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.

Seasonal and temporal variations in the population and biting habit of mosquitoes on the Atlantic coast of Lagos, Nigeria.

OBJECTIVES: To determine the hourly density of vector mosquitoes in coastal Nigeria, compare seasonal human-biting and sporozoite rates in the vector density, locate breeding sites of mosquitoes, and determine larval population at breeding sites. MATERIALS AND METHODS: Indoor and outdoor mosquitoes of a coastal Nigerian community were caught during early and late wet seasons and in the harmattan period, a time of dusty wind from the Sahara on the western coast of Africa. Larvae were collected from various locations during the study period. The mosquitoes were physically characterized and their salivary glands dissected for sporozoite rate. Larvae density was calculated. Human-biting rate was calculated for Anopheles gambiae complex. RESULTS: Of the 4,317 female A. gambiae complex collected during the night bait catches, 3,543 (82.1%) were from outdoors and 774 (17.9%) from indoors during the three seasons. The maximum human-biting rate approached 25/h and the sporozoite rate was almost 3.0%. These vector mosquitoes were mainly outdoor biting and midnight feeding. Of the 1,269 Anopheles mosquitoes collected with pyrethrum spray catches, 1,245 (98.1%) were A. gambiae complex. There was no significant difference in the entomological inoculation or sporozoite rates during the three seasons of study. There was a preponderance of A. gambiae complex larvae from larval collection. CONCLUSION: Findings from this study should be useful in the implementation of Integrated Vector Management for the control of malaria in coastal and noncoastal areas of Nigeria.

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.

Amodiaquine treatment of uncomplicated malaria in children, in an area of chloroquine-resistant Plasmodium falciparum in north-central Nigeria.

The efficacy of amodiaquine against Plasmodium falciparum malaria was assessed in an area of confirmed chloroquine resistance in the cool, north-central plateau of Nigeria, using a 14-day protocol. The patients were all children aged <5 years of age. The drug proved highly efficacious, giving a cure 'rate' of 100% on day 14 and mean fever- and parasite-clearance times of 1.11 and 3.11 days, respectively. It was also well tolerated. Following treatment, packed-cell volumes (PCV) generally increased (65% of patients) but remained constant (12%) or even decreased (23%) in some patients; the overall improvement in PCV was not statistically significant (P >0.05). The results justify the use of amodiaquine to treat P. falciparum malaria in those who have failed treatment with chloroquine and the second-line drugs (e.g. sulfadoxine-pyrimethamine) currently used in Nigeria. As the amodiaquine would be better employed as one part of a combination than on its own, there is a need to identify suitable partner compounds.