Steve:

Strangely, I just saw Derlet a little while ago and asked him the incubation period for Giardia. He said 2 to 4 weeks and thought a few days too quick.

I've got a bunch of other articles from Bob Derlet and his colleagues at Sierra Nature Notes:
www.sierranaturenotes.com

There's also a really interesting data set that I keep meaning to publish, which I'll try to put at the top of my list. It was a fairly well done study in Yosemite by a guy named Atwill at UC Davis. Only the data is published, not a paper. Anyway, grad students (undoubtedly!) collected, weighed and took samps of horse manure between Tuolumne Meadows and Glen Aulin. They examined for both giardia and chryptosporidium. They also estimated the number of poops per mile as well as the number of cysts per poop.

Incidence of giardia from sample total over 4 years was (this is from memory) about 3 to 6% of all samples. Crypto was, I think 2 to 3%. Belding ground squirrel droppings were also sampled and found to have something like 60% giardia -- HOWEVER it was not the type that humans are susceptible to. The horse/mule giardia was the type that humans are susceptible to and is thousands of cysts per poop, to say nothing of the increased nitrogen and phosphorous loading of meadows and streams.

George

None of the views expressed here in any way represent those of the unidentified agency that I work for or, often, reality. It's just me, fired up by coffee and powerful prose.