This page is the sequence of decisions you will actually face, in the order they arrive. Not what the organism is — that's the next page — but what to do on Tuesday, and what to ask for, and what to stop doing.
The most common history with this infection is not "we never tested." It is: the fecal test came back normal, several times. That isn't your vet being careless. A routine fecal exam looks for the wrong thing, and even a test that is looking for this organism misses it most of the time.
A direct fecal smear — the cheap, quick look under the microscope — finds the organism only about 14% of the time when it is genuinely there. Culture does better, at roughly half. PCR is the sensitive test, and it is the one to ask for by name.
Two details decide whether that PCR is worth anything:
Ask for a fecal loop sample, not a swab of whatever was in the box. Across 1,717 cats, samples taken with a fecal loop were about twice as likely to test positive as samples taken by washing the colon out, and there was no difference between a colonic flush and a plain voided stool.4 The likely reason is that the organism sits in the mucus on the colon wall, and a loop scrapes the wall.
Sample on a bad day. The organism is shed intermittently. A cat tested during a good week may test negative and still be infected.
This is the least-discussed and most important decision on this page, and it cuts against the instinct to do something.
In a study following cats with this infection, diarrhea usually resolved on its own, typically within about two years — and cats that were treated actually took longer to get better.2 That finding has been repeated in guideline advice ever since.
The catch, and it is a big one: the diarrhea resolving does not mean the infection is gone. Untreated cats generally remain carriers, sometimes for life, and diarrhea can return after a stressful event.
So the honest position is that this is a genuine trade-off your vet has to weigh with you:
Only one drug has been shown to actually eliminate the infection: ronidazole. It is not licensed for cats in the United States, it has to be compounded, and it has a narrow safety margin. Your vet prescribes it and decides the dose. What follows is why the protocol looks the way it does, so you can recognize a plan that is up to date.
The current recommendation is once daily, not twice. Earlier studies used twice-daily dosing, and much of what you'll find online still says so. Then a pharmacokinetic study found the drug is absorbed almost completely and has a half-life of about 10.5 hours, with drug still present two days later5 — which led the original authors to withdraw the twice-daily recommendation in 2014 because it accumulates.6
Expect that it may not work. Across repeat testing of treated cats, roughly one in four still tested positive afterwards.47 Failure has several different causes and they are worth distinguishing: reinfection from another cat, an underdose, a poor-quality compounded product, or genuine resistance, which has been documented.8
Retest after treatment — and late enough for it to mean something. A normal-looking stool is not evidence of clearance. Infection has been seen to relapse as long as 20 weeks after the last dose, so an early negative is an early reading, not a result. This matters enormously before you introduce a treated cat to other cats: cured, or just quiet?
Most cats with chronic diarrhea have already been through several diets, a probiotic or two, and a course of metronidazole before anyone tests for this. That is completely understandable. It is also now thought to be part of the problem.
Cats with this infection have measurably disrupted gut bacterial populations compared with uninfected cats — the technical word is dysbiosis.9 The organism appears to compete with the fermenting bacteria the colon depends on. Every new diet and every course of antibiotics resets that community again.
Which is why current guidance recommends stopping the trial-and-error cycle and giving the gut one stable diet rather than a rotating series of experiments. Note the irony: the probiotic you were sold may not be useless — one was shown to inhibit the organism Laboratory only in a dish, and has never been tested in infected cats.
Metronidazole deserves its own line, because it is the most commonly reached-for drug and it does not work here: stool may firm up while the cat is on it, and the diarrhea returns when it stops, because the organism was never eliminated.
This is where the disease stops being a nuisance and becomes a household problem. Cats with no symptoms at all can carry and shed the organism, so "the other cats are fine" is not evidence that they are uninfected.
The organism is fragile outside the cat and is not a hardy environmental survivor. It spreads through shared litter trays, paws, grooming and close housing rather than through the air or the furniture. That is genuinely good news: litter hygiene is the main lever you control.
The ideal is testing every cat and separating infected from uninfected. Where that is impractical on cost grounds — which is most households — the guidance offers a fallback: treat the cats with symptoms, keep treated and untreated cats apart until treatment is confirmed to have worked, and keep trays scrupulously clean throughout.
