Moving from control to elimination of schistosomiasis in sub-Saharan Africa: time to change and adapt strategies

Abstract

Schistosomiasis is a water borne parasitic disease of global importance and with ongoing control the disease endemic landscape is changing. In sub-Saharan Africa, for example, the landscape is becoming ever more heterogeneous as there are several species of Schistosoma that respond in different ways to ongoing preventive chemotherapy and the inter-sectoral interventions currently applied. The major focus of preventive chemotherapy is delivery of praziquantel by mass drug administration to those shown to be, or presumed to be, at-risk of infection and disease. In some countries, regional progress may be uneven but in certain locations there are very real prospects to transition from control into interruption of transmission, and ultimately elimination. To manage this transition requires reconsideration of some of the currently deployed diagnostic tools used in surveillance and downward realignment of existing prevalence thresholds to trigger mass treatment. A key challenge will be maintaining and if possible, expanding the current donation of praziquantel to currently overlooked groups, then judging when appropriate to move from mass drug administration to selective treatment. In so doing, this will ensure the health system is adapted, primed and shown to be cost-effective to respond to these changing disease dynamics as we move forward to 2020 targets and beyond.

Keywords: Control; Diagnostics; Elimination; Mapping; Mass drug administration; Preventive chemotherapy; Schistosomiasis; Sub-Saharan Africa.

Fig. 1

Evolution of the number of people treated for schistosomiasis and treatment coverage in the WHO African region, between 2006 and 2015

Fig. 2

Intense water contact leading to high transmission dynamics of schistosomiasis in Barombi Kotto. Barombi Kotto, a village located in the South-West region of Cameroon, is divided in two parts; a mainland and an island. This photograph shows a view of the island from the shore of the mainland, and illustrates the intense water exposure of populations. There is no school in the island. All children leaving in the island go to school in the mainland. Therefore, they have contact with water at least twice per day, as they must cross the lake out and in. This frequent water exposure leads to rapid and high reinfections with schistosomiasis, that occur even from the same day of treatment in schools. Furthermore, there is no water supply in the island; the whole population relies on water from the lake, and 100% of people are at high risk of infections. The transmission dynamics and reinfection patterns are significantly different between populations from the island and those living on the mainland. Particular attention should be paid to such hotspots that require more regular and intensified interventions

Fig. 3

Comparison of district endemicity level/classification using either the mean prevalence of schistosomiasis per district (a) or the higher school prevalence within the district (b) in Cameroon

Fig. 4

The changing of health district endemicity category for schistosomiasis in Cameroon, from lower-risk (rose bars) to moderate-risk (red bars) and high-risk (dark red bars), when moving from using the current recommended mean district prevalence (Mean) to using the maximum school prevalence within the district (Max). The number of districts per category are reported in the corresponding bars