The effects of variable spatial aggregation on lymphatic filariasis transmission

Abstract

BACKGROUND: Elimination of lymphatic filariasis (LF) is a World Health Organization goal, with several countries at or near prevalence thresholds. Where LF cases remain after mass drug administration, they tend to be spatially clustered, with an overdispersed individual worm burden. Both individual and spatial heterogeneities can cause aggregation of infection; however, few studies have investigated the drivers of heterogeneity and implications for disease elimination. METHODS: We used a spatially explicit lymphatic filariasis model to investigate LF transmission in American Samoa at three spatial scales - a territory-level model, a village model with 64 groups and a subvillage model with 316 groups. RESULTS: To reproduce American Samoan survey data, models with less spatial structure required increased individual-level bite aggregation. Threshold behaviour was present in the territory model but less evident in the models with spatial structure. As such, mass drug administration was most effective in the territory model, while in the spatially structured models, successive rounds of mass drug administration only gradually increased the likelihood of elimination. With the addition of spatial structure, residual infections remained in limited groups, and infection resurgence was slowed. CONCLUSIONS: Due to the impacts on potential intervention and surveillance strategies, it is critical that studies incorporate individual and spatial sources of heterogeneity to accurately model transmission and inform potential policy decisions.