Which organism serves as the intermediate snail host in Schistosoma mansoni transmission?

• A. Freshwater fish
• B. Snails
• C. Mosquitoes
• D. Rodents

Answer: (B)

The key idea is that Schistosoma mansoni needs a snail to complete part of its life cycle. After the parasite reaches freshwater, the eggs hatch into miracidia that must enter a suitable freshwater snail and infect it. Inside the snail, the parasite multiplies asexually and releases free-swimming cercariae, which then leave the snail and can penetrate human skin to establish infection. Because this snail stage is essential for producing the cercariae that infect humans, snails are the intermediate hosts in this life cycle. In this parasite, Biomphalaria species are the typical snail hosts.

Other options don’t fit because they aren’t the stage where this parasite undergoes its essential multiplication before reaching the human host. Freshwater fish, mosquitoes, and rodents are involved in the lifecycles of different parasites or as incidental hosts, not as the required intermediate host for Schistosoma mansoni.