Fig. 1

Map of a Africa, b The Gambia and c the Farafenni area. In b the star represents the capital Banjul and the solid dot represents Farafenni. In c the location of the five surveys was as follows: 1988 Kataba hamlets (solid black fill), 1990 Fula hamlets and surrounding villages (dashed lines), 2003 the catchment area of the Farafenni hospital (solid black line), 2008 Dibba Kunda, Bambali, Sara Kunda and Pallen Wollof (dotted–dashed lines) and 2011 the Farafenni area (dotted line)