Longitudinal survey on the distribution of Biomphalaria sudanica and B. choanomophala in Mwanza region, on the shores of Lake Victoria, Tanzania: implications for schistosomiasis transmission and control

Abstract

BackgroundSchistosomiasis is hyper-endemic in the Lake Victoria basin; with intestinal schistosomiasis plaguing communities adjacent to the lake, where the intermediate host snails live. The two intermediate host species of Schistosoma mansoni in the Mwanza region are Biomphalaria sudanica, found on the banks of the lakes, and B. choanomphala, found in the lake itself. There are few longitudinal surveys documenting changing abundance and differential transmission patterns of these Biomphalaria snails across seasons and years. We undertook 15 field surveys at 26 sites over four years to determine the parameters that influence Biomphalaria abundance, presence of S. mansoni-shedding snails and impact of schistosomiasis treatment interventions on transmission potential in the Mwanza region.ResultsStatistical analysis revealed seasonal difference in the abundance of B. sudanica with the highest number of snails found in the dry season (Kruskal-Wallis χ 2 = 37.231, df = 3, P < 0.0001). Water measurements were not associated with B. sudanica abundance; however, high levels of rainfall did have a negative effect on B. sudanica [coefficient effect -0.1405, 95% CI (-0.2666, -0.0144)] and B. choanomphala abundance [coefficient effect -0.4388, 95% CI (-0.8546, -0.0231)] potentially due to inundation of sites “diluting” the snails and influencing collection outcome. Biomphalaria sudanica snails were found at all sites whereas B. choanomphala were far more focal and only found in certain sites. Shedding Biomphalaria did not show any variation between dry and rainy seasons; however, a decrease in shedding snails was observed in year 4 of the study.Conclusions Biomphalaria sudanica is uniformly present in the Mwanza region whereas B. choanomphala is far more focal. Seasonality plays a role for B. sudanica abundance, likely due to its habitat preference on the banks of the lake, but not for B. choanomphala. The decrease in shedding Biomphalaria abundance in Year 4 could be linked to ongoing schistosomiasis treatment efforts in the neighbouring human populations. The highest number of shedding Biomphalaria was observed at sites with high levels of human movement. Prioritising snail control at such sites could greatly reduce transmission in these high-risk areas.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-017-2252-z) contains supplementary material, which is available to authorized users.

A search for snail-related answers to explain differences in response ofSchistosoma mansonito praziquantel treatment among responding and persistent hotspot villages along the Kenyan shore of Lake Victoria

“…11,22 Gouvras et al, found B. choanomphala to be focally distributed in the Tanzanian waters of the lake, including in some more protected inlets. 34 They noted, as did we, a general trend for this species to be more common in western than eastern locations, and considered a possible gradual change in substratum conditions from west to east might be responsible. Many other factors such as levels of dissolved oxygen, pollution and seasonal effects like rainfall may also influence B. choanomphala populations.…”

“…Many other factors such as levels of dissolved oxygen, pollution and seasonal effects like rainfall may also influence B. choanomphala populations. 33,34 We did not observe any association between the abundance of B. choanomphala and water hyacinths. Although hyacinth mats can cover areas of the lake where deeper water and hence B. choanomphala populations are present and could have adverse shading or other effects, our impression was that hyacinth mats along the more exposed shores away from the Gulf were more prone to be dispersed by winds.…”

“…We found B. sudanica at all 20 collecting sites and it was typically abundant as it is along much of the Lake Victoria shoreline. 9,11,22,34 We recovered approximately five times as many B. sudanica as B. choanomphala .…”

“…An alternative approach to detect prepatent infections is to employ molecular xenomonitoring techniques to detect nascent infections at the time of collection. 24,28,34,47 Such techniques can be very sensitive and specific, but are prone to error if the infection is very young at the time of assay, and some detected infections may have been doomed to fail, so transmission potential could potentially be overestimated. 28 Also, extractions of individual snails for PCR-based or other types of molecular detection of prepatent infections becomes prohibitively expensive and time-consuming when large numbers of snails need to be screened.…”

“…22,26–33 Shoreline and bottom conditions can also play important roles in efficacy of transmission. 9,22,24,32,34…”