Giardia in cats: large North American study finds 6.3% prevalence and substantial zoonotic potential

A large molecular survey of more than 157,000 domestic cats in the United States and Canada found Giardia duodenalis in 6.3% of faecal samples. Among infected cats, 18.5% carried assemblages A or B, the genetic groups considered to have zoonotic potential because they can infect humans as well as other mammals.

The retrospective study analysed 157,616 feline faecal samples submitted to a diagnostic laboratory between March 2022 and August 2023. Samples came from 49 US states, Washington DC and five Canadian provinces. Overall, 9,962 cats tested positive for G. duodenalis. Of these, 1,840 (18.5%) were positive for assemblages A/B, which are of particular One Health interest because, unlike the more cat-adapted assemblage F, they can occur in humans and other mammalian hosts.

Kittens had the highest prevalence

Age was one of the strongest risk factors. Kittens under one year of age had a 10.7% prevalence, compared with just 1.3% in senior cats aged 10 years or older. Among Giardia-positive kittens, 19.7% carried potentially zoonotic A/B assemblages. The decline in prevalence with age is consistent with greater susceptibility and exposure in younger animals, although the study does not establish whether immunity, behaviour or husbandry explains the difference.

Male and intact cats had higher odds

Male cats had 20% greater odds of Giardia detection than females. Neutering status also interacted with age. Among cats older than one year, intact animals generally had higher odds of infection than neutered cats. In young adults aged 1–6 years, intact cats had around 1.65 times greater odds of Giardia detection.

Infection varied by season and region

Summer had the lowest prevalence, at 4.6%, compared with 7.1% in spring, 7.7% in autumn and 8.1% in winter.

Geographically, the US Midwest had the highest detection rate, at 7.8%, followed by the Northeast at 6.4%, West at 6.1% and South at 5.9%. Cats in the Midwest had 1.36 times higher odds of testing positive than cats in the South.

Zoonotic potential does not mean proven transmission

Finding assemblages A or B in cats demonstrates potential zoonotic compatibility, but it does not prove that those cats transmitted infection to people. Most feline Giardia infections are also subclinical. Nevertheless, infected cats may shed cysts into the environment, creating potential exposure through contaminated litter, surfaces, water or hands.

The results therefore support routine faecal screening, particularly in kittens and other higher-risk cats, alongside hygiene measures in multi-cat households, shelters and environments shared with vulnerable people.