Tritrichomonas foetus Diarrhea in Cats: Signs and Treatment
Tritrichomonas foetus is a protozoan parasite that colonises the large intestine of cats, causing chronic, foul-smelling large-bowel diarrhea that persists for months despite standard antibiotic treatment. It is especially common in young cats from multi-cat households and catteries, and requires a specific drug — ronidazole — that most vets don't reach for first.
Last reviewed: June 2026
What Is Tritrichomonas foetus in Cats?
Tritrichomonas foetus is a flagellate protozoan that colonises the ileum and large intestine of cats, causing persistent large-bowel inflammation. It is transmitted via the fecal-oral route and spreads readily through shared litter boxes. Young cats in catteries and multi-cat households are most commonly affected. As described in Quesenberry & Carpenter's Ferrets, Rabbits & Rodents: Clinical Medicine and Surgery and broader feline medicine texts, T. foetus is notorious for being misdiagnosed because it is not detected on routine fecal float tests.
Recognising the Signs
The diarrhea pattern is characteristic and distinguishes T. foetus from most other intestinal disorders.
Hallmark signs:
- Chronic, semi-formed to liquid large-bowel diarrhea lasting weeks to months
- Foul-smelling, often mucoid or bloody feces
- Fecal urgency — cats may not reach the litter box in time
- Fecal soiling around the anus and perineum
- Normal appetite and energy in otherwise healthy cats — weight loss is uncommon
- Rectal prolapse in severe or long-standing cases
Key distinction from small-bowel diarrhea: Large-bowel diarrhea shows urgency, mucus, and small amounts of blood; small-bowel diarrhea produces larger volumes, causes weight loss, and is less likely to contain visible mucus.
Holliday et al., 2009, JFMS evaluated diagnostic methods in cats with chronic large-bowel diarrhea and confirmed that fresh fecal culture (InPouch TF) outperformed direct smear, while PCR was the most sensitive test overall.
Diagnosis and Treatment
Diagnostic options (in order of sensitivity):
- Fecal PCR — most sensitive; recommended first-line when T. foetus is suspected
- Fecal culture (InPouch TF) — good sensitivity; requires fresh, promptly submitted feces
- Direct saline smear — identifies motile trophozoites but has low sensitivity; negative result does not exclude infection
Treatment:
- Ronidazole (oral, 14 days) — the only consistently effective treatment; as described in Plumb's Veterinary Drug Handbook, dosing must be precise because ronidazole has a narrow safety margin and can cause neurological side effects (tremors, ataxia) if dosed too high or continued too long
- Metronidazole and tinidazole — commonly prescribed for protozoan diarrhea but consistently ineffective against T. foetus; using them delays appropriate diagnosis and treatment
- Litter box hygiene: scoop daily and disinfect with dilute bleach (1:30); isolate affected cats during treatment in multi-cat homes
The AAFP-AAHA Feline Life Stage Guidelines, 2021 emphasise that parasitic gastrointestinal disease should be considered in any cat with chronic large-bowel signs, and species-specific diagnostic testing is required for accurate identification.
When to See a Vet
Call your vet today if:
- Your cat has had loose or liquid stool for more than 2–3 weeks, especially if mucoid or bloody
- Your cat is straining to defecate and passing only small amounts with urgency
- You have a young cat from a breeder, shelter, or multi-cat household with persistent diarrhea
Go to the ER immediately if:
- Your cat's rectum is visibly prolapsed and cannot be replaced with gentle pressure
- Your cat is lethargic, refusing food, or showing signs of significant dehydration
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Frequently Asked Questions
Can people get Tritrichomonas foetus from cats? No. The feline intestinal strain does not infect humans. Hand-washing after litter box cleaning is always recommended as general hygiene but is not required due to any specific zoonotic risk from this organism.
Why doesn't metronidazole cure T. foetus? T. foetus is naturally resistant to metronidazole. Using it first leads to weeks or months of treatment delay and continued diarrhea while the underlying organism remains untreated. Ronidazole is the treatment of choice.
How much does treating T. foetus cost? Vet exam and fecal PCR testing typically run $100–250 combined. Ronidazole (14-day course) costs $40–100. If colonoscopy or advanced diagnostics are needed, costs can reach $600–1,200. Delayed diagnosis — repeated ineffective antibiotic courses — is typically the biggest driver of total expense.
Will my cat's diarrhea come back after ronidazole treatment? Some cats clear the infection permanently after one course. Others are reinfected in multi-cat environments if environmental decontamination is inadequate. Re-testing with fecal PCR 4–6 weeks after treatment confirms eradication.
Is T. foetus contagious to dogs? No. T. foetus does not cause intestinal disease in dogs. It is highly contagious between cats, so multi-cat households need to test and treat all affected cats and clean litter boxes thoroughly during and after treatment.
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