There's a common myth that a dog's mouth is cleaner than a human's.
That might make us feel better about being licked or slobbered on, but the truth is that a dog's mouth holds as many microbes as our own – we even share some.
A new study has found that a parasite typically found in humans is also present in domestic dogs and, to a lesser extent, in cats.
The results were published in Veterinary Research Communications, led by scientists at Wrocław University of Environmental and Life Sciences in Poland.
The parasite Trichomonas tenax is a mouth scavenger that feeds on microbes tucked between the teeth and around the gums.
Most people don't even know their mouth is home to this parasite, and while it doesn't always cause health problems, it has been linked to some periodontal infections.
T. tenax can be spread through saliva, and some studies have found it in dogs, cats, and horses, too.
The new research from Poland can't establish whether our pets are getting these oral parasites directly from us, but that is a possibility.
Scientists collected oral swabs from 238 household animals in Poland, including 151 dogs and 87 cats.
When they cultured the samples, they found trichomonads in 17 percent of the dog swabs and nearly 6 percent of the cat swabs.
T. tenax was one of the only parasite species detected in both dogs and cats. It was the second most frequently identified trichomonad.
Among dogs, pets over 9 years of age and those with severe tartar deposits on their teeth were more likely to host oral trichomonads.
Housing conditions may also play a role, though the difference wasn't statistically significant.
For instance, all the cats with oral trichomonads were indoor animals, with limited contact with the outside world but closer contact with humans.
"Therefore," the authors suggest, "the primary source of infection seems to be humans, for whom T. tenax is considered a typical species."
That may be why dogs have higher rates.
Compared to cats, researchers in Poland explain, our dogs "generally have greater opportunities for contact with other individuals and the external environment, which is associated with a higher risk of infection."
"Additionally," they add, "cats are typically more reserved in interactions with conspecifics and often have limited exposure to the external environment… "
For now, this is only a hypothesis, but the sharing of oral germs could have health implications for both us and our pets.
Oral trichomonads, for example, can produce proteins and enzymes that damage collagen fibers and promote soft tissue loss.
Sometimes, they are even found outside the mouth, in the tonsils or lymph nodes.
"Despite clear evidence of their role as pathogens, trichomonads remain poorly understood in dogs and cats," write researchers in Poland, led by Agnieszka Piekara-Stępińska.
"These domestic animals are particularly important due to their close contact with humans, which increases the risk of transmission of pathogens with a zoonotic potential."
Zoonotic potential means the ability for a pathogen to jump from one species to another.
Toxoplasma gondii is perhaps the most famous of parasites that can jump between our pets and us.
Cats are its typical host, but it can also end up in our own brains and stay there for decades.
Most cases don't cause health issues, but in children or those with suppressed immune systems, severe flu-like symptoms can occur, as well as blurred vision and brain inflammation.
The parasite's presence in humans is associated with schizophrenia.
To find out whether T. tenax is also associated with health issues, however, will require more research.
The trouble is, scientists have measured widely different rates of trichomonad infections in our pets, depending on where in the world they look, and what subset of pets they consider.
A previous study in Poland, for instance, found a lower rate of infection among dogs, and in the neighboring Czechia, researchers found oral trichomonad prevalence reaches nearly 38 percent in dogs and 10 percent in cats.
That latter study, however, also included shelter pets, which makes the two sets of figures harder to compare directly.
More rigorous studies are needed to pinpoint which of our pets are most affected by human-associated oral parasites, and how these microbes may be related to health.
The research has been published in Veterinary Research Communications.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.