Mapping the Geographical Distribution of Lymphatic Filariasis in Zambia

Background Past case reports have indicated that lymphatic filariasis (LF) occurs in Zambia, but knowledge about its geographical distribution and prevalence pattern, and the underlying potential environmental drivers, has been limited. As a background for planning and implementation of control, a country-wide mapping survey was undertaken between 2003 and 2011. Here the mapping activities are outlined, the findings across the numerous survey sites are presented, and the ecological requirements of the LF distribution are explored. Methodology/Principal findings Approximately 10,000 adult volunteers from 108 geo-referenced survey sites across Zambia were examined for circulating filarial antigens (CFA) with rapid format ICT cards, and a map indicating the distribution of CFA prevalences in Zambia was prepared. 78% of survey sites had CFA positive cases, with prevalences ranging between 1% and 54%. Most positive survey sites had low prevalence, but six foci with more than 15% prevalence were identified. The observed geographical variation in prevalence pattern was examined in more detail using a species distribution modeling approach to explore environmental requirements for parasite presence, and to predict potential suitable habitats over unsurveyed areas. Of note, areas associated with human modification of the landscape appeared to play an important role for the general presence of LF, whereas temperature (measured as averaged seasonal land surface temperature) seemed to be an important determinant of medium-high prevalence levels. Conclusions/significance LF was found to be surprisingly widespread in Zambia, although in most places with low prevalence. The produced maps and the identified environmental correlates of LF infection will provide useful guidance for planning and start-up of geographically targeted and cost-effective LF control in Zambia.
Author Summary Lymphatic filariasis (LF) is a debilitating mosquito borne parasitic infection which worldwide affects more than 120 million people. It is also widespread in Sub-Saharan Africa. A World Health Organization coordinated Global Programme to Eliminate LF has targeted LF for elimination as a public health problem by the year 2020, with annual mass drug administration (MDA) being the primary measure for this endeavor. An important first step before initiating MDA is the geographical mapping of infection in order to delimit the target areas. Past case reports have indicated that LF occurs in Zambia, but knowledge on its distribution and prevalence has been limited. Here we report on a country-wide survey carried out to map the geographical distribution and prevalence pattern across Zambia by screening adult volunteers for specific circulating filarial antigens (CFA). The CFA prevalences observed at the numerous survey sites are presented and mapped to give an indication of LF distribution in the country. The observed geographical variation is furthermore examined using a species distribution modeling approach to explore environmental requirements for LF presence, and to predict potential suitable habitats over unsurveyed areas. The findings provide a firm background for planning and start-up of LF control in Zambia.