Study finds some pet cats carry Giardia strains with zoonotic potential

A large retrospective study is putting fresh numbers behind an old veterinary question: how often do pet cats carry Giardia duodenalis, and how often are those infections potentially relevant to human health? In a dataset of 157,616 domestic cat stool samples from the U.S. and Canada, researchers found Giardia in 6.3% of cats, with 18.5% of positive samples carrying assemblages A or B, the genotypes associated with zoonotic potential. The paper was published in PLOS One last week and drew broader public attention after mainstream coverage framed the finding as roughly “1 in 15” cats carrying the parasite. (journals.plos.org)

The new study builds on a long history of mixed messaging around feline Giardia and zoonotic risk. Cats are most often infected with host-adapted assemblage F, while people are more commonly infected with assemblages AII and B. Earlier reviews and clinical guidance have generally concluded that cat-to-human transmission appears uncommon, even though cats can occasionally carry potentially zoonotic strains. That’s also the position reflected in current CAPC guidance, which says human infections are primarily acquired from other humans and that transmission from cats appears to be rare, except with some assemblage A infections. (capcvet.org)

What makes this study notable is its scale and its use of molecular testing through a commercial reference lab. According to the PLOS One paper and PubMed record, the analysis used anonymized fecal qPCR submissions from March 2022 through August 2023 via Antech’s KeyScreen GI Parasite PCR panel. That let investigators look not only at overall Giardia detection, but also at the prevalence of assemblages A and B among positive cats. In practical terms, about 1.2% of all cats tested carried one of those potentially zoonotic assemblages. The dataset also allowed researchers to examine associations with age, season, geography, and reproductive status. (journals.plos.org)

Outside coverage and commentary have added useful nuance. ABC News quoted study co-author Dr. Anne Kimmerlein, global veterinary epidemiologist at Mars Veterinary Health, saying the findings can support earlier detection, treatment, and conversations with pet parents about protecting both pets and households. CIDRAP, meanwhile, emphasized the other side of the message: potentially human-relevant strains were found in only a small share of all samples, reinforcing that the headline risk can be overstated if genotype details are left out. (abcnews.com)

That nuance matters for clinicians. Giardia remains a common differential for feline diarrhea, especially in younger or group-housed cats, and molecular tools are improving visibility into which strains are circulating. But the presence of assemblage A or B in cats doesn’t automatically mean onward transmission to people is likely, nor does a positive PCR necessarily settle treatment or follow-up decisions. CAPC notes that asymptomatic cats may not always require treatment, and that PCR and ELISA can remain positive after therapy, so follow-up is better guided by fecal flotation 24 to 48 hours after treatment completion. Reinfection, not true drug failure, is often the bigger management problem. (capcvet.org)

Why it matters: For veterinary professionals, this study sharpens risk communication more than it changes the basic clinical playbook. It supports routine fecal screening and reinforces the value of discussing litter box hygiene, environmental control, and household vulnerability with pet parents, particularly when young children, older adults, or immunocompromised people are involved. It also suggests that genotype-aware diagnostics may become more relevant in referral, shelter, breeding, or public health-adjacent settings, where knowing whether a cat carries a host-adapted or potentially zoonotic assemblage could affect counseling. (journals.plos.org)

What to watch: The next question is whether these prevalence data translate into changes in guidelines or testing patterns. Watch for follow-up analyses from Mars/Antech or independent groups on regional trends, age-related risk, and whether assemblage-level results meaningfully improve case management, as well as any updates from parasite guidance bodies on when genotype testing is worth the added cost. (journals.plos.org)