The Question: I’ve heard that it might not be healthy to swim in a pond with an active beaver colony in it, as something is in the water from the beaver that can be bad for humans. Do you know if this is so? We have a large beaver pond with an active colony, and deep water at least 30 feet out from the dam, and we’d love to swim in it. Are there any tiny worm-like parasites we should be worried about as well? I couldn’t find any information online about this.

Submitted by: Faye, Minnesota, USA

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The Short Answer: Before I begin to answer, I should say that I am not a doctor and not a qualified public health official. There are dozens or maybe hundreds of diseases that can be contracted from “surface waters,” meaning lakes, rivers, ponds, etc. Some of them are caused by worms, but giardiasis and cryptosporidiosis, the two most commonly associated with beavers (Castor canadensis in North America, Castor fiber in Eurasia), are caused by protozoan intestinal parasites (Giardia sp. and Cryptosporidium sp. respectively), which are spread through feces. Because beavers poop in the water, they can release the infectious cysts of the organisms. If you swallow the water, you may become infected. Unfortunately, you only have to ingest a small number of cysts to become infected.

Beaver“Common knowledge” associates giardiasis and beavers so closely that people often call the disease “beaver fever.” However, common knowledge isn’t always correct, and it’s not clear that beavers very often contaminate people with Giardia. In fact, it seems that Giardia species tend to specialize. So the Giardia that most commonly infects beaver is different from the one that infects people. However, most Giardia have some ability to infect organisms other than their primary host, and you can find human-type Giardia in beavers. But the evidence suggests that when it is found in swimming waters and in beavers, the most common source of the human-type Giardia is not the beavers but people. Typically, it’s from untreated sewage that gets into the water or from fecal matter washing off people when they are in the water. For that reason, health officials recommend that people with diarrhea stay out of swimming waters.

Special Note:  Heidi Perryman of, wishes I had defended beavers more strongly.  For her commentary, see below.

Swimming with or without beavers is not the most common way to get “beaver fever.” The largest number of cases are among small children in day care centers, who pass it among each other.

So if you want to know if a swimming area is safe, rather than look for a swimming hole with no beaver, you should probably look for one with no people – especially small people. The situation for cryptosporidiosis is similar. And while these diseases tend to be “self-limiting,” they are not fun, often causing severe diarrhea. One common hiker’s saying is that “While beaver fever won’t kill you, you might wish you were dead.”

When I asked Lihua Xiao, who is Chief of the Molecular Epidemiology Laboratory/WBDP of the Division of Foodborne, Waterborne, and Environmental Diseases for the United States Centers for Disease Control, for his opinion about swimming with beavers, he said flatly “I would not swim in a pond where beavers are active.”

I think that is a somewhat extreme view that would restrict you from the joy of swimming in most of the waters of Minnesota, but Dr. Xiao is one of the world’s foremost experts on these diseases and the organisms that cause them, and I am not. So you should weigh our opinions appropriately.

If you do decide to swim in your beaver pond, you should, as much as possible, avoid swallowing the water, and you should never drink untreated surface waters. The recommendation is to boil or use filters that specifically say they will remove Giardia. The cysts of Giardia and Cryptosporidium are somewhat resistant to chlorine, so don’t count on that. Boiling and filtration will also kill or remove most other waterborne diseases and parasites at the same time, including any worm-like parasites.

Fear the muskrat: A study that tested beaver and muskrat (Ondatra zibethicus) in Pennsylvania and New Jersey for Giardia and Cryptosporidium found that out of 10 beaver tested, none were infected with Cryptosporidium and only one was infected with Giardia. The muskrat were a different story. Nearly 90% were carrying one or the other of the parasites and nearly a third were carrying both. So on the basis of this study, it might make even more sense to avoid waters with muskrat than with beavers. Of course, beaver and muskrat are often found together in Minnesota and most other places, so that doesn’t help much.

To Get More Info: The Center for Disease Control’s Giardia page:

Lifecycle of Cryptosporidium from CDC:

Lifecycle of Giardia from CDC:

Very informative page from the Washington State Department of Fish & Wildlife on beaver, how to live with them, and how to discourage them if that’s your goal:

Additional commentary from Heidi Perryman of If the question is “Can beaver ponds carry disease?” the answer is yes, and so can any other body of water you might feel like swimming in. If you’re asking “Do beaver ponds cause beaver fever?” the answer is also sometimes.  Beavers can pass on giardiasis just like deer, muskrat or even humans. They are not, however, more likely to carry it. One researcher even remarked that in other countries there is no association with beavers and giardiasis because in other languages the words “beaver” and “fever” don’t happen to rhyme!

From the beaver’s point of view the answer to the question “Can I swim with beavers?” is: please don’t. Beavers would rather not have you around while they’re working and would definitely rather not have your dog around. This is especially true in the summer months when there are young beavers to protect. Every year I read at least one story about someone whose pet was attacked by a beaver in June or July. Remember, any animal that can chop down a tree with its teeth can do harm to your dog. (Rosell, F. Rosef, 0 & Parker H ) 2001. Investigations of Waterborne Pathogens in Eurasian Beaver (Castor fiber) Acta vet. scand. 2001, 42, 479-482.)


Monis, P.T. & Thompson, R.C.A. “Cryptosporidium and Giardia-zoonoses: fact or fiction?” Infection, Genetics and Evolution 3 (2003): 233–244.

Bitto, A, & Aldras, A. (2009). Prevalence of giardia and cryptosporidium in muskrats in northeastern Pennsylvania and New Jersey. Journal of environmental health, 71(8), 20-26.

Feng, Y, & Xiao, L. (2011). Zoonotic potential and molecular epidemiology of giardia species and giardiasis. Clinical microbiology reviews, 24(1), 110-40.

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