Confirmed local endemicity and putative high transmission of Schistosoma mansoni in the Sesse Islands, Lake Victoria, Uganda

Abstract

The Sesse Islands, in the Ugandan portion of Lake Victoria, have long been considered a low transmission zone for intestinal schistosomiasis. Based on observations of high prevalence of Schistosoma mansoni infection in the northern-most islands of this archipelago, a follow-up survey was conducted to ascertain whether transmission was endemic to this island group, combining parasitological and malacological surveys. Prevalence of intestinal schistosomiasis was again observed to be high, as was intensity of infections which, combined with low reported incidence of treatment, suggests that chemotherapy-based control initiatives are not being maximally effective in this region as high levels of population movement between islands and districts are confounding. The local disease transmission was confirmed by the observations of high abundance of Biomphalaria, as well as field-caught snails shedding S. mansoni cercariae. DNA sequencing of 12 cercariae revealed common mitochondrial cox1 haplotypes, as well as, novel ones, consistent with the high genetic diversity of this parasite in Lake Victoria. Intestinal schistosomiasis is firmly endemic in parts of the Sesse Islands and more broadly, this island group provides an insight into the future challenges to be faced by the Ugandan National Control Programme in regularly reaching these rather remote, inaccessible and largely itinerant communities.

Figure 1

Sketch map of the Sesse Islands (inset) in relation to Lake Victoria, with close up views of the findings of the mapping surveys undertaken in Bufumiri sub-county in January 2010 (left) and November 2010 (right). The islands within this sub-county lie in a NW-SE orientation covering a 25 km distance. With one or two exceptions, the prevalence of intestinal schistosomiasis is broadly high (>50%) across the within sampled children from the 12 shoreline communities.

Figure 2

Sketch map of the islands within Bufumiri sub-county and distribution and abundance of Biomphalaria at each of the 15 sampled sites. Note that at site B005, snails could be found in marshy fringes (B005a) as well as within the lake proper (B005b). Snails shedding schistosome cercariae were found at 3 sites (B006, B008 and B010) clearly affirming the local risk-of-infection with S. mansoni for people conducting water contact at these sites.