Dynamics of malaria vector indices in two vegetation zones within North Eastern Adamawa State, Nigeria.

Studies profiling community and zonal malaria entomological risk indices are required to identify high risk areas where targeted control resources are most needed or likely to have the greatest impact on reducing risk of malaria infection. This study presents a first report on malaria vector risk indices in two vegetation zones within Adamawa state, Nigeria. Endophilic mosquitoes were collected for one year in selected communities in the Guinea and Sudan savanna zones within the State. Plasmodium falciparum Sporozoite and human blood meal ELISA assays were carried out on the female Anopheles mosquitoes collected. Sibling species composition of the An. gambiae complex were determined using PCR assays. Mean numbers of mosquitoes in the Guinea savanna communities were significantly (t = 7.73, DF = 11, p < 0.001) higher than the Sudan. Man-biting rates (F = 2.76, p = 0.13) of Anopheles mosquitoes were higher in the Guinea but not significantly different from Sudan savanna. Sporozoite rates of mosquitoes within the Guinea savanna were 2.7 times higher than the Sudan. The predominant Anopheles coluzzii species encountered in the state had higher overall human blood indices (0.63) and sporozoite rates (6.9%) compared to An. gambiae (0.39, 1.9%) and An. arabiensis (0.58, 2.3%) respectively. Overall annual human blood indices (0.59) of mosquitoes in Adamawa were lower compared to reports from other States. Prevalence and higher transmission risks indices of endophilic An. coluzzii mosquitoes reveal the need for LLIN and management of relatively permanent An. coluzzii breeding sites in the State. Widespread cattle rearing lifestyle and lower human blood indices of mosquitoes in the study area suggest the need to investigate cattle blood indices of the mosquitoes in the state. Higher entomological risk indices in the Guinea Savanna zone provide baseline information for prioritization of malaria vector control supplies within the State.

Dynamics of malaria vector indices in two vegetation zones within North Eastern Adamawa State, Nigeria

@#Studies profiling community and zonal malaria entomological risk indices are required to identify high risk areas where targeted control resources are most needed or likely to have the greatest impact on reducing risk of malaria infection. This study presents a first report on malaria vector risk indices in two vegetation zones within Adamawa state, Nigeria. Endophilic mosquitoes were collected for one year in selected communities in the Guinea and Sudan savanna zones within the State. Plasmodium falciparum Sporozoite and human blood meal ELISA assays were carried out on the female Anopheles mosquitoes collected. Sibling species composition of the An. gambiae complex were determined using PCR assays. Mean numbers of mosquitoes in the Guinea savanna communities were significantly (t = 7.73, DF = 11, p < 0.001) higher than the Sudan. Man-biting rates (F = 2.76, p = 0.13) of Anopheles mosquitoes were higher in the Guinea but not significantly different from Sudan savanna. Sporozoite rates of mosquitoes within the Guinea savanna were 2.7 times higher than the Sudan. The predominant Anopheles coluzzii species encountered in the state had higher overall human blood indices (0.63) and sporozoite rates (6.9%) compared to An. gambiae (0.39, 1.9%) and An. arabiensis (0.58, 2.3%) respectively. Overall annual human blood indices (0.59) of mosquitoes in Adamawa were lower compared to reports from other States. Prevalence and higher transmission risks indices of endophilic An. coluzzii mosquitoes reveal the need for LLIN and management of relatively permanent An. coluzzii breeding sites in the State. Widespread cattle rearing lifestyle and lower human blood indices of mosquitoes in the study area suggest the need to investigate cattle blood indices of the mosquitoes in the state. Higher entomological risk indices in the Guinea Savanna zone provide baseline information for prioritization of malaria vector control supplies within the State.

Test for association between dieldrin resistance and 2La inversion polymorphism in Anopheles coluzzii from Lagos, Nigeria.

