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.