CHARACTERIZATION OF THE POPULATION OF VECTOR SNAILS FROM MWEA IRRIGATION SCHEME FOR SCHISTOSOMA MANSONI TRANSMISSION

Abstract

Schistosomiasis is recognized as one of major neglected tropical diseases. Freshwater snails of the genus Biomphalaria serve as obligatory intermediate hosts of Schistosoma mansoni. The disease affects more than 249 million people worldwide with more than 90% of those requiring treatment residing in Africa. The use of synthetic molluscides to control vector snails is increasingly becoming unpopular due to its adverse effects on the environment and its associated high costs. Currently there are no vaccines proven effective. It is therefore important to target the aquatic stages of schistosome life cycle to compliment control measures. This study was set out to identify and characterize schistosome resistant snails from Mwea Irrigation Scheme. The study objectives were: To determine population proportion of resistance snails; To determine morphological characteristics of S. mansoni resistant and susceptible snails; To investigate evolutionary development Biomphalaria sp; and To assess transmission levels of resistant traits in snails under laboratory conditions. Snails infected with S. mansoni were determined by direct light illumination method and parasite rDNA observed by agarose gel electrophoresis. Morphological characterization of resistant and susceptible snails was determined through shell morphometric. PCR products of Snails rDNA were used for custom sequencing. Sequencing datasets were analyzed to generate the snails’ phylogenetic tree. Experimental snails were infected with S. mansoni and infections rates determined up to F2 progeny. It was found that some Biomphalaria pfeifferi snails are refractory to transmission of cercaria. The mean value of infection rates for the Field, F1 and F2 snails were 36.6 ± 3.72, 1.93 ± 1.46, 0.36 ± 0.049 respectively with the infection rates decreasing from the field snail samples through to F2 snail samples. The snail morphological parameters had no significant differences in resistant and susceptible populations. The snails had mean shell height of 9.9 and 10.0 mm and shell width of 9.5 mm and 9.5 mm for resistant and susceptible snails respectively. At molecular level, PCR detection of cercaria in snails after exposure to miracidia larvae produced specific amplification of target size band of approximately 400 bp in rDNA ITS gene. Some snails had higher positive titres of cercaria DNA while others were negative. There was active transmission of resistant traits under laboratory conditions. The exhibited resistance traits in B. pfeifferi could not be attributed to morphological characteristics. The genetic lineage of B. pfeifferi was monophyletic with clusters of closely related isolates. Genus Biomphalaria is still undergoing population expansion through random mutation.