The assortment of paracentric chromosomal inversion 2La is associated with the maintenance of dieldrin resistance in laboratory colonies of the malaria vector Anopheles gambiae. This association has not been tested in field populations. The aim of this study was to test the association between inversion 2La and dieldrin resistance in a field population of An. coluzzii in Nigeria. Field collected immature stages of Anopheles were raised to adults and exposed to 4% dieldrin according to WHO criteria. Knockdown was recorded at 10 min intervals for 1 hour and final mortality was recorded 24 hours post exposure. Species and inversion 2La diagnostic PCR assays were conducted on the resistant and susceptible mosquitoes. The mosquitoes were highly resistant to 4% dieldrin (17.1% knock down and 25.7% final mortality; KDT50 and KDT95 calculated as 170 and 1, 514 minutes respectively). Frequencies of 2La in both the resistant and susceptible cohorts assorted within HardyWeinberg estimates (χ2=1.32, p=0.8 for dead/susceptible mosquitoes and χ2=2.54, p=0.5 for survivors or resistant mosquitoes). However, a higher number of heterozygous mosquitoes were observed in the resistant cohort compared to the susceptible, with significant variation in karyotype frequencies (χ2=11.08, DF=2, p<0.05) and a significantly higher frequency of the 2La inversion arrangement in the resistant cohort (Pearson's χ2 = 4.58, p = 0.03.). These data are the first to associate paracentric chromosome inversion 2La and dieldrin resistance in field population of An. coluzzii. Dieldrin resistance shows a weak but significant association with 2La whose assortment is affected by positive heterosis. Variation in the assortment of 2La inversion arrangements between resistant and susceptible cohorts of this An. coluzzii population suggests that dieldrin resistance is at least partially linked to inversion 2La which may explain the persistence of dieldrin resistance in this population despite a significant absence of selection for resistance to this insecticide.

Test for association between dieldrin resistance and 2La inversion polymorphism in Anopheles coluzzii from Lagos, Nigeria

@#The assortment of paracentric chromosomal inversion 2La is associated with the maintenance of dieldrin resistance in laboratory colonies of the malaria vector Anopheles gambiae. This association has not been tested in field populations. The aim of this study was to test the association between inversion 2La and dieldrin resistance in a field population of An. coluzzii in Nigeria. Field collected immature stages of Anopheles were raised to adults and exposed to 4% dieldrin according to WHO criteria. Knockdown was recorded at 10 min intervals for 1 hour and final mortality was recorded 24 hours post exposure. Species and inversion 2La diagnostic PCR assays were conducted on the resistant and susceptible mosquitoes. The mosquitoes were highly resistant to 4% dieldrin (17.1% knock down and 25.7% final mortality; KDT50 and KDT95 calculated as 170 and 1, 514 minutes respectively). Frequencies of 2La in both the resistant and susceptible cohorts assorted within Hardy- Weinberg estimates (χ2=1.32, p=0.8 for dead/susceptible mosquitoes and χ2=2.54, p=0.5 for survivors or resistant mosquitoes). However, a higher number of heterozygous mosquitoes were observed in the resistant cohort compared to the susceptible, with significant variation in karyotype frequencies (χ2=11.08, DF=2, p<0.05) and a significantly higher frequency of the 2La inversion arrangement in the resistant cohort (Pearson’s χ2 = 4.58, p = 0.03.). These data are the first to associate paracentric chromosome inversion 2La and dieldrin resistance in field population of An. coluzzii. Dieldrin resistance shows a weak but significant association with 2La whose assortment is affected by positive heterosis. Variation in the assortment of 2La inversion arrangements between resistant and susceptible cohorts of this An. coluzzii population suggests that dieldrin resistance is at least partially linked to inversion 2La which may explain the persistence of dieldrin resistance in this population despite a significant absence of selection for resistance to this insecticide.

Species composition of the Anopheles gambiae complex across eco-vegetational zones in Bayelsa State, Niger Delta region, Nigeria.

BACKGROUND & OBJECTIVES: Correct vector identification is an important task in the planning and implementation of malaria vector control programmes. This study was designed to provide baseline information on the species composition and distribution of members of the Anopheles gambiae complex in three eco-vegetational zones in Bayelsa state, Nigeria. METHODS: Adult mosquitoes were collected by pyrethrum spray catch (PSC) in randomly selected houses during September 2009-August 2010. Anopheles mosquitoes were identified using standard morphological keys. Mosquitoes identified as An. gambiae s.l. were used for species specific PCR-assays. RESULTS: Out of 203 Anopheles gambiae s.l. successfully amplified, 180 (88.7%) were Anopheles gambiae s.s., 14 (6.9%) were An. melas and 9 (4.4%) were An. arabiensis. The variation in the sibling species composition of An. gambiae s.l. was not significant (p >0.05). Anopheles gambiae s.s. was predominant in all the collections with three sibling species occurring in all the eco-vegetational zones. INTERPRETATION & CONCLUSION: The observation of An. melas in the fresh water swamp forest of Yenagoa is of importance in malaria epidemiology. These findings are of importance in the planning and implementation of malaria vector control strategy in the three eco-vegetational zones of Bayelsa state.

