Trichomonads: 1 in 6 dogs may carry this parasite usually found in human mouths

A new study highlights the relatively frequent presence of oral trichomonads in dogs and, to a lesser extent, cats. The identification of Trichomonas tenax, a protozoan traditionally associated with humans, also raises questions about possible transmission between companion animals and their owners.

Researchers examined oral samples from 238 companion animals, including 151 dogs and 87 cats. Samples were collected from the gingival and dental surfaces between June 2024 and December 2025. Positive cultures were subsequently analysed by PCR and sequencing to determine the trichomonad species involved.

Trichomonads detected in 17.2% of dogs

Oral trichomonads were detected in:

• 26 of 151 dogs, or 17.2%

• 5 of 87 cats, or 5.7%

The difference between dogs and cats was statistically significant. These figures suggest that oral colonisation by trichomonads may be relatively common in dogs and potentially underestimated in routine veterinary practice.

T. brixi and T. tenax identified

Two species were identified: Trichomonas brixi and Trichomonas tenax. Among the sequenced canine samples, approximately 65.2% were identified as T. brixi and 34.8% as T. tenax.

In cats, the distribution was different, with T. tenax accounting for 80% of the positive samples and T. brixi for the remaining 20%.

The study also reports the detection of T. brixi in Polish dogs and cats. This relatively recently described species appears so far to be mainly associated with carnivores and has not been demonstrated in humans.

A protozoan shared with humans

T. tenax is best known as an inhabitant of the human oral cavity and has frequently been associated with periodontal disease. Its detection in dogs and cats living in close contact with humans raises the possibility of interspecies circulation.

The authors therefore highlight its potential zoonotic relevance, although the present study does not demonstrate direct transmission between animals and humans. No samples were obtained from the owners, making it impossible to establish whether identical strains were circulating within households or to determine the direction of any possible transmission.

One observation nevertheless deserves attention: all cats positive for trichomonads in the study were indoor animals. The authors suggest that close and prolonged contact with humans could potentially contribute to exposure to T. tenax, although this hypothesis remains to be confirmed.

Older dogs and dental calculus at higher risk

In dogs, two factors were significantly associated with trichomonad detection: advanced age and severe dental calculus. Dogs aged nine years or older were more frequently positive, as were animals with substantial calculus accumulation. This association is biologically plausible. Periodontal disease alters the oral environment, producing changes in pH, microbial composition and oxygen availability that may favour anaerobic protozoa.

The accumulation of organic material around periodontal pockets could also provide favourable conditions for trichomonad persistence.

No statistically significant association between infection and age, lifestyle or dental status was demonstrated in cats, although the relatively small number of positive animals limits interpretation.

Are oral trichomonads pathogenic?

The presence of trichomonads does not necessarily mean that they are the primary cause of periodontal disease. Their pathogenic role remains debated.

However, several studies have associated oral trichomonads with periodontal lesions. Some species can produce proteases and haemolysins capable of damaging collagen fibres and soft tissues, suggesting that they could participate in inflammatory processes or aggravate pre-existing periodontal disease.

T. tenax has also occasionally been detected outside the oral cavity. In humans, it has been reported in the salivary glands, tonsils, lymph nodes and, more rarely, in lower respiratory tract infections and pulmonary abscesses, particularly in patients with underlying disease or impaired immunity.

These observations support its classification as an opportunistic organism rather than a consistently pathogenic parasite.

Large differences between countries

Published prevalence estimates vary considerably between studies. Previous investigations have reported trichomonads in approximately 4.9% of Polish dogs, while studies in the Czech Republic found prevalence levels reaching 37.8% in dogs and 9.8% in cats.

Research conducted in Japan reported infection in approximately 14.6% of dogs, whereas prevalence in one study from eastern China was around 2.8%.

Such differences probably reflect variation in study populations, animal age, dental health, husbandry conditions and diagnostic methods. Direct comparisons between countries should therefore be made cautiously.

The Polish study does not establish that dogs or cats represent an important source of human T. tenax infection. It does, however, demonstrate that a protozoan commonly associated with humans is also circulating among companion animals living in close contact with people.

Molecular comparison of T. tenax isolates collected from people and their pets living in the same household could determine whether transmission really occurs between species and, if so, in which direction.