Giardia duodenalis is a microscopic, flagellated parasite that often calls the human and animal gut its home. In most cases, it coexists harmlessly with its host, and most infected pets and people show no signs of illness. However, when it becomes pathogenic, it can cause diarrhea, digestive issues or weightloss.
Cats can carry a pathogenic version of the gut parasite that also infects humans, but how common is it among pet cats, and how often do they harbor such strains?
A recent study investigated these questions by analyzing stool test data from 157,616 pet cats across 49 U.S. states and five Canadian provinces between March 2022 and August 2023.
The results showed that 6.3% of the cats tested positive for Giardia duodenalis. Among the infected cats, 18.5% carried assemblages A or B, genetic groups that can infect both humans and animals. The remaining majority carried strains associated primarily with cats, particularly assemblage F, which generally do not pose a risk to humans.
The findings are published in PLOS One.
Giardia and its hosts
Giardia duodenalis has found its way across the globe and can hitch a ride through contaminated water, food, surfaces and objects, or pass directly from one person to another. Once inside the small intestine, it sheds its protective shell and starts multiplying, splitting into two. As the parasites grow in number, they begin absorbing nutrients from their host.
In the U.S., the parasite shows up in 1.4% of people. Among animals, reported infection rates range from 3.7% to 100% in cattle, 1.1% to 56.8% in dogs and 0.2% to 44.4% in cats.
A 2023 study proposed splitting Giardia into eight separate species, labeled A through H. Most show clear host preferences: C and D favor dogs, E hoofed animals, F cats, G rodents and H pinnipeds such as seals. Assemblages A and B break the pattern, infecting humans along with a wide range of other mammals, making them the most important for public health.
Earlier research on Giardia in cats relied on small sample sizes or focused on very specific locations, such as individual shelters or a few select states. This made it difficult to determine how widespread the parasite truly was across North America.
To get a clearer picture of how widespread the parasite is, researchers conducted a large-scale retrospective analysis, examining existing test records. Although they did not test the samples themselves, the data came from a highly sensitive DNA-based test called the KeyScreen GI Parasite PCR panel, which detects Giardia duodenalis DNA. Among the samples that tested positive, the team screened for assemblages A and B, the genetic groups that can infect humans and animals.
Pet parasite pattern
Of the cats that tested positive for Giardia, 18.5% carried assemblages A or B. Kittens had the highest detection rate at 10.7%, compared with just 1.3% in senior cats. Among the kittens that tested positive, roughly one in five carried A/B types.
The data also revealed that geography and season affected infection rates. Rates were lowest in the summer (4.6%) and higher during winter (8.1%) and fall (7.7%). Giardia was most common in the U.S. Midwest, where 7.8% of cats tested positive. The picture changed among cats that tested positive: Human-transmissible strains were most common in the Northeast (21.6%) and the West (20.0%).
The researchers note that people with pet cats, especially those in households with young children, older adults or anyone who is immunocompromised, should get routine fecal screening for their cats, particularly for parasite strains that affect both animal and human health. Knowing which Giardia type a cat carries could help veterinarians have more informed conversations about treatment measures needed to maintain household health. https://phys.org/news/2026-10-house-cats-gut-parasite-infect.html
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