Evidence of multiple pyrethroid resistance mechanisms in the malaria vector Anopheles gambiae sensu stricto from Nigeria.

Pyrethroid insecticide resistance in Anopheles gambiae sensu stricto is a major concern to malaria vector control programmes. Resistance is mainly due to target-site insensitivity arising from a single point mutation, often referred to as knockdown resistance (kdr). Metabolic-based resistance mechanisms have also been implicated in pyrethroid resistance in East Africa and are currently being investigated in West Africa. Here we report the co-occurrence of both resistance mechanisms in a population of An. gambiae s.s. from Nigeria. Bioassay, synergist and biochemical analysis carried out on resistant and susceptible strains of An. gambiae s.s. from the same geographical area revealed >50% of the West African kdr mutation in the resistant mosquitoes but <3% in the susceptible mosquitoes. Resistant mosquitoes synergized using pyperonyl butoxide before permethrin exposure showed a significant increase in mortality compared with the non-synergized. Biochemical assays showed an increased level of monooxygenase but not glutathione-S-transferase or esterase activities in the resistant mosquitoes. Microarray analysis using the An. gambiae detox-chip for expression of detoxifying genes showed five over-expressed genes in the resistant strain when compared with the susceptible one. Two of these, CPLC8 and CPLC#, are cuticular genes not implicated in pyrethroid metabolism in An. gambiae s.s, and could constitute a novel set of candidate genes that warrant further investigation.

Identification of three members of the Simulium damnosum (Diptera Simuliidae) group in South Western Nigeria.

Identification of the specific Simuliium species at play in the transmission of Onchocerca volvulus is important because it helps to explain the epidemiology and clinical presentation of the disease and guides in monitoring of long term impact of ivermectin treatment on onchocerciasis transmission. The study therefore sought to use molecular markers for vector identification. A combination of Polymerase Chain Reaction (PCR) assay and Restriction fragment Length polymorphism (RFLP) was used to identify members of Simulium damnosum complex in forest area of south western Nigeria. The results revealed the presence of three members of the Simulium damnosum complex: S. yahense, S. sirbanum and S. damnosum ss. S. yahense was the predominant and major vector in the study sites. This provides baseline data for future monitoring vector of human onchocerciasis in the area.

Anopheles gambiae s.s. breeding in polluted water bodies in urban Lagos, southwestern Nigeria.

BACKGROUND & OBJECTIVE: Urban malaria is on the rise in Nigeria due to rapid industrialisation and developmental activities. A study was undertaken in Lagos, Nigeria to study the Anopheles breeding in polluted water bodies. METHODS: Anopheles larval breeding habitats were surveyed and water samples from 24 larval breeding sites from four strategic areas in urban Lagos were analysed. The relationship between eight abiotic variables (pH, dissolved oxygen, conductivity, hardness, nitrate, total dissolved solids, turbidity and oil) and density of Anopheles larvae were investigated. The levels of heavy metals (Zn, Co, Cu, Pb, Mn, Fe, Hg and Ni) pollution were analysed by Atomic Absorption Spectrophotometry. RESULTS: Morphological and PCR analysis of 2358 anopheline larvae revealed only the presence of two members of the Anopheles gambiae complex consisting of 93.1% Anopheles gambiae s.s. and 6.9% An. arabiensis. Multiple regression analysis revealed a significant association between occurrence of An. arabiensis larvae and two independent variables: pH and turbidity but not for An. gambiae s.s. The levels of three heavy metals: Fe, Cu and Pb from more than half of the sites surveyed were three times higher than the values obtained in natural breeding sites of An. gambiae s.s. from a rural area of Lagos. Over 85% of An. gambiae s.s. larvae were found in water bodies characterised by low dissolved oxygen (<3 mg/L), high conductivity (>900 uS/cm), turbidity (>180 FAU), oil (>11 mg/L) and heavy metals: Fe, Cu, and Pb (>0.4 mg/L). INTERPRETATION & CONCLUSION: These results indicate that An. gambiae s.s. is adapting to a wide range of water pollution in this urban area. The survival of the mosquito in widespread polluted water bodies across Lagos metropolis could be responsible for the rise in the incidence of malaria.

