Hi Joycelyn,

It is possible for schistosomes to adapt to larger intermediate hosts, but only if this improved the targeting of cercariae to definitive hosts. This usually means that the snail occupies just the right ecological niche. As larger snails tend to occupy a wider ecological niche than smaller ones, there may not be strong pressure for a move towards larger snails. This issue is touched upon in this paper http://www.sciencedirect.com/science/article/pii/S0065308X1073013X

Schistosoma malayensis infects snails which are less than 2 mm long - so small snails will do just fine. Small snails tend to occur at higher population densities, so the population can still produce enough cercariae.

Also, in schistosomiasis terminology the vectors are the definitive hosts (cattle, people, cats, rodents, etc) because they tend to spread the disease around more than snail movements do. Your question mentioned the snails, so I assumed you meant the snails.

I hope this helps.

Stephen

Hi Joycelyn

I agree with Stephen. The most important for schistosome success is the compatibilty/specificity  between schistosome specie and the respective intermediate host snail. This is a key for schistosomiasis endemicity.

Silvana 

Thank you Stephen and Silvana for shedding light! Very helpful inputs!