Anthropogenic infection foci: The role of gravel pits, quarries, and other anthropogenic activities in the transmission of parasites in Zambia
Trematodes are complex life cycle multi-host parasites that utilize molluscs (often aquatic snails) as their first intermediate host. Anthropogenic activities that affect aquatic snails also have a high potential to affect the transmission of trematode parasites, some of which are of medical and veterinary importance. This study focuses on the geographic distribution and prevalence of two trematode-caused diseases, (human schistosomiasis and bovine fasciolosis), as well as the distribution and density of susceptible snail hosts in rural Zambia. The central questions motivating this study are: What are the infection patterns in this area of rural Zambia? Does infection prevalence depend on susceptible host distribution and density? What environmental factors are important in predicting susceptible snail distributions? Data on snail densities and biodiversity were collected both in March (rainy season) and July (dry season) from 18 water bodies along a gradient of infection as determined by data previously collected by the Zambian Schistosomiasis Control Initiative and records from local abattoirs. Snails were pooled into susceptible species (pulmonates) and non-susceptible species (prosobranchs) and evaluated against environmental variables (stream health indicators, total dissolved organic carbon (TDOC), total dissolved nitrogen (TDN), and water body type).
For both diseases, infection prevalence was highest around the town of Mansa and decreased towards the shores of lake Bangweulu. Correspondingly, susceptible pulmonate snails were more common around Mansa than the lake, whereas non-susceptible prosobranch snail populations exhibited an inverse trend. A generalized linear model (Poisson distribution) was employed to determine the underlying reasons for the prevailing snail, and hence disease, distribution. Using AIC, the full factorial model that included both water body location (Mansa vs. lake shore) and the TDOC:TDN ratio was the best supported, with a significant (p < 0.05, R2 = 0.68) interaction between water body location and water TDOC:TDN. Splitting the data by water body location indicated that, for water bodies around Mansa, the water TDOC:TDN ratio was a significant predictor of pulmonate snail distribution (p<0.05, R2 = 0.42) whereas water bodies along the lake were low in pulmonate snails regardless of water nutrient stoichiometry. Additionally, man-made structures (gravel pits, cassava processing pits) were associated with increased pulmonate snail density (Two-Way ANOVA, p<0.05) compared to natural ponds and streams, regardless of the season. Disease prevalence is driven by snail densities, which in turn are correlated with water column TDOC:TDN. Water column stoichiometry may be indicative of food quantity/quality, sedimentation rates, or disturbance.