Dynamics of knockdown pyrethroid insecticide resistance alleles in a field population of Anopheles gambiae s.s. in southwestern Nigeria.

BACKGROUND & OBJECTIVES: Pyrethroid insecticide resistance in the malaria vector Anopheles gambiae Giles is mainly associated with reduced target site sensitivity arising from a single point mutation in the sodium channel gene, often referred to as knockdown resistance (kdr). This resistance mechanism is widespread in West Africa and was reported for the first time in Nigeria in 2002. Here we present changes in the susceptibility/resistance status of the molecular 'M' and 'S' forms of An. gambiae and the frequency of the kdr alleles from 2002-05. METHODS: Adult anophelines were sampled quarterly inside human dwellings from January 2002 to December 2005 and adults reared from wild larvae were identified using morphological keys. Samples belonging to the An. gambiae complex were subjected to PCR assays for species identification and detection of molecular 'M' and 'S' forms. Insecticide susceptibility tests were carried out using standard WHO procedures and test kits only on 2-3 days old adult An. gambiae s.s. reared from larval collections. The kdr genotypes were determined in both live and dead specimens of An. gambiae s.s. using alleles-specific polymerase chain reaction diagnostic tests. RESULTS: The overall collection showed that the molecular 'S' form was predominant (> 60%) but the proportions of both forms in the mosquito populations from 2002-05 were not statistically different. Both forms also occurred throughout the period without apparent relationship to wet or dry season. Insecticide susceptibility tests did not show any significant increase in the resistance status recorded for either Permethrin or DDT from 2002-05, rather, an improvement in the susceptibility status of the mosquitoes to these insecticides was observed from 2004-05 relative to the tests performed in 2002-03. CONCLUSION: The proportion of the molecular 'M' and 'S' form of An. gambiae and the kdr frequencies have not increased significantly from 2002 when it was first reported in Nigeria. However, the findings on susceptible mosquitoes exhibiting the kdr gene need further investigation. Further monitoring of this may provide additional information on the ongoing debate on the possibility of restriction in gene flow and reproductive barriers in these sympatric taxa.

Molecular differentiation of three closely related members of the mosquito species complex, Anopheles moucheti, by mitochondrial and ribosomal DNA polymorphism.

Distinction between members of the equatorial Africa malaria vector Anopheles moucheti (Evans) s.l. (Diptera: Culicidae) has been based mainly on doubtful morphological features. To determine the level of genetic differentiation between the three morphological forms of this complex, we investigated molecular polymorphism in the gene encoding for mitochondrial cytochrome oxidase b (CytB) and in the ribosomal internal transcribed spacers (ITS1 and ITS2). The three genomic regions revealed sequence differences between the three morphological forms similar in degree to the differences shown previously for members of other anopheline species groups or complexes (genetic distance d = 0.047-0.05 for CytB, 0.084-0.166 for ITS1 and 0.03-0.05 for ITS2). Using sequence variation in the ITS1 region, we set up a diagnostic polymerase chain reaction (PCR) for rapid and reliable identification of each subspecies within the An. moucheti complex. Specimens of An. moucheti s.l. collected in Cameroon, the Democratic Republic of Congo (DRC), Uganda and Nigeria were successfully identified, demonstrating the general applicability of this technique.

Behaviour and population dynamics of the major anopheline vectors in a malaria endemic area in southern Nigeria.

