PCR Screening for Intestinal Parasites

One of the strange little side effects of the COVID-19 pandemic is that almost everyone now knows the term “PCR test.” For a while, PCR testing was something people talked about constantly, usually while trying to figure out if they could still go to work, school, or Thanksgiving dinner. The good news is that all of that attention helped make PCR machines much more common and much less expensive. Veterinary laboratories are now able to use this technology in ways that used to be too costly for routine testing.

One of the most useful places we are seeing this is in screening stool samples for intestinal parasites. What used to be a more specialized DNA test is becoming a much more practical way to look for the little freeloaders living in or passing through our pets’ intestinal tracts.

PCR stands for polymerase chain reaction, which sounds like something you either learned in high school biology and forgot immediately or something that requires a lab coat and dramatic music. The simple version is that PCR testing looks for the DNA of an organism. For intestinal parasite screening, a small stool sample is run through a machine that searches for genetic material from a panel of parasites common to our area. Many of these panels check for more than twenty different parasites at the same time.

This is different from a traditional fecal test, where the lab is usually looking under a microscope for parasite eggs, cysts, or the parasites themselves. Traditional stool testing is still very useful, but parasites can be sneaky. They do not always shed eggs every day, and their reproductive cycles do not always line up politely with the day your dog leaves us a sample. Because PCR testing can detect very small amounts of DNA, we are now catching infections that would have been missed in the past. This is especially helpful for pets with intermittent soft stool, diarrhea, weight loss, or other vague digestive issues.

One of the more interesting things we are learning is that a positive PCR result does not always mean the parasite is actually making the dog sick. Sometimes we find DNA from parasites that normally live in rodents or other wildlife. These parasites are simply passing through the pet after it ate rodent feces or some other exciting snack that no one would ever authorize. In a large dog, that may mean there is a secret buffet somewhere in the yard. In a small dog or cat, it may even mean eating mouse feces inside the house long before the humans realize they have tiny tenants.

In those cases, the pet may not need treatment for that parasite at all. However, the result still tells us something very important: the pet is being exposed to things the owner did not know about. This can be a big deal. Many pets with off-and-on diarrhea have historically been suspected of having food allergies, inflammatory bowel disease, or other chronic digestive problems, especially when everyone is confident that the dog “never gets into anything.” A PCR result can sometimes reveal that the dog does, in fact, have access to something unexpected.

Instead of jumping straight to expensive diets, repeated medications, ultrasound, endoscopy, or other advanced testing, we may be able to start with behavioral and environmental changes. More supervision outside, cleaning up areas where rodents may be active, checking for mice in the house, leash-walking, or training dogs to stop eating mystery items can make a real difference.

PCR screening is not just a fancier parasite test. It can also be a very useful clue about what our pets are actually doing when they think no one is watching.