DDT resistance in An. culicifacies (and other species) was a major barrier to effective vector control in several areas - particularly in parts of the northwestern states. Yet in other areas DDT still seemed to be effective. There was also increasing evidence from other studies suggesting differences in An. culicifacies found in different areas - particularly differences in seasonal prevalence and man-biting activity. We now know - as Sarala Subbarao discusses here - that An. culicifacies s.l. represents a complex of at least four sibling species. But in this case, one of the most important findings is that DDT resistance is mainly associated with species B which proves to be a very poor vector of malaria. Such findings, made possible by careful cytogenetic studies, have very important consequences for malaria epidemiology and control policies.

A DNA probe to distinguish the species Anopheles quadriannulatus from other species of the Anopheles gambiae complex

DNA probes used previously to distinguish the species Anopheles gambiae sensu stricto, An.arabiensis, An.melas and An.merus were tested against An.quadriannulatus. Using these DNA probes, An.gambiae s.s. and An.quadriannulatus were indistinguishable. A genomic library was constructed for An.quadriannulatus. Differential screening of this genomic library with An.gambiae s.s. and An.quadriannulatus genomic DNAs identified a species-specific, repeated DNA sequence. When used as a hybridization probe, this DNA sequence clearly distinguished An.gambiae s.s. from An.quadriannulatus. A simplified protocol for the use of DNA probes is described which may be used to identify material squashed directly on to nitrocellulose filter paper.