BACKGROUND & OBJECTIVES: Anopheline mosquitoes consist of a large number of species each of which differs from another in population, resting and feeding behaviour in relation to the prevailing conditions in the locality. A longitudinal study was carried out to investigate the population dynamics, resting and feeding behaviour of the major anopheline species found in a rain forest zone of Nigeria. METHODS: Mosquitoes resting and biting indoors were collected using WHO standard techniques and supplemented with outdoor-biting collections in the study areas between January and December 2004. Samples were sorted and identified microscopically for morphological features while molecular identification was carried out using polymerase chain reaction (PCR) techniques. RESULTS: PCR-based tests showed that both indoor and outdoor collections constitute three groups of Anopheles mosquitoes, An. gambiae s.l. Giles (68.6%), An. funestus Giles (30.7%) and An. moucheti Evans (0.7%). Of the 1342 female Anopheles mosquitoes collected indoors, 799 were caught resting and 543 were caught biting. The outdoor-biting population accounted for 28.8% of the total collections (n=1885). There was no significant difference (p > 0.05) in the biting activities (indoors and outdoors) of these species in four villages. However, An. arabiensis and An. moucheti were more exophagic with >60% of their biting occurring outdoors while An. gambiae and An. funestus were more endophagic with >55% of their biting occurring indoors. The human-vector contact with An. gambiae and An. funestus (indoors) was about 73.3 and 66.7%, respectively as against 27.7% in An. arabiensis and 25.3% in An. moucheti. This gave the corresponding man-biting rates (MBR) of 17.5 bites/man/night for An. gambiae, 14.6 bites/man/night for An. funestus, 6.7 bites/man/night for An. arabiensis and 4.3 bites/man/night for An. moucheti. Moreover, the number of An. gambiae and An. funestus caught resting indoors was significantly higher than the other two species (p < 0.05). The wet season collections showed thatAn. gambiae caught were more than 67% of the total catch while An. arabiensis was predominant in the dry season (chi2 = 75.44, df = 3, p < 0.01). INTERPRETATION & CONCLUSION: The present study highlights some aspects of the behaviour of anopheline mosquitoes in southern Nigeria which is an important component of epidemiological study of malaria. This information provides basis to the understanding of the role played by the identified anopheline species in malaria transmission and a baseline to formulate malaria control programme.

Impact of urbanisation on bionomics and distribution of malaria vectors in Lagos, southwestern Nigeria.

BACKGROUND & OBJECTIVES: The patterns of annual, rainfall as well as average daily temperature have not changed drastically in the study area since 1960 when detailed baseline entomological surveys were carried out. However, the increase in human population from 1.2 to 10 million has resulted in both expansion of land and tremendous ecological and environmental change. This has led to drastic changes in vectors' densities as well as species' composition while the preferred larval habitat of malaria vectors has shifted to water reservoirs. A longitudinal study was carried out to investigate the impact of urbanisation on bionomics and distribution of malaria vectors in Lagos, a mega city in Nigeria. METHODS: Mosquitoes were collected indoors and outdoors using WHO standard techniques in the selected areas between January and December 2000. Specimens were identified using the morphological keys and PCR assays. ELISA tests were used for Plasmodium falciparum sporozoite infection. RESULTS: The Anopheles gambiae species-specific PCR identified 56% (435/777) of the An. gambiae s.1. as An. gambiae s.s. and 44% (342/777) as An. arabiensis. The molecular M and S forms represented 35.3 and 64.7% of the An. gambiae s.s. population, respectively. The An. funestus species-specific PCR identified 60% (239/401) of the An. funestus group as An. funestus s.s. and 40% (162/401) as An. rivulorum. The biting activity of An. funestus s.s. both indoors and outdoors attained a peak at 0200 and 2200 hrs, respectively, with a significant increase in the "pre-bed time" collections indoors (Chi2 = 6.15, df = 1, p < 0.05) and outdoors (Chi2 = 6.28, df = 1, p < 0.05). The overall outdoor collection was significantly higher (Chi2 = 28.23, df =3, p < 0.05) than that recorded indoors. The overall sporozoite rates for An. gambiae s.s., An. arabiensis and An. funestus were significantly different in both localities (Chi2 = 0.58, df = 2, p < 0.01). Infection rates in both indoor and outdoor collections were also different statistically (Chi2 = 0.67, df = 2, p < 0.01). INTERPRETATION & CONCLUSION: Large number and species of anopheline mosquitoes collected in the study area may be associated with the availability of aquatic breeding sites. A phenomenon leading to an increase in man-vector contact and a high level of risk to the crowded urban population is observed.

Distribution of the molecular forms of Anopheles gambiae and pyrethroid knock down resistance gene in Nigeria.

