Compatibility of Schistosoma Mansoni and Its Intermediate Host Snails (Biomphalaria Spp) in Relation to Transmission of Intestinal Schistosomiasis in Kenya

Abstract

Introduction: Schistosomiasis afflicts an estimated 240 million people worldwide, with majority of the cases occurring in sub-Saharan Africa. In Kenya, an estimated 6 million people are infected, with >30 million people at risk of infection. Schistosoma mansoni, the most widespread among the human infecting Schistosoma species depends on the freshwater planorbid snails in the genus Biomphalaria for its transmission. In this study compatibility and vectorial competencies of 3 Biomphalaria spp were examined and snail-related factors of S. mansoni transmission were investigated in Lake Victoria, western Kenya Materials and Methods: Reciprocal cross infection experiemnt using field derived B. pfeifferi from Kirinyaga county and B. sudanica from Kisumu county in 3 categories based on age/size was done using sympatric and allopatric S. mansoni and then, using lab-raised F1 generation of Lake Victoria derived B. sudanica and B. choanomphala snails with sympatric and allopatric combinations of S. mansoni in 3 different miracidia dose (1, 5 and 10). On 4 occasions over a 2-year period, relative abundance of B. sudanica and B. choanomphala, prevalence of S. mansoni in the snail population, hyacinth intrusion, and acquisition of S. mansoni worms by sentinel mice at 20 sampling sites (two per village) were determined. Results: It was observed that S. mansoni developed faster and consistently had higher infection rates in B. pfeifferi (39.6-80.7%) than in B. sudanica (2.4-21.5%) regardless of the parasite source or snail size/age. Cercariae production was greater for B. pfeifferi as compared to B. sudanica. It was also observed that B. choanomphala was more susceptible to S. mansoni than B. sudanica (12.7 - 80.8% versus 5.2 – 18.6%) and on average, B. choanomphala produced more cercariae than B. sudanica and increase in the miracidia dose was not associated with an increased cercariae production for both snail taxa. It was also observed that shoreline-associated B. sudanica did not

differ in relative abundance or prevalence of S. mansoni infections between PHS and RESP villages (P>0.05). However, water hyacinth intrusions were associated with increased B. sudanica abundance (P<0.05). The deep-water B. choanomphala, on the other hand, was significantly more abundant in the PHS villages than in the RESP villages (P<0.05). Conclusion/Recommendations: Results of this study suggest that B. pfeifferi is more likely to become infected by S. mansoni and to shed more cercariae than B. sudanica and B. choanomphala, suggesting that the risk per individual snail of perpetuating transmission in Kenyan streams or lacustrine habitats may differ considerably between these 2 habitat types. The high susceptibility of B. choanomphala to S. mansoni and its presence in all sampling sites in the PHS villages suggests that it could be providing an alternative mode of transmission that may favor greater persistence of S. mansoni in PHS villages. The study established evidence of transmission in all ten villages studied and thus recommends for an integrated, basin-wide control plan to counteract rapid reinfections facilitated by large snail populations and movements of infected people around Lake Victoria.