Dogs can get trichomoniasis, but it’s not the same infection humans get as a sexually transmitted disease. In dogs, trichomonad parasites infect the gastrointestinal tract rather than the reproductive system, and the primary symptom is chronic diarrhea. Two different single-celled parasites cause the condition in dogs, and neither is the species responsible for human trichomoniasis.
Which Parasites Cause It in Dogs
The human STI is caused by a parasite called Trichomonas vaginalis, which does not infect dogs. Instead, dogs harbor two different trichomonad species: Pentatrichomonas hominis and Tritrichomonas foetus. Of the two, P. hominis is far more common. In one study of dogs with diarrhea and confirmed trichomonad infection, 17 out of 19 positive cases were P. hominis, while only one was T. foetus (and one dog had both).
The role each parasite plays is still debated. P. hominis was long considered a harmless organism that simply lives in the large intestine without causing problems. More recent research has confirmed it can cause gastrointestinal symptoms, including diarrhea, in dogs, cats, cattle, and humans. T. foetus is better known for causing severe diarrhea in cats and reproductive disease in cattle, but it appears to be relatively uncommon in dogs.
How Common Is It
Prevalence varies widely depending on the population tested. A large survey of 294 dogs in China found P. hominis in 22.5% of dogs and T. foetus in 5.8%. Rates differed significantly by environment: dogs in working or service settings had the highest P. hominis rates (31.3%), followed by street dogs (23.8%) and dogs visiting veterinary hospitals (16.0%). Breed and seasonal factors also influenced infection rates, with mixed-breed dogs showing higher T. foetus rates (16.3%) compared to purebred dogs (4.2%).
These numbers come from screening studies that tested dogs regardless of symptoms, meaning many infected dogs may carry the parasite without showing signs of illness. The infection is not restricted to puppies or young dogs, as was once assumed.
Symptoms to Watch For
The hallmark sign is chronic or recurring diarrhea. Unlike a brief stomach bug, trichomonad-related diarrhea tends to linger or come back repeatedly. The stool may contain mucus, and dogs often strain during bowel movements. Some dogs show no symptoms at all and simply carry the organism.
One complicating factor is that dogs with trichomonad infections frequently have other gut infections at the same time. In research examining dogs with confirmed trichomoniasis, most also tested positive for at least one additional pathogen, including Campylobacter, Clostridium toxins, Cryptosporidium, or Giardia. This makes it difficult to pin symptoms on the trichomonads alone and means your vet may need to test for multiple infections if your dog has persistent diarrhea.
How Dogs Get Infected
P. hominis spreads through the fecal-oral route. A dog picks up the parasite by ingesting contaminated feces, contaminated water, or material from a soiled environment. Shared water bowls, communal living spaces like kennels or shelters, and areas where multiple dogs defecate all increase the risk. Dogs that groom each other or eat feces (a common canine habit) are also more exposed.
Trichomonads are fragile organisms. They exist only in an active feeding form, without the tough protective cyst stage that parasites like Giardia use to survive outside a host for long periods. This means they don’t persist in the environment as long, but direct contact with fresh fecal material or recently contaminated surfaces is still enough for transmission.
How It’s Diagnosed
Diagnosis isn’t always straightforward. The simplest method, examining a fresh stool sample under a microscope, has a sensitivity of only about 14%. That means it misses the vast majority of infections. The parasites can also be easily confused with Giardia or other harmless organisms under the microscope, leading to misdiagnosis.
PCR testing on a fecal sample is the most reliable method. It can identify the exact species of trichomonad present and doesn’t require the sample to be perfectly fresh, since DNA remains stable under various storage conditions. Fecal culture is another option and more sensitive than a direct smear, but results can take up to 11 days and the sample must be freshly collected. If your dog has chronic diarrhea that hasn’t responded to standard treatments, asking your vet about PCR-based testing for trichomonads is reasonable.
Treatment Options and Challenges
Treatment typically involves antiparasitic medications in the same drug family used for other protozoal infections. However, treating trichomoniasis in dogs comes with some important caveats. The medication most studied for this purpose can cause neurological side effects in dogs at doses as low as 10 mg per kilogram of body weight per day. These side effects include wobbliness, tremors, agitation, and changes in alertness, and they can appear up to nine days after starting treatment. The good news is that these symptoms resolve once the medication is stopped.
There’s also a dosing dilemma: lower doses may not fully clear the infection, while higher doses increase the risk of side effects. Treatment effectiveness for standard antiparasitic drugs against trichomonads generally falls in the 85 to 95% range, but reinfection from a contaminated environment can make it seem like treatment failed. If your dog is being treated, watch closely for any neurological changes and report them immediately.
Because many dogs with trichomonad infections also carry other gut pathogens, successful treatment often requires addressing all of the infections present, not just the trichomonads.
Can Your Dog Give It to You
P. hominis, the species most commonly found in dogs, is considered a zoonotic organism, meaning it can potentially spread between animals and humans. The most common genetic variant of P. hominis (called CC1) has been identified in dogs, cats, monkeys, foxes, and humans, showing it does not stick to a single host species. Transmission would occur through the fecal-oral route, so contact with infected dog feces followed by inadvertent ingestion is the theoretical risk.
In humans, P. hominis infections have been linked to gastrointestinal symptoms including diarrhea, and some research has associated it with more serious conditions like irritable bowel syndrome. That said, confirmed cases of dog-to-human transmission are rare, and practical risk for most pet owners is low. Basic hygiene, picking up after your dog promptly, washing your hands after handling feces, and keeping living areas clean, reduces the risk substantially.
Reducing the Risk of Infection
Since trichomonads spread through fecal contamination, prevention centers on sanitation. Pick up feces promptly from your yard and on walks. Clean shared water bowls regularly and avoid letting your dog drink from communal water sources at parks or kennels when possible. If you have multiple dogs and one is diagnosed, have the others tested as well, since asymptomatic carriers are common.
Dogs in group housing, whether shelters, boarding facilities, or multi-dog households, face higher exposure risk. In these settings, regular cleaning of surfaces and prompt removal of feces are especially important. Discourage coprophagia (feces eating) if your dog has this habit, as it’s one of the most direct transmission routes.