Poop. We all do it. Pets do it too. It’s normal, but also gross. When you carefully scoop up the latest handiwork of your pet’s large intestine into an old newspaper bag, you can’t help but immediately scan for a neighbor’s trashcan to toss it into. And since we’re all so averse to turds, why is your veterinarian sending you a postcard begging you for fresh feces? Why are we greedily requesting that mushy bag like it’s full of brown gold?

We’re not trying to embarrass you by making you drive to the clinic with a steamy bag of poop on your dashboard. We’re looking for parasites!

But, you’ll contend, I never see worms in her poop! Ah yes, we know that. We do more than just look at it. Not to denigrate your own diagnostics capabilities, but we run analysis on the samples checking for microscopic evidence of parasites.

But the poop looks normal! I also understand the viewpoint that, if there are no gastrointestinal symptoms, it may not seem necessary to test for little creepy crawlies. Instead, why not put the money towards something your dog will actually appreciate?

Why are fecal screens done?

The poop that you awkwardly place on our front desks annually is part of our efforts to keep your pet healthy. It’s screened for parasites that are common causes of both animal and human disease. The reason you don’t routinely do this at your own wellness visits is that gastrointestinal parasitism is not a common human health problem in more developed countries. Animals, however, often with a slightly lower aversion to consuming their own feces, are still frequently infested by these internal freeloaders.

There are all sorts of strange life forms that can live inside of your dog or cat. Watch this informative music video for an overview. Although often shed asymptomatically or causing mild signs, some can cause severe disease including death. Roundworms, one of the more common dog parasites, can kill puppies if not treated. Killing puppies, I think we can all agree, is bad, but roundworms are actually even worse than that. If a human gets exposed to the infectious larvae, they can invade and damage tissue including the eye (warning, this photo will definitely gross you out).

Being capable of detecting gruesome disease is only one aspect of why fecal screens are done. There needs to be some level of the disease within the population to make screening tests useful. You can make arguments about where the cutoff level is based on how severe the disease is, how susceptible the population is, and what the risk is to environmental and human health. But in the case of intestinal parasites in pets, the prevalence of disease alone may be enough to justify widespread fecal testing.

There are a number of studies that have estimated parasite prevalence in pets. The populations investigated differ somewhat, but the most recent one involved over 3,000 dogs across the US and found that 20% of the dogs had at least one parasite. And since there are close to 80 million owned dogs in the US, that roughly equates to a crapload of parasites. The prevalence varies by parasite, geographical region, age and underlying health of the dog, and whether or not a heartworm preventative was being used (which also treats many intestinal worms), among other factors. For cats, the numbers also vary wildly, but the most recently published article showed that about 25% of feline fecal samples were positive for parasites.

The point is that intestinal parasites are still common in pets. One of the most common parasitic infections in pets in North America is Giardia, with the most recent study showing a prevalence of about 13% in dogs. In humans in the US, for comparison, the number of reported cases is less than 0.001%. Dogs are waaaaaay more likely to actually be hosting and transmitting these organisms than people. Even though people in the US don’t commonly carry parasites, exposure to animal parasites can be quite high. As much as 5% of the population have antibodies to a common dog and cat parasite, Toxocara (the one in the scary photo from before).


You can’t usually see most of the stages of parasites in feces, so it’s probably not worth your while to comb through each defecatory production in search of worms (believe me, some people try). Some animals are also asymptomatic carriers, so even if your pet has “normal” poops, it doesn’t mean they’re “clean”.

As any ferret-legger will tell you, lifestyle choice can be a risk factor. For dogs, going to the park and running off leash increases the chances of contracting a parasite. But unless you are raising hermetically-sealed Labradors, there are enough parasites and enough modes of transmission that it’s hard to completely eliminate any exposure. Even the use of monthly heartworm preventatives doesn’t guarantee that your dog is parasite-free, since organisms like Giardia aren’t treated by those medications.

There is the issue, as in all screening tests, of the risk of overdiagnosis leading to unnecessary medical care. But in general, the treatments for gastrointestinal parasites are pretty benign, and the tests accurate enough that this problem is relatively minor. And especially given the high prevalence of these diseases, the risk-benefit ratio seems in favor toward screening.

Conclusion: It doesn’t hurt to check, what else are you going to do with that poop?

Truthfully, no one has ever rigorously evaluated the question of whether or not doing an annual fecal test makes your pet’s individual health any better. There is no mandate that says you must deliver warm dog poop to your vet’s office every year, but parasites in dogs are still common enough in developed countries that it’s not a bad idea. If you have a young dog, visit the dog park, don’t use a heartworm preventative medication, have a known coprophage, or listen to Nickelback, doubly so. You are never going to have zero risk unless your dog is a Tamagotchi. If you want an idea of what parasites might be lurking in that dog turd you just stepped in, check out this industry-funded organization’s interactive map. And pick up that poop, people!



  • Greg Bishop is a veterinarian in Oregon who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.

Posted by Greg Bishop

Greg Bishop is a veterinarian in Oregon who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.