Yeah,
I am meaning that feces itself or water quality does not cause swimmer’s itch.
Certainly plays a role in the disease ecology and transmission cycle. The thing is, people hear “duck poo” and they think that’s what causes it. It’s a fairly removed role, not a correlation most people are going to think about.
Better, I think, to tell people it comes from the snails. It’s why we tell people 24 hours for Lyme disease when it’s actually more like 48. Simpler, safer, they aren’t going to learn the entire disease cycle. Points that stick.
The correlation with water quality is anecdotal, not really explored in depth. Any study I have seen is that cleaner water correlates with less disease presence. The disease can only exist with the presence of suitable snails, which tend to thrive more in eutrophic water systems. Eutrophic water systems are generally lower in DO (dissolved oxygen) than oligotrophic systems, DO being of huge importance in determining water quality. That being said, most snails don’t require a high level of DO, and they do thrive off the organic material in highly eutrophic water bodies. So, snails don’t mean clean or dirty water in terms of pollution, just that snails can live there. So generally, snails = snails and not much else. However, I’ve done a lot of biomonitoring, and there are a few (2, IIRC) families of snails on our MI “procedure 51” list (which is stream/river auditing). They don’t have a huge impact on the score. Also, if you look at the values for a Hilsenhoff biotic index you’ll see only a handful of families are weighed, and they’re mostly highly tolerant. I’m not even sure if they contain vectors for cercarial dermatitis.