The Diarrhoea That Keeps Coming Back
A cat with persistent large bowel diarrhoea — loose, often malodorous stools, sometimes with fresh blood or mucus — that fails to respond to dietary changes or standard treatments is a frustrating clinical puzzle. In multi-cat households and breeding catteries, when multiple animals are affected, Tritrichomonas foetus (T. foetus) moves swiftly to the top of the differential diagnosis list. This protozoan parasite of the feline large intestine is more common than many owners and even some vets appreciate, and it requires a specific diagnostic approach to identify.
What Is Tritrichomonas Foetus
T. foetus is a flagellated protozoan — a single-celled organism that moves using whip-like appendages. It colonises the ileum, caecum, and colon of cats, disrupting normal mucosal architecture and causing chronic large bowel inflammation. The same organism has historically been associated with reproductive disease in cattle, but the feline strain is genotypically distinct and does not cause reproductive problems in cats.
Unlike Giardia, T. foetus does not form cysts. It exists only in its active trophozoite form, which means it does not survive long in the external environment. Transmission requires relatively direct faecal-oral contact, typically through shared litter trays or contaminated bedding. This is why prevalence is highest where cats live in close proximity to one another.
Who Gets It and How Common Is It
Studies in UK and US purebred cat populations have reported prevalence rates of ten to thirty per cent in cattery environments. Young cats under two years of age are most commonly affected, though any age is susceptible. Pedigree breeds, particularly those that have passed through multiple breeders or shows, are over-represented in case series — almost certainly reflecting the higher density and contact rate of those populations rather than any breed-specific susceptibility.
Risk Factors
- Living in a cattery or multi-cat household
- Recent acquisition from a breeder, rescue, or shelter
- Attendance at cat shows
- Sharing litter trays with an infected cat
- Concurrent gastrointestinal infection (Giardia co-infection is not uncommon)
Clinical Signs: What T. Foetus Looks Like
The hallmark presentation is chronic, intermittent, large bowel diarrhoea in an otherwise relatively well cat. The distinction from small bowel diarrhoea is clinically important — large bowel disease produces frequent small volumes with straining (tenesmus), mucus, and sometimes fresh blood, without the profound weight loss or vomiting more typical of small intestinal disease.
Typical Features
- Semi-formed to liquid stools with pronounced odour
- Mucus coating on faeces — sometimes mistaken by owners for worms
- Visible fresh blood in some cases
- Urgency and occasional faecal incontinence, particularly in kittens
- Perianal inflammation from frequent defaecation
- Relatively normal appetite, body condition, and energy in mild to moderate cases
The waxing and waning nature of the diarrhoea — improving spontaneously then returning — is characteristic and often leads owners and vets through multiple rounds of dietary modification and empirical treatment before the diagnosis is established.
Diagnosis: Why Standard Tests Miss It
This is where T. foetus differs most significantly from other parasitic causes of diarrhoea in cats. It is not detected by routine faecal flotation, which identifies the eggs and cysts of helminths and some protozoa. Because T. foetus produces no cyst form, it simply does not appear.
Diagnostic Options
Direct faecal smear with microscopic examination can identify trophozoites in fresh faeces (no more than two hours old, unrefrigerated), but sensitivity is low and requires an experienced microscopist. The organism must be distinguished from Giardia trophozoites and normal commensal flagellates.
The InPouch culture system — a commercial medium developed specifically for T. foetus — offers higher sensitivity and is used in many reference laboratories. A fresh faecal sample is inoculated into the medium and examined over several days.
PCR testing of faecal samples is now the gold standard for sensitivity and specificity, and most veterinary reference laboratories offer it as a routine service. Multiple samples taken on different days increase detection rates, since shedding can be intermittent.
Always discuss which test is most appropriate with your vet based on availability, turnaround time, and whether concurrent pathogens need ruling out simultaneously.
Treatment and Long-Term Outlook
Ronidazole is the only antiprotozoal with demonstrated efficacy against T. foetus in cats and represents the standard treatment. It requires a prescription and precise dosing — the margin between a therapeutic and a neurotoxic dose in cats is narrow. Neurological side effects, including tremors and seizures, have been reported with overdose and should prompt immediate cessation and veterinary contact. Treatment typically lasts fourteen days.
Metronidazole, despite being the most commonly used antiprotozoal in cats, is not effective against T. foetus and should not be substituted. If a cat with presumed T. foetus improves on metronidazole, coincidental resolution or a concurrent susceptible organism is the more likely explanation.
The Natural History Without Treatment
There is some evidence that a proportion of cats — perhaps the majority — will eventually experience resolution of clinical signs without treatment as they mature, though this can take two years or more. During this period, affected cats remain infectious to housemates. In multi-cat environments, allowing natural resolution without treatment is rarely practical or fair to either the cats or their owners.
Environmental Control
Since T. foetus trophozoites survive only briefly outside the host, standard cleaning protocols are effective. Daily litter tray cleaning, dedicated trays for infected cats during treatment, and thorough disinfection of shared bedding reduce re-exposure risk significantly.
Practical Summary
- Consider T. foetus in any cat or kitten with large bowel diarrhoea that has failed to respond to diet changes or standard treatments
- Request PCR faecal testing specifically — routine faecal flotation will not detect this organism
- Submit multiple samples on different days if the first test is negative but clinical suspicion remains high
- Ronidazole is the appropriate treatment — do not substitute metronidazole
- Keep litter trays for infected cats separate and clean daily during and after treatment
- Discuss timing of retesting with your vet following treatment to confirm clearance before relaxing isolation measures