We investigated the distribution of the molecular M and S forms of Anopheles gambiae and the knock down resistance (kdr) gene associated with pyrethroid and DDT resistance in A. gambiae s.s. at 13 localities across Nigeria. Two-three days old adult female mosquito reared from larval collections were tested using standard WHO procedures, diagnostic test kits and impregnated papers to assess their pyrethroid resistance status. Specimens were identified by PCR assays and characterized for the kdr gene. DNA from adult A. gambiae s.s. collected from human dwellings were also tested for the presence of the kdr gene. The overall collection was a mix of the molecular M and S forms across the mangrove (63:37%), forest (56:44%), and transitional (36:64%) ecotypes, but almost a pure collection of the S form in the Guinea and Sudan-savanna. Results of insecticide susceptibility tests showed that mosquitoes sampled at seven localities were susceptible to permethrin, deltamethrin, and DDT, but populations of A. gambiae resistant to these insecticides were recorded at six other localities mainly in the transitional and Guinea-savanna ecotypes. The kdr gene was found only in the molecular S forms, including areas where both forms were sympatric. The overall kdr frequency was low: <47% in forest, 37-48% in the transitional, and 45-53% in Guinea-savanna. The data suggest that pyrethroid resistance in A. gambiae in Nigeria is not as widespread when compared to neighbouring West African countries.

Identification of three members of the Anopheles funestus (Diptera: Culicidae) group and their role in malaria transmission in two ecological zones in Nigeria.

The role of the Anopheles funestus group in malaria transmission was investigated in two ecological zones in Nigeria. Sampling was carried out at four sites each around Ibadan (forest) and Ilorin (savanna). Human landing catches were supplemented with indoor and outdoor resting collections. PCR was used to identify 1848 A. funestus group mosquitoes to species level (749 in the savanna, 1099 in the forest) and three species were identified. In the forest, A. funestus s.s. predominated (55.4%), followed by A. rivulorum (27.6%) and A. leesoni (17.0%). Anopheles funestus was found mostly indoors. Anopheles rivulorum and A. leesoni predominated in outdoor collections (P<0.001). Only Anopheles funestus s.s. was found in the savanna. ELISA analysis of 803 blood meal-positive specimens showed that over half of the blood meals were taken from humans in both ecotypes. The human blood index in A. funestus from the two study areas was similar. Anopheles funestus s.s. was the only species found positive for Plasmodium falciparum using ELISA, with overall infection rates of 2.3% and 1.0% in the forest and savanna respectively. The presence of three A. funestus species in Nigeria emphasizes the desirability of correct species identification within a malaria vector control programme.

Independent mutations in the Rdl locus confer dieldrin resistance to Anopheles gambiae and An. arabiensis.

Substitutions of a conserved alanine residue in the Rdl locus coding for a gamma-aminobutyric acid (GABA) receptor subunit with serine or glycine confer resistance to dieldrin in various insect species. Here, we show that alanine to glycine substitution in the Rdl locus of the malaria vector, Anopheles gambiae, is genetically linked to resistance to dieldrin. An alanine to serine substitution developed independently in a dieldrin resistant strain of An. arabiensis. An allele-specific polymerase chain reaction (PCR) assay was able to differentiate dieldrin resistant and susceptible mosquitoes.

Restriction fragment length polymorphism method for the identification of major African and Asian malaria vectors within the Anopheles funestus and An. minimus groups.

The African Anopheles funestus and the Asian An. minimus groups are closely related and are probably considered distinct only because of their geographic separation. This study aimed at improving two identification methods based on polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) already developed for either group. Each PCR-RFLP, either on the internal transcribed spacer 2 (ITS2) for the An. minimus group, and domain 3 (D3) for the An. funestus group, was applied to the other group for the standardization of one identification method applicable on both continents. The ITS2 fragment digested by Bsi ZI showed the highest diagnostic power. This assay allowed the discrimination of at least 13 Anopheles species within the subgenus Cellia from two continents (Africa and Asia), among which are five major malaria vectors. Moreover, digestion of the D(3) with Msp I showed intragenomic variations within An. funestus populations. Two types of D3 copies (M and W) occurred in specimens from southern Africa. The populations from West-Central Africa presented only type W and East-Malagasy populations exhibited type M. Since An. funestus shows a great capacity of adaptation, these molecular variations, along with behavioral and ecologic ones, reinforce the hypothesis of a species complex that will need to be further investigated.

Anopheles arabiensis and An. quadriannulatus resistance to DDT in South Africa.

