I get variations of this question not uncommonly from veterinarians:

A client just came in and said they/their kid was diagnosed with Dientamoeba fragilis infection and their physician said to test/treat their dog. What should I do?

On one hand, it’s nice to see physicians thinking about animal exposures. On the other hand, it sometimes ends up with the physician making a testing or treatment suggestion (or directive) for an animal that doesn’t make sense (or at least needs to be considered with a lot more nuance).

Here’s the basic need-to-know about D. fragilis:

• It’s a microscopic, protozoal parasite that lives in the large intestine. It’s shed in feces, and people get infected through fecal-oral exposure (aka eating poop, albeit in minute quantities).
• It’s found in humans worldwide, and people are its normal host. Compared to many other intestinal parasites, it’s actually fairly common in people even in high-income countries. For example, a study of children in Toronto daycares found that 8.6% were shedding the parasite, along with 4.0% of the daycare workers (Keystone et al. 1984).
• Many, if not most, people carrying this parasite in their intestine are healthy. It’s role in causing disease (if any) isn’t clear. If it does cause any illness, it’s pretty non-specific things like diarrhea, abdominal pain, nausea, fatigue and loss of appetite.
• It’s been found in a few animal species, including non-human primates (e.g. gorillas), budgies and pigs. However, it seems rare and data are quite limited.

One study (Chan et al. 2016) reported finding D. fragilis using PCR in a single dog and a single cat . Whether the dog and cat were truly infected or the test just cross-reacted with a different protozoan isn’t clear. If it was true infection, the low prevalence in dogs/cats compared to humans and the lack of evidence that infected dogs/cats have much of a parasite burden make it pretty unlikely that dogs/cats are a significant source of human infection. The aforementioned daycare study from Toronto (Keystone et al. 1984) reported an association between the parasite and cat ownership, but it’s far from certain that there was any causal relationship; the analysis was pretty basic and the relationship between cat ownership and other identified risk factors wasn’t explored. (The authors indicated that the association could have even occurred by chance.)

I would consider D. fragilis a parasite that’s not normally found in dogs and cats, and is not well adapted to them if they are exposed. If a parasite infects people and is shed in feces, there’s a reasonable chance dogs and cats would be exposed (one way or another), but not necessarily that they’d get infected. Even if infection did happen, it could very well be at such a low level that transmission wouldn’t occur (i.e. a dead-end infection).

At most, dogs and cats are rarely infected with D. fragilis, and there’s no evidence that they’re infectious to people. I never say never with diseases that we haven’t studied much but, it clearly spreads quite commonly human-to-human, so pets are not likely to play a relevant role in the transmission cycle. If anything, the risk is human-to-pet transmission, not the other way around. 

Can we test pets for D. fragilis, if requested?

Routine fecal flotation would detect this parasite if there was a reasonable burden. PCR could also be used, but there are no commercially available tests for animals (although a human PCR test would presumably work – but few human labs will accept diagnostic samples from animals).

Should we test pets for D. fragilis, if requested?

The key question for me with any test is, “what would I do with a positive vs a negative result?”

If a pet tests positive for D. fragilis, I’d say “Interesting. Your dog presumably got this from you or another person with whom you have contact. Odds are really low that the dog will infect anyone. It’s a don’t-eat-poop disease, so let’s just focus on basic hygiene and safe fecal handling, and it will go away on its own.“

If a pet tests negative for D. fragilis, I’d say “It’s probably negative, but testing isn’t 100% sensitive. Regardless, we don’t want you to give it to your pet and we don’t want your pet to give you any of the many bacteria and parasites that might be in its intestine, so let’s just focus on basic hygiene and safe fecal handling.”

Not a lot of difference either way.

Should we just treat pets for D. fragilis is someone in the household has it? 

If I had to treat a D. fragilis infection in a dog – where we found the parasite AND had reason to believe it was causing disease (both pretty unlikely) – I’d most likely use metronidazole. If the parasite isn’t causing disease I’d want to avoid using an antibiotic like that, because it’s not an innocuous drug and it will negatively impact the dog’s normal intestinal microbiota, potentially for weeks.

Time is the cheapest, safest and ultimately most effective treatment in a healthy dog that happens to be infected with this parasite. More than that though, if people in the house are infected, they’re better off focusing on their hygiene (i.e. not eating poop) and eliminating their own infection, rather than worrying about the pets.