Freshwater snail diversity in relation to Schistosomiasis in the middle Zambezi basin in Lusaka Province

Abstract

This study was carried out at the Middle Zambezi basin in Lusaka province to determine freshwater snail diversity, its relationship to the presence of schistosomiasis and the impact of prevailing environmental factors on snail diversity. The study was carried out during the cool-dry season (July), hot-dry season (October) and hot-wet season (February), from July 2006 to February 2007. Twenty 400 metre transects were established on three rivers, ten streams and one reservoir. Snails were sampled every four metres on each transect using a scoop net. Each transect was divided into five 80 metre segments. Environmental data consisting of physical, chemical and biotic data was obtained at the mid point of every segment. Physical data collected consisted of water depth, temperature and velocity. Chemical data collected consisted of conductivity, pH and calcium concentration. Biological data collected was vegetation by type and genera, and phytoplankton diversity. Fifteen species of freshwater snails were collected from the study area. Pulmonate species consisted of Bulinus globosus, Bulinus forskalli, Bulinus canescens, Biomphalaria pfeifferi, Segmentorbis angustus, Gyraulus costulatus, Lymnaea natalensis, Physa acuta and Ferrissia burnupi burnupi. The prosobranch species consisted of Bellamya cappillata, Cleopatra nsendwensis, Gabbiella kisalensis, Gabbiella sp, Lanistes ovum and Melanoides tuberculata. Linear regression was used to determine the relationship between snail diversity (species richness) and the number of infected children at the schools in close proximity to the water sampling points. There was a significant relationship between snail diversity (in the presence of the intermediate snail host) and schistosomiasis prevalence. Snail diversity explained 62% of the variance in schistosomiasis prevalence. Results of the study demonstrated that in the presence of high species diversity of snails, transmission of Schistosoma haematobhim is lower. Multiple regression analysis (Best Sub Set) revealed that water velocity was the most important factor affecting snail diversity in the rivers, while conductivity was the most important factor in the streams. Snail diversity was significantly different among the different vegetation types. All the floating vegetation types were associated with higher snail diversity whereas Phytoplankton diversity was not significantly associated with snail diversity.