Millions of US Cats Carry 'Beaver Fever' Parasite Transmissible to Humans, Study Finds

A significant percentage of pet cats in the United States are carriers of Giardia, a parasite responsible for an illness commonly known as 'beaver fever,' which can spread to humans and cause severe gastrointestinal distress. According to a new study by U.S. veterinary researchers, approximately one in 15 domestic cats harbor the parasite.

Giardia is transmitted to humans through the ingestion of microscopic cysts found in feces, contaminated water, or on dirty hands and surfaces. In the United States, over a million people contract beaver fever annually. Symptoms typically include frequent diarrhea, stomach cramps, nausea, and dehydration, which can persist for two to six weeks. In severe or prolonged cases, the illness can lead to joint pain, symptoms resembling irritable bowel syndrome, weight loss, and impaired nutrient absorption, potentially lasting for years.

The study, published in PLOS One, analyzed over 157,000 feline stool samples collected from across the United States and Canada. Researchers utilized a sensitive DNA test to detect the presence of Giardia. Of the samples tested, more than 6% were found to be positive for the parasite. These infected cats were distributed throughout the U.S., with higher concentrations observed in the Northeast, including states such as Maryland, Pennsylvania, New Jersey, New York, and Massachusetts.

Further analysis revealed that nearly one in five of the Giardia-infected cats carried strains known as 'assemblages A and B.' These are the specific types of the parasite that can readily transmit to humans and cause beaver fever. The remaining positive cats primarily carried assemblage F, a strain that rarely affects people.

Based on these findings, the study estimates that potentially five million cats across the U.S., including those in densely populated urban areas, may be carrying some form of Giardia. Of this group, as many as one million cats could be hosts to strains that easily infect humans, potentially leading to prolonged illness from contact with contaminated surfaces.

Veterinary researchers emphasize the importance of thorough cleaning protocols for households with cats diagnosed with Giardia. This includes meticulously cleaning litter boxes, food bowls, and water dishes. Special caution is advised for households with young children or immunocompromised individuals, who should have limited contact with pets undergoing treatment until re-testing confirms they are clear of the parasite.

Giardia's nickname, 'beaver fever,' originated in the 1970s when outbreaks were linked to untreated water sources in areas inhabited by beavers. Beavers can contaminate streams and lakes with the parasite's cysts, which are hardy and can survive in cool, moist environments for extended periods. For pet cats, contamination typically occurs when they come into contact with fecal matter, such as by touching a used litter box or soiled fur, and then transfer the cysts to surfaces or their own mouths. Humans can also contract the illness by drinking or coming into contact with water or food contaminated by infected cat feces. Ingesting even a small number of cysts can be sufficient to cause infection.

The study also noted demographic patterns among infected cats. The parasite was found more frequently in younger cats and kittens, and slightly more often in males. Senior cats showed the lowest rates of infection. Unexpectedly, intact cats (those not spayed or neutered) were found to be less likely to carry Giardia compared to their spayed or neutered counterparts.

Researchers advised pet owners to be particularly vigilant as the seasons change from summer to fall. The study indicated that Giardia infections in cats tend to be lowest during the summer months and increase through the fall, winter, and spring seasons.