Mind the weather: a report on inter-annual variations in entomological data within a rural community under insecticide-treated wall lining installation in Kwara State, Nigeria

Abstract

Background: Entomological indices within a specific area vary with climatic factors such as rainfall, temperature and relative humidity. Contributions of such weather parameter fluctuations to the changes in entomological data obtained within a community under implementation of a promising vector control intervention should be taken into account. This study reports on inter-annual changes in entomological indices within two rural communities, one of which was under insecticide-treated durable wall lining (DL) installation.

Methods: Community-wide DL installation was followed by monthly meteorological data and pyrethrum spray mosquito collections for 2 years in intervention and a similar neighbouring community (control). Human blood meal and sporozoite ELISA tests were conducted on female mosquitoes collected alongside PCR identification of subsamples. Mosquitoes collected at the intervention site were tested in cone susceptibility assays against subsamples of installed DL materials collected on a 6-monthly basis for 2 years. Deltamethrin susceptibility of Anopheles mosquitoes from the intervention site was determined before and after DL installation. Entomological indices in the first and second years were compared within each site.

Results: Rainfall in the study area increased significantly (t = -3.45, df = 11, P = 0.005) from first to second year. Correlation between rainfall and Anopheles densities in both sites were significant (r = 0.681, P < 0.001). Mosquitoes collected at the intervention site were susceptible (100%) to deltamethrin at baseline but resistant (92%) in the second year. However, subsamples of installed DL materials remained effective (100% mortality) against Anopheles mosquitoes from the intervention site throughout the 6-monthly cone assay exposures. Monthly pyrethrum spray collections showed significant increase in Anopheles densities from first to second year in the control (6.36 ± 1.61 vs 7.83 ± 2.39; t = -3.47, df = 11, P = 0.005), but not in the intervention (2.83 ± 1.86 vs 4.23 ± 3.31; t = -2.03, df = 11, P = 0.067) community. However, mean annual mosquito man-biting rates increased significantly in both intervention (0.88 ± 0.18 vs 1.06 ± 0.38; F_{(1, 10)} = 9.50, P = 0.012) and control (1.45 ± 0.31 vs 1.61 ± 0.34; F_{(1, 10)} = 10.18, p = 0.010) sites along with increase (≥ 1.6 times) in sporozoite rates within intervention (0-2.13%) and control (2.56-4.04%) communities.

Conclusions: The slight increase in vector density, induced by significant increase in rainfall, led to increased sporozoite infection and significantly increased man-biting rates within the intervention site. These reveal the need for incorporation of integrated vector management strategies to complement DL installation especially in regions with high rainfall and mosquito density. Promising vector control tools such as DL should be evaluated on a long-term basis to reveal the possible effect of weather parameters on control performance and also allow for holistic recommendations.

Keywords: Durable wall lining; Malaria vectors; Rainfall variations.

Fig. 1

Relationship between rainfall and numbers of female Anopheles in the study communities

Fig. 2

Mean ± standard error (SE) annual rainfall and numbers of female Anopheles mosquitoes collected during first and second years in the study communities. Means with the different letters within the same site are significantly different (Student’s t-test, P < 0.05)