Discussion

In Kenya, an estimate of 6 million people are infected with schistosomiasis  with >30 million people at risk of infection. Schistosoma mansoni is commonly transmitted by Biomphalaria pfeifferi, an inhabitant of streams and small water bodies, and B. sudanica, which is mostly found along lakeshores, mainly in Lake Victoria. Recent studies have accentuated the role of infected snails in maintaining transmission as some snails can survive for over a year shedding cercariae daily. We sought to determine if these two snail species may differ with respect to the efficiency with which they support S. mansoni infections. We exposed field-derived B. pfeifferi (Kirinyaga, central Kenya) and B. sudanica (Kisumu, western Kenya) to S. mansoni derived from human subjects from Kirinyaga or Kisumu. The reciprocal cross infection design allowed us to ascertain if local adaptation effects might influence infection outcomes. Juvenile (<6 mm shell diameter), young adult (6-9 mm) and adult snails (> 9 mm) were exposed, all to one miracidium/snail. Overall, B. pfeifferi consistently had higher infection rates than B. sudanica (39.6 - 80.7% vs. 2.4 - 21.5%), regardless of the source of S. mansoni or the size of the snails used. Allopatric B. pfeifferi - S. mansoni combinations had higher infection rates than sympatric combinations while B. sudanica showed the opposite trend. Infection rates were inversely proportional to snail size. Mean daily cercariae production was greater for B. pfeifferi exposed to sympatric than allopatric S. mansoni (62 –2465 and 100 – 1232, respectively), and this trend increased with snail size. Overall mean daily cercariae production amongst all B. sudanica was low (50–590) with no significant differences between sympatric or allopatric combinations, or among the different snail sizes (p < 0.05). In conclusion B. pfeifferi is more likely to become infected and to shed more cercariae than B. sudanica, suggesting that the per snail risk of perpetuating transmission in Kenyan streams and lacustrine habitats may differ considerably, with noteworthy implications for understanding transmission dynamics and planning control efforts.