Great answers to this week's case! All were correct:
"I think these itchy creatures are Schistosome cercaria - probably S. mansoni based on the [presumably] hepatomegalic abdomens of local bathers"
"I agree - they are the cercariae of S. mansoni (transmitted by the Biomphlaria Spp snail. "
Thank you to everyone who wrote in - both online and off.
These are indeed cercariae of Schistosoma spp. With the history I gave, you are correct in assuming that these belong to S. mansoni. However, it is important to remember that S. haematobium is also present in many parts of Africa. To my knowledge, you can not tell the cercariae of different species apart morphologically at this level.
The CDC has a nice map showing the distribution of S. haematobium (aka urinary schistosomiasis), S. mansoni (aka African hepatic/intestinal schistosomiasis), and where they overlap:
Click here for CDC Map
You'll notice from the map that much of Africa's population is at risk for schistosomiasis.