Variance partition reveals contrasting random effect contributions on the density and species composition of malaria-transmitting mosquitoes

Spatial-temporal variation exists in the density and species composition of malaria-carrying mosquitoes, which will in turn influence that of disease transmission. While there has been extensive research on the season effect and other main drivers on the vectors’ populations, the heterogeneity induced by random effects are less studied. To investigate the relative contributions of the between-house, between-village, and between-year variations, as well as other house-level covariates such as inhabitant size and bed net usage, intensive Pyrethroid Spray Catches (PSC) sampling was conducted across a 60-month period between 2012-2019 from four villages in the Sudano-Sahelian region of Burkina Faso.

For mosquito density, measured by the female Anopheles gambiae s.l. counts, our modelling showed that the between-house variation was the largest component, followed by the between-year then between-village variations, after accounting for seasonality and other covariates. Density increased with the number of habitants within a household but was unaffected by bed net usage. A subset of female mosquitoes from the species complex was genotyped for species identification. The composition of An. coluzzii and An. gambiae , the two dominant vectors in the region, varied hugely across villages without a clear seasonal tread. The between-village variance contributed up to 76% of the total random variation, followed by the between-year variance. The between-house variation was estimated to be insignificant. Neither household size nor bed net usage had any impact on their proportion.

In short, the relative importance of the random components in mosquito density was in the reverse order from species composition. The variance estimates and relative strengths help parameterise potential field trials for novel vector control programmes and monitoring.