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

ABSTRACT

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.