You do not need to argue the literature. You need six sentences.
Many general practitioners have not seen a case, and that is not a failing — it is why this page exists. Be the person who brings a request rather than a verdict. There is a dedicated diagnostic laboratory at North Carolina State University that runs this PCR, and the senior researcher on the infection will take consultations from veterinarians — not from owners. If your vet wants to check the current position, they can.
Print the one-page consult summary → It carries the test to request, the dated dosing history, the signs that mean stop, and ten sources at the foot — so the conversation starts from the literature rather than from a forum post.
Chronic diarrhea in a cat has many causes, and most of them are not this one: food intolerance, inflammatory bowel disease, Giardia, cryptosporidium, worms, thyroid disease, lymphoma. What makes trichomonosis worth ruling out early is a specific combination — a young cat, a cattery or shelter background, and large-bowel signs: soft or watery stool that smells markedly worse than usual, straining, urgency, mucus, sometimes blood.
The reason it sits undiagnosed for months is not that it is exotic. It is that a routine fecal exam finds it about 14% of the time when it is there. Why the test misses it →
A single-celled, flagellated protozoan parasite that lives in the mucus lining the ileum and colon. It has no cyst stage, it does not survive long outside the cat, and it spreads by fecal–oral contact — shared litter trays, grooming after stepping in stool, dense housing. It is not a worm and it is not a bacterium, so wormers and most antibiotics do nothing to it.
It also has two names: the feline organism was proposed in 2013 as a separate species, Tritrichomonas blagburni. Most labs still report T. foetus. They are the same organism. The full explanation, including why the name is contested →
By PCR on feces, ideally from a fecal loop sample, taken while the cat is actually having diarrhea, with the litter kept out of it. PCR is roughly 94% sensitive; culture is around half that; a direct smear is about 14%.
Across 1,717 cats, loop samples were about twice as likely to be positive as colonic-flush samples. A colonic flush is still better than a stool from the tray, but a loop is the better ask. How to get a real diagnosis →
Four very different reasons, and they call for different next steps. One: it was never this organism. Two: the drug dose, duration or formulation was inadequate — a lower dose, a missed course, or a compounded product that did not contain what the label claimed. Three: reinfection from another cat in the house that was never tested. Four: genuine resistance, which is documented but less common than the first three.
There is also a fifth possibility that gets missed: the diarrhea may no longer be caused by the parasite at all, even though the cat once had it. Why a resolved stool is not a cure →
Yes — with eyes open. The organism spreads through stool and shared trays, not through the air or the furniture, so a household managed carefully is not a hazard. What matters is tray hygiene, enough trays that cats are not queueing at a soiled box, and knowing who is actually positive rather than assuming the cats without diarrhea are clear.
The dangerous assumption is the common one: that only cats with diarrhea are infected. Roughly a quarter of positive cats in sanctuary populations had normal stool. What that means for a multi-cat house →
By treating density as the enemy and hygiene as the main lever. That means isolating by both test status and symptom status, disinfecting trays rather than furniture, deciding deliberately whether to treat symptomatic cats only or every in-contact cat, and budgeting for a second course in roughly a quarter of treated animals.
The hardest part is not medical. It is deciding whether to hold a cat through treatment and retest, or rehome with disclosure. The outbreak playbook →
That one drug works, that it is not licensed for cats, and that the recommended dose was revised downward more than a decade ago because the original twice-daily regimen accumulated and risked neurological toxicity. That roughly a quarter of confirmed cases still fail it. And that in a follow-up study, diarrhea often resolved on its own within about two years, with treatment lengthening that time.
None of that means do not treat. It means the decision is a genuine trade-off rather than an obvious one. The evidence, in order, with the dated dosing history →
It can, and this is discussed far too little. A course of treatment plus a meaningful retest can mean weeks in a spare room or a pen. Cats are not adapted to confinement, and the welfare literature on single-cage housing is not encouraging.
We treat this as a real factor in the decision, not an afterthought. Isolation, and the cat's side of it →
