Tritrichomonas blagburni (formerly T. foetus) is a protozoan parasite that colonizes the large intestine of cats, causing chronic, foul-smelling diarrhea that is resistant to standard treatments. Young cats in catteries or shelters are most commonly affected, and diagnosis requires specific testing not included in routine fecal exams.
Last reviewed: June 2026
What Is Tritrichomonas and How Do Cats Get It?
Tritrichomonas blagburni is a single-celled flagellated protozoan that lives in the large intestine of cats. It spreads through direct fecal-oral contact — sharing litter boxes, mutual grooming, or close housing are the primary transmission routes. Prevalence is highest in purebred cats, young cats (under one year), and cats in multi-cat environments. As described in Ettinger's Textbook of Veterinary Internal Medicine, the organism attaches to and disrupts the colonic epithelium, impairing water absorption and provoking a local inflammatory response.
Unlike Giardia, which is a small intestinal pathogen, Tritrichomonas targets the large intestine — a distinction that matters for diagnosis and treatment.
Signs of Tritrichomonas Infection in Cats
The hallmark is chronic, intermittent large-bowel diarrhea persisting for weeks to months with these distinctive features:
- Soft to liquid stool with a distinctively foul, pungent odor
- Mucus and occasionally fresh blood (hematochezia) in or coating the stool
- Increased urgency and straining (tenesmus) — cats may defecate outside the litter box
- Mild perianal irritation from frequent loose stools
- Appetite and weight generally preserved — cats typically remain bright and alert
Gookin et al., 2004, JVIM documented the clinical and diagnostic features in 117 cats and confirmed that large-bowel diarrhea with mucus is the predominant presentation, with many cats maintaining normal demeanor despite months of symptoms.
Diagnosing Tritrichomonas: Why Standard Tests Miss It
Tritrichomonas is missed by standard fecal flotation because the trophozoites do not produce oocysts. Testing options:
- Saline smear of fresh stool — can reveal motile trophozoites microscopically, but sensitivity is low and requires immediate processing
- Fecal PCR — the most sensitive and specific test; a fresh fecal sample goes to a reference laboratory; results in 3–5 days; this is the recommended diagnostic test
- InPouch TF culture — a commercially available culture kit; less convenient than PCR
Standard Giardia antigen tests and fecal flotations are negative for Tritrichomonas. Co-infection with Giardia is common — both should be tested when large-bowel diarrhea is present in a young cat. Stockdale et al., 2008, JFMS confirmed fecal PCR as the gold standard for Tritrichomonas detection in cats.
Treatment
Ronidazole is the only drug with documented efficacy against Tritrichomonas in cats, as described in Plumb's Veterinary Drug Handbook. Standard metronidazole and fenbendazole are not effective against this organism. Ronidazole is given at carefully controlled doses for 14 days; neurological side effects (ataxia, seizures) are possible at higher doses, so strict adherence to the prescribed dose is essential.
Many cats experience spontaneous resolution over 2 years without treatment, but shedding and transmission to housemates continue throughout. Environmental decontamination — disinfecting litter boxes with quaternary ammonium or dilute bleach — is important to prevent reinfection.
When to See a Vet
Call your vet today if:
- Your cat has had soft, mucousy, or liquid stool with an unusually foul odor lasting more than 1–2 weeks
- You see fresh blood or mucus coating the stool
- Your cat is straining to defecate or defecating outside the litter box
- Multiple cats in your household are developing similar diarrhea
Go to the ER immediately if:
- Your cat is lethargic, not eating, or vomiting in addition to diarrhea — dehydration develops rapidly
- Your cat shows neurological signs during ronidazole treatment (wobbling, tremors, seizures) — stop the medication and seek immediate care
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Frequently Asked Questions
How is Tritrichomonas diarrhea different from regular cat diarrhea? Tritrichomonas causes large-bowel diarrhea — stool with mucus, sometimes fresh blood, a distinctively pungent odor, and straining. The diarrhea is chronic, lasting weeks to months, and recurs despite standard antibiotics. Cats typically remain bright and alert with normal appetite, which distinguishes it from acute infectious diarrheas. A fecal PCR test is required to confirm the diagnosis.
Can humans catch Tritrichomonas from cats? Current evidence does not support human infection with Tritrichomonas blagburni from cats — it is considered host-specific. Standard hygiene, including handwashing after handling cat feces, is always recommended regardless of the diagnosis.
Will Tritrichomonas go away without treatment? Many cats improve spontaneously over 1–2 years, particularly older cats as immune maturity increases. However, affected cats continue shedding the organism and may infect housemates throughout that period. Ronidazole treatment is recommended when the diagnosis is confirmed to shorten symptoms and reduce environmental contamination.
How much does testing and treatment cost? A vet exam typically runs $50–100. Fecal PCR testing costs $60–120 at most reference laboratories. A course of ronidazole is typically $30–80. Total cost for uncomplicated diagnosis and treatment is usually $150–300. Testing multiple cats in a multi-cat household adds to the total.
Can other cats in the house get it? Yes — Tritrichomonas spreads readily between cats sharing litter boxes or through close contact. If one cat is diagnosed, testing all cats in the household is strongly recommended. Litter box disinfection with dilute bleach is necessary to prevent reinfection after treatment.
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