The area around the spring where my friend got giardia is a very popular camping area--I wouldn't be at all surprised if there was seepage into the groundwater!

From what I've read, giardia started with humans (a lot of people are immune but are carriers) and that's how it spread to the wildlife. At least some wild animals have been found to be carriers also, the mountain beaver of the northwest being one of those. You're right, though, it's not fair to blame it on the animals. But they are continuing the process that humans started.

The sites I checked for links to scientific studies were Katadyn and MSR. They may have something, but I sure didn't find it!