The malaria control programme of KwaZulu-Natal Province, South Africa, includes Mamfene and Mlambo communities. Western-type houses there are currently sprayed with deltamethrin, whereas traditional houses are sprayed with DDT for malaria control. In 2002, mosquitoes of the Anopheles gambiae complex (Diptera: Culicidae) were collected from DDT-sprayed houses, by window exit traps, and from man-baited nets outdoors. Larval collections were also carried out at Mzinweni Pan near Mlambo. Species of the An. gambiae complex were identified by rDNA polymerase chain reaction assay. The majority of samples collected by window trap and baited nets were identified as the malaria vector An. arabiensis Patton, with a few An. merus Dönitz and An. quadriannulatus (Theobald). The larval collections were predominantly An. quadriannulatus with a small number of An. arabiensis. Standard WHO insecticide susceptibility tests using 4% DDT and 0.05% deltamethrin were performed on both wild-caught females and laboratory-reared progeny from wild-caught females. Wild-caught An. arabiensis samples from window traps gave 63% and 100% mortality 24-h post-exposure to DDT or deltamethrin, respectively. Wild-caught An. arabiensis samples from man-baited net traps gave 81% mortality 24-h post-exposure to DDT. The F1 progeny from 22 An. arabiensis females showed average mortality of 86.5% 24-h post-exposure to DDT. Less than 80% mortality was recorded from five of these families. Biochemical analyses of samples from each of the families revealed comparatively high levels of glutathione-S-transferases and non-specific esterases in some families, but without significant correlation to bioassay results. Wild-caught An. quadriannulatus larvae were reared through to adults and assayed on 4% DDT, giving 47% (n = 36) mortality 24-h post-exposure. Finding DDT resistance in the vector An. arabiensis, close to the area where we previously reported pyrethroid-resistance in the vector An. funestus Giles, indicates an urgent need to develop a strategy of insecticide resistance management for the malaria control programmes of southern Africa.

Absence of the kdr mutation in the molecular 'M' form suggests different pyrethroid resistance mechanisms in the malaria vector mosquito Anopheles gambiae s.s.

Field tests conducted on adult Anopheles mosquitoes using standard WHO procedures, diagnostic kits and test papers in south-western Nigeria showed pyrethroid (deltamethrin and permethrin) resistance in adult populations of Anopheles gambiae sensu stricto. The knock-down resistance (kdr) mutation involved in pyrethroid resistance was only found in the molecular S form of An. gambiae s.s. even in area where both molecular M and S forms occurred in sympatry. The absence of the kdr mutation in the M form suggests an additional pyrethroid resistance mechanism in An. gambiae s.s.

Dynamics of the malaria-vector populations in coastal Lagos, south-western Nigeria.

An entomological study was carried out in coastal Lagos, south-western Nigeria, to explore the role of Anopheles gambiae s.l. and An. moucheti in the transmission of Plasmodium falciparum in this holo-endemic area. Mosquitoes were caught, on human bait at night and by pyrethrum-spray catches during the day, twice a month throughout 2000. Of the 1812 Anopheles mosquitoes collected, An. gambiae s.l., the predominant vector species, represented 78.7% while the other 21.3% were An. moucheti. The results of a PCR-based test identified 56.8% of the mosquitoes of the An. gambiae complex collected as An. gambiae s.s., 36.9% as An. melas and 6.3% as An. arabiensis. Anopheles gambiae s.s. was predominantly recorded in the wet season, biting females being collected from May to October, with a peak in July. Anopheles melas and An. moucheti were present throughout the year-long study whereas An. arabiensis was mainly found in the dry season. The results of ELISA-based analyses of bloodmeals indicated that An. gambiae s.s., An. melas and An. moucheti were predominantly anthropophagic whereas An. arabiensis was largely zoophagic. Among all of the females investigated, 3.6% of the An. gambiae s.s., 1.9% of the An. melas, 1.8% of the An. moucheti and 0% of the An. arabiensis were found to be infected with P. falciparum (i.e. carrying the parasite's circumsporozoite antigen). The corresponding proportions for the females collected during the dry season were 1.3%, 2.3%, 2.7% and 0%. The entomological inoculation rates for An. melas and An. moucheti were significantly higher during the dry season than at other times of the year. Taken together, these results indicate that An. melas and An. moucheti maintain transmission of P. falciparum during the dry season, while the biting population ofAn. gambiae s.s. is relatively small.