Urogenital schistosomiasis transmission on Unguja Island, Zanzibar: characterisation of persistent hot-spots

Abstract

BackgroundElimination of urogenital schistosomiasis transmission is a priority for the Zanzibar Ministry of Health. Preventative chemotherapy together with additional control interventions have successfully alleviated much of the disease burden. However, a persistently high Schistosoma haematobium prevalence is found in certain areas. Our aim was to characterise and evaluate these persistent “hot-spots” of transmission and reinfection in comparison with low-prevalence areas, to support the intervention planning for schistosomiasis elimination in Zanzibar.MethodsPrevalences of S. haematobium were annually determined by a single urine filtration in schoolchildren from 45 administrative areas (shehias) in Unguja in 2012, 2013 and 2014. Coverage data for biannual treatment with praziquantel were available from ministerial databases and internal surveys. Among the 45 shehias, five hot-spot (≥ 15 % prevalence) and two low-prevalence (≤ 5 %) shehias were identified and surveyed in mid-2014. Human-water contact sites (HWCSs) and the presence of S. haematobium-infected and uninfected Bulinus globosus, as well as safe water sources (SWSs) and their reliability in terms of water availability were determined and mapped.ResultsWe found no major difference in the treatment coverage between persistent hot-spot and low-prevalence shehias. On average, there were considerably more HWCSs containing B. globosus in hot-spot than in low-prevalence shehias (n = 8 vs n = 2) and also more HWCSs containing infected B. globosus (n = 2 vs n = 0). There was no striking difference in the average abundance of SWSs in hot-spot and low-prevalence shehias (n = 45 vs n = 38) and also no difference when considering SWSs with a constant water supply (average: 62 % vs 62 %). The average number of taps with a constant water supply, however, was lower in hot-spot shehias (n = 7 vs n = 14). Average distances from schools to the nearest HWCS were considerably shorter in hot-spot shehias (n = 229 m vs n = 722 m).ConclusionThe number of HWCSs, their infestation with B. globosus and their distance to schools seem to play a major role for a persistently high S. haematobium prevalence in children. In addition to treatment, increasing access to reliably working taps, targeted snail control at HWCSs near schools and enhanced behaviour change measures are needed to reduce prevalences in hot-spot areas and to finally reach elimination.Trial registration ISRCTN48837681.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1847-0) contains supplementary material, which is available to authorized users.

Environmental Predictors of Schistosomiasis Persistent Hotspots following Mass Treatment with Praziquantel

“…9 Although MDA with PZQ reduces the average prevalence and intensity of Schistosoma infection, areas where the schistosomiasis burden remains high despite repeated rounds of MDA, known as "persistent hotspots" (PHSs), have been documented in multiple settings. [10][11][12][13] Possible causes of PHSs include insufficient treatment coverage, reduced rates of infection cure (or egg count reduction) among those treated, 13,14 sustained transmission within and from untreated populations, environmental conditions, and socioeconomic conditions that lead to frequent human contact with contaminated water sources. [15][16][17] A better understanding of how these and other factors shape PHS risk would enable the more efficient allocation of interventions for schistosomiasis control.…”

Economic evaluations of human schistosomiasis interventions: a systematic review and identification of associated research needs

“…Despite treatment, there are hotspots where infection remains at persistently high levels 14,[134][135][136] . Hotspots can be caused by various programmatic factors such as poor treatment coverage/ adherence, movement of infected individuals and intense water contact.…”

Comparative analysis of time-based and quadrat sampling in seasonal population dynamics of intermediate hosts of human schistosomes

“…Time-based sampling. A time-based snail sampling technique was employed analogously to the one previously used in malacological studies in the country [52], and to the one typically employed in recent schistosomiasis control trials in SSA [31,39]. The sampling protocol consisted of systematically scooping pre-defined areas of the investigated habitats for 30 minutes using a 2mm metal mesh where water depths permitted it [40].…”

“…On a practical level, seasonal variations in snail populations enable the timing of chemotherapy and snail control measures due to context-specific parasite transmission windows during the year [28][29][30]. Beyond the requirement of snail host presence for transmission to occur, incidence has been more consistently linked to the density of infected snails rather than total snail abundance [31,32]. However the temporal variation of infected and total snail densities can be linked in some cases when accounting for the duration of pre-patency during which snails are infected but not yet shedding cercariae, schistosome larvae that infect humans [33].…”

“…Most snail population sampling protocols used today were established by research efforts between the 1950's and the 1970's fostered by a strong focus on transmission control through snail population management [10,37,38]. Recent quantitative estimates of snail abundance in the context of schistosomiasis control trials in SSA have principally relied on methods involving active habitat sampling principally using time-based protocols, as opposed to passive methods such as snail traps [31,39]. Time-based protocols involve systematically collecting snails for a limited time duration within a pre-defined area, presenting the advantage of being relatively cheap, rapid and easily implemented [40].…”