The effect of Pesticide Pollutants on Transmission of Schistosomiasis in Western Kenya

Abstract

Schistosomiasis is caused by trematodes that utilise planorbid snails as their intermediate hosts in Africa. The study aimed to determine the effect of pesticides on the transmission of the disease. This was done by first investigating the effect of two commonly used pesticides, imidacloprid, a neonicotinoid, and diazinon, an organophosphate, on the host snails, Biomphalaria and Bulinus, to determine their tolerance. The tolerance of the snail’s competitors and predators was also investigated. Using laboratory experiments, sensitivity distribution graphs were used to determine that the host snails were the most tolerant taxa to the pesticides chosen. Data from the field showed that the snails were present in moderately polluted sites or greater, with a distinct lack of presence in pristine waters. Binomial and mixed effects general linear models were used to determine a minimum effects model of variables that show the distribution of host snails, and were put into a principal component analysis which showed an antagonistic relationship between pesticide pollution and competitors of host snails. This led the study to conclude that pesticide pollution in the field increases the abundance of host snails by reducing the number of competitors, due to the competitor’s higher sensitivity to the pesticides. The effect of pesticides on the parasite itself were then investigated by studying the effects of the same two pesticides on the free-living life-stages of Schistosoma mansoni. The dose-response calculations revealed the tolerance to be 50-60 times the concentration of what was found in the field. Miracidia were further exposed to sub-lethal doses at 20% the EC50s, where it was shown that these concentrations could reduce the proportion of infected snails. However, these concentrations were 500-600 times greater than those in the field. Therefore, the study here shows that pesticide pollution has the potential to increase snail hosts dominance over their competitors in polluted environments, while not affecting the parasite at environmentally relevant concentrations, leading to an overall increase in risk of disease transmission. As pesticide pollution could increase the risk of schistosomiasis, it must be monitored within habitats to determine the change in risk across seasons. The study evaluated the potential of the SPEARpesticide bioindicator for use in Kenya and found it to be promising. Thus, the government of Kenya could potentially use the bioindicator to cheaply and quickly assess pesticide pollution in the rivers of Kenya and use the information to map out potential areas where the risk of schistosomiasis could increase in the coming years.