Beaver Fever
Dan Onion, MD, MPH
Mt. Vernon/Vienna Health Officer
293-2076; dkonion@gmail.com
October, 2016
Tom Ward, our local weatherman among many other things, called me with a story I could leave for spring to describe. But it is so interesting, I have to tell you about it now. He called because he'd learned that a local summer resident on Flying Pond, had one of her two Labrador retrievers diagnosed by her veterinarian with "beaver fever". Tom wondered if that could affect humans as well and if there were ways to eliminate such risk.
Tom told me that one of the two dogs habitually swam for long periods of time and drank Flying Pond water constantly while doing so. The other dog, not so much. Only the water-drinking dog got sick, with vomiting, chronic cramps and diarrhea. The vet presumably made the diagnosis by examining the stool, as we physicians would in a person as well, and finding Giardia cysts or antigens (diagnostic protein pieces of the parasite) there. Such stool samples in infected individuals are often negative much of the time, so multiple stool tests over several days may be necessary to diagnose it.
"Beaver fever" is caused by the single-celled animal, Giardia lamblia. It is a common parasite of beavers as well as muskrats, dogs often, and sometimes deer. It can't be eliminated from those wild populations without eliminating the carrier populations, an impractical solution. It is spread by ingestion of cysts found in infected individuals' feces. Many dogs, including ones I know, like to roll in animal poop; often they later lick themselves to clean up. So it is no surprise they can easily ingest the parasite. It is possible this dog got it from drinking surface water, in this case from the lake; that is the most common way people get it.
Even infected, many dogs and people have no or few symptoms, but those who do, get chronic nausea, vomiting and diarrhea often along with fatigue and weight loss. The onset of symptoms is often delayed by a week or two after ingestion. The parasite lives primarily in the duodenum, the uppermost portion of the small bowel, right after the stomach. There it causes fat malabsorption, which is the cause of the diarrhea and weight loss, and sometimes actual bowel obstruction, hence causing vomiting. It does not cause fever, so "beaver fever" is really a misnomer. The disease in dogs and humans is insidious in onset and can last months or longer, sometimes even if treated with oral antibiotics. People with immunosuppression from chemotherapy drugs, immunodeficiency diseases, or immunosuppression from human immunodeficiency virus infection (AIDS) all are made much sicker by Giardia than immuno-competent individuals.
So to Tom's question: "Is there a way to get it out of Flying Pond?" First of all, the dog may not have gotten it from the pond but rather from rolling in dog or beaver poop. But it would be very hard to prevent Giardia from being in pond water. Its cysts can tolerate cold and live in moist environments for months outside their hosts' bodies. The wild animal hosts can usually go where they want. Town water reservoirs have to avoid human and animal contamination, micro-pore filter their water, and work to control beaver and muskrat populations in their reservoirs. People with springs as a water source also must work to protect them from fecal contamination. People and dogs shouldn't drink water from "pure mountain streams or lakes while swimming or not. The more lake water they drink, the greater their risk of ingesting Giardia cysts. Maybe that's why the non-drinker dog didn't get sick. Giardia is endemic in Maine and many northern regions, and probably will always be. So, "don't drink the water" from our lakes or streams.