Giardiasis, known by the amusingly lyrical name of “beaver fever”, is an infection caused by tiny animals known as protozoans. Giardia protozoans attach themselves to the wall of the mammalian intestine in dense thickets, holding on with a sucker disk on their rear surface. They absorb nutrients here and cause such symptoms as diarrhea, bloating, nausea, vomiting, and other non-fatal but unpleasant digestive problems.
The infection will pass in time even without treatment, though getting rid of it as soon as possible is the best route to take both for the patient's own comfort and for the safety of those who might otherwise be infected by protozoans shed by the infected person.
Giardia organisms themselves are fairly fragile and, in their normal state, will die rapidly outside a host. However, they can encase themselves in a cyst to travel passively from host to host. This cyst allows the enclosed organism to survive for anywhere for several weeks to several months, depending on the water temperature and other factors.
These cysts are particles of a regular size, and therefore can be removed easily by any filter with an absolute pore size of 1 micron or less. Chlorination is no protection from Giardia because the cyst armors the occupant from many environmental effects, including most chemicals. The sole certain way to remove cysts is the straightforward one of mechanically filtering them out.
The Irony of “Beaver Fever”
For many years, it was believed that Giardia originated with beavers and muskrats – a logical connection, to be sure, since many of these large rodents are infected, and live in the water besides. Ironically, however, it appears that beavers are actually innocent of causing Giardia, and humans are the actual disease vectors, while muskrats and beavers pick the ailment up from contact with humans.
Beavers and muskrats who live in remote areas where human feces are absent from the water have been conclusively shown to be free of Giardia infection. Furthermore, even in generally sewage-polluted waters, those animals who live downstream of the sewage source (a campground, a town, etc.) are far more likely to be infested with the parasites than those upstream from it.
Of course, beaver ponds form a “sink” that traps particulate matter like Giardia cysts and infected human feces, so swimming in or drinking the water from them is riskier than open, flowing water.
The presence of beavers or muskrats is therefore not a sure sign of Giardia in the water. Rather, the presence of humans and their waste products in the water are what present an actual risk of giardiasis. However, beavers and muskrats do serve as a “marker” – if the local beaver and muskrat populations are known to be infected with Giardia, then that is a sure sign that infected humans are regularly defecating in the water supply, and proper filtration should be used such as Multipure water filters that are NSF tested.