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 χ2 = 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.ConclusionsBiomphalaria 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.
Highlights
Schistosomiasis is hyper-endemic in the Lake Victoria basin; with intestinal schistosomiasis plaguing communities adjacent to the lake, where the intermediate host snails live
Biomphalaria sudanica is uniformly present in the Mwanza region whereas B. choanomphala is far more focal
Of these 42,816 were identified either as B. sudanica or as B. choanomphala with 58 Biomphalaria not identified to the species level
Summary
Schistosomiasis is hyper-endemic in the Lake Victoria basin; with intestinal schistosomiasis plaguing communities adjacent to the lake, where the intermediate host snails live. Schistosomiasis is a Neglected Tropical Disease (NTD) affecting over 200 million people worldwide with an atrisk population of 700 million people [1] The aetiology of this disease is a small blood-dwelling digenetic trematodes of the genus Schistosoma, characterised by a complex, obligate indirect life-cycle, involving an intermediate aquatic snail host and transmission through direct water contact. Schistosoma mansoni infects over 83 million people [1, 3] across sub-Saharan Africa, the Middle East, parts of South America and the Caribbean Islands [4]. It causes intestinal schistosomiasis potentially resulting in severe liver fibrosis, spleen damage and pulmonary hypertension [5]. Several species of both genera are involved in Schistosoma infections across sub-Saharan Africa [6]
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