Nearly 1 In 5 Cats With This Gut Parasite Carry Strains That Can Infect Humans, Study Finds

Healthy-Looking Kittens Can Carry a Parasite That Also Infects People

A cat can look perfectly healthy, purr on the couch, and still be carrying a parasite that also infects people. A new analysis in PLOS One checked more than 157,000 cat stool samples from across the U.S. and Canada and found Giardia, a gut parasite that can cause diarrhea but often causes no symptoms at all, in 6.3% of them. Of the infected cats, 18.5% carried strains with the potential to infect humans too.

Most cats with Giardia carry a cat-only version. Two strains, labeled A and B by scientists, can infect people and other mammals, and the lab test used here flagged both. Researchers describe this as zoonotic potential, meaning the parasite could move between animals and humans. Nobody tracked whether any cat actually passed it to an owner, so the results show what is possible, not what is happening in living rooms.

Kittens under a year old were the likeliest to carry those strains, with close to 20% of infected kittens testing positive for them. The authors say that supports routine stool testing for cats, especially in homes with young children, older adults, or anyone with a weakened immune system.

Giardia in Cats Was Detected in Samples From 49 States and Five Provinces

Most earlier work on Giardia in cats focused on specific places or small groups of cats, with different tests along the way. Here, the researchers dug through records from Antech Diagnostics, a commercial reference lab. Its stool test looks for the parasite’s DNA and can tell which strain is present. This look-back approach, called a retrospective study, made an enormous sample possible, covering tests run between March 2022 and August 2023.

Samples came from 49 U.S. states, Washington D.C., and five Canadian provinces. The cats ranged from 1 day old to 26 years old, were split nearly evenly between males and females, and 71% were spayed or neutered. A positive result means the parasite’s DNA was found, not that the cat was sick. The 6.3% rate sits on the higher end of earlier U.S. estimates.

Because the records were already collected, nothing was known about whether a cat lived indoors or outdoors, had diarrhea, or had recently traveled. The team compared infection rates across age, sex, neuter status, season, and region.

Giardia in Cats Is Most Common in Kittens and Least Common in Summer

Age showed the clearest pattern. About 11% of kittens under a year old were infected, versus just 1.3% of cats over 10. The authors point to immune defenses that strengthen with time and to changes in housing and activity as cats age, which could mean less exposure.

Sex mattered, too. Males tested positive more often than females, 6.9% versus 5.7%, possibly because of roaming or outdoor time, which the study did not measure. Cats not spayed or neutered were also infected more often than fixed ones, with one twist: among kittens, unfixed ones were less likely to be infected. The authors called one explanation plausible but untested. Shelter kittens are sometimes neutered as early as 8 weeks old, and the stress of shelter life plus surgery on an immature immune system could leave them more vulnerable.

Timing and geography played a part as well. Summer had the lowest rate, 4.6%, versus roughly 7% to 8% the rest of the year. Giardia spreads through hardy, shell-like cysts that survive longer in cool, damp conditions, so hot, dry months may give the parasite a harder time. The Midwest had the highest overall rate, 7.8%, and the South the lowest, 5.9%.

Northeast Cats With Giardia Were Most Likely to Carry Strains That Could Infect People

Among infected cats, the chance of carrying a strain with human potential also varied by place. The Northeast led at 21.6%, while the South trailed at 16.3%. That echoes human data, since Giardia infections in people have been reported more often in the northern U.S. Winter had the highest share by season, and senior cats had the lowest by age, at 13.5%.

Sex and neuter status made no clear difference here, despite mattering for infection overall. The authors expect unfixed males to have less contact with people because of their lifestyle, which could keep those strains from showing up more often in these cats.

People get Giardia too. The paper notes that roughly 1.4% of people in the U.S. carry it. Earlier genetic studies, limited to a few states, reported strain A alone in 21% to 35% of infected cats, higher than this study’s figure, but the authors say their far broader sample likely gives a better estimate of how common these strains really are.

Researchers also raised a two-way possibility. Some strains found in cats may have come from humans in the first place, through contact with people or a cat eating feces, a back-and-forth scientists call reverse zoonosis. In that case, owners and pets could be sharing the parasite in both directions. Future studies using finer genetic tools could sort out which direction the parasite travels.

Who is passing what to whom remains an open question. What the data do show is that a healthy-looking kitten can carry a parasite that also infects people, and a stool test is how to find out.

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Disclaimer: This article is for informational purposes only and is not medical or veterinary advice. Readers with concerns about a pet’s health or their own should consult a veterinarian or other qualified health professional. The findings come from one study and do not prove cause and effect.

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