Not a worm. Not a bacterium. A single cell with whip-like tails that swims in the mucus lining the large intestine, and irritates it badly enough to cause diarrhea that can go on for months.
Tritrichomonas foetus lives in the last stretch of a cat's gut — the ileum, caecum and colon — sitting in the mucus on the surface rather than burrowing into the tissue. It does not form cysts, which matters: there is no tough dormant stage waiting in the environment for you to disinfect away. It is fragile outside the cat and dies fairly quickly, which is why this spreads through shared litter trays and close contact rather than through the air or the furniture.
Where it does damage, it does it by adhering to the cells lining the colon and releasing enzymes that break down the barrier — a cysteine protease called CP30 has been identified as a probable virulence factor.1 It also appears to compete with the fermenting bacteria your cat's colon depends on, disrupting that community in ways that may explain part of the diarrhea.2
The classic picture is a young cat — often under a year old, often from a cattery or shelter — with chronic or intermittent large-bowel diarrhea that smells markedly worse than ordinary cat diarrhea. Straining, urgency, mucus, and sometimes blood. Weight loss in more severe cases.
But the picture is unreliable in both directions:
Which is the first thing worth internalising: the presence or absence of diarrhea is a poor guide to who is carrying this organism. Testing is what tells you.
Any cat that lives closely with other cats. The strongest risk factors that keep coming out of the epidemiology are dense housing and youth — not a breed, and not an outdoor life.
| Population | Prevalence |
|---|---|
| Cats in large-scale hoarding operations, USA | ≈39% |
| Pedigreed cattery cats, USA | ≈31% |
| Long-term sanctuary cats with diarrhea | ≈26% |
| Long-term sanctuary cats with normal stool | ≈10% |
| Mixed pet-cat populations | ≈4–10% |
| Purebred vs non-purebred cats, one Polish survey | 60.7% vs 7.8% |
No breed is exempt, and no breed is implicated. The risk factors that keep appearing in the data are housing density and age — multi-cat households, catteries, shelters and rescues, and young cats in particular. That is a property of how a cat is kept, not of which cat it is. A domestic shorthair in a busy foster home carries more risk than a pedigree cat living alone.
Pedigreed and cattery cats dominate prevalence studies because of how they are housed, not because of heredity. If you have searched for your cat's breed and found nothing specific, that is because there is no breed-specific finding to find — and any page that offers you one is guessing. If you breed cats, the cattery path starts here →
For years the organism in cats was assumed to be the same species as the venereal parasite of cattle, Tritrichomonas foetus. Then researchers looked more closely. The feline organism behaves differently in cattle than the bovine one does, and the two differ genetically. In 2013 a group led by Heather Walden proposed that the feline organism is a separate species, Tritrichomonas blagburni, named for the parasitologist Byron Blagburn.4
That proposal is not universally accepted. Critics have argued the original case did not cleanly demonstrate separate species, and later whole-genome work has been read both ways. So the names are used inconsistently:
| Source | Name used |
|---|---|
| ABCD (European cat-disease guidelines) | T. blagburni throughout |
| TROCCAP (international parasitology guidance) | T. foetus |
| The Gookin laboratory, NC State | T. foetus / blagburni — deliberately both |
| Most commercial diagnostic laboratories | T. foetus |
| Cornell shelter medicine guidance | T. foetus, "tritrich" |
It is not Giardia, though the two look similar under a microscope and are often confused. It is not Pentatrichomonas hominis, another look-alike that cats carry without apparent harm — distinguishing them was one of the corrections that established this infection in the first place.5 And in cats it is not a venereal disease, despite the family name: a study looking for reproductive-tract colonisation in at-risk cats found none, suggesting venereal transmission is not a meaningful route in this species.6
That last point surprises people who read the bovine literature and assume it carries across. It mostly does not. Cats get this by eating it, not by mating.
If the rest of this page is more than you want today, this is the version to keep.
