Tritrichomonas Infection

Tritrichomonas foetus is a microscopic, single-celled parasite that lives in the large intestine of cats and causes one of the most frustrating forms of chronic diarrhoea in the species. Unlike many gut infections it does not form a hardy cyst stage, so it relies on close, direct contact between cats to spread — which is why it is so strongly associated with catteries, shelters and multi-cat homes.

The classic picture is a young cat, often a pedigree kitten from a busy breeding household, with soft, foul-smelling, semi-formed diarrhoea that comes and goes for weeks or months. The large-bowel pattern means frequent straining, urgency, and sometimes fresh blood or mucus. The anus can become red and inflamed, and in some cats faeces leak involuntarily. Affected cats usually stay bright and keep eating and gaining weight, which helps distinguish it from more serious disease.

Diagnosis is a common stumbling block. The parasite is easily confused with Giardia under the microscope, and it is shed inconsistently, so routine faecal examinations frequently miss it. A faecal PCR test is the most reliable way to confirm the infection, and it is worth testing for other parasites at the same time, as co-infections are common.

Treatment is genuinely difficult. Ronidazole is the only drug shown to clear the organism reliably, but it must be dosed carefully by a vet because it can affect the nervous system if overdosed. Many cases also need dietary management and time — even after the parasite is gone, the irritated gut can take weeks to return to normal.

Encouragingly, a proportion of cats eventually clear the infection on their own over many months, and most treated cats do well. The key messages are patience, an accurate diagnosis, and attention to hygiene and crowding, which are the real drivers of infection in group-living cats.