Additional information at this link:
http://pythiosis.com/description-of-pythiosis/
According to what I am reading, the spores can also attach to grass. We all know how prone dogs are to eat grass on occasion.
Pythiosis is also a fairly new phenomenon in the U.S. Even if you live in a cooler climate, if you let your dog drink out of streams and ponds, there are other things like giardia he can pick up. Make sure your pooch has plenty of clean, fresh water in his bowl, so he’s not as tempted to drink from unsafe sources.
Keeping fresh water is a good practice. In theory, it should work...however...
I have yet to have a dog that would not by pass a bowl of fresh water and head for the nearest birdbath, stock tank or other nasty water for a drink.
All I can think of is it must taste better to them.