The Challenges of Diagnosing Giardia in Cats and Dogs

Published: October 29, 2013
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Diagnosing Giardia infections in dogs and cats is not always a straightforward endeavor. Owners commonly associate Giardia with diarrhea, but the list of diseases that can cause pets to develop that symptom is seemingly endless, and not every animal with Giardia in its intestinal tract becomes sick.

A microscopic fecal exam should be the first diagnostic test. It is simple, inexpensive, and can reveal several of the causes of diarrhea in pets, including Giardia … sometimes.

I say “sometimes” because Giardia is notoriously hard to diagnose with a single fecal examination. The parasites are shed intermittently, so pick the wrong pile of poop to sample and you may miss them. The diagnostic accuracy of fecal examinations can be improved by looking at multiple samples taken over the course of several days and by using zinc sulfate fecal flotation solution and a centrifuge, but even then the incidence of false negatives can be quite high. In my opinion, a fecal examination can only tell you two things when it comes to Giardia:

1. The pet has Giardia, or

2. The pet may have Giardia

A negative test result does not mean a pet does not have Giardia. Capiche?

If the fecal examination is negative, but I’m still suspicious that Giardia is the cause of a pet’s diarrhea, I will run a Fecal ELISA (enzyme-linked immunosorbent assay). These are now available as bench-top snap tests (or samples can be sent out to a laboratory) and have a much lower incidence of false negative results in comparison to microscopic fecal exams.

I only run Giardia ELISAs on pets with a negative fecal exam who have symptoms consistent with the disease, however. The reason is simple. As I said previously, Giardia microorganisms do not cause disease in every individual. Overusing the test risks diagnosing pets with giardiasis (what vets call the disease caused by Giardia) when they are either sick from another cause or aren’t actually sick at all.

The last step in this diagnostic conundrum is taking into account the pet’s history. Symptoms associated with Giardia infections are much more common when an animal is in a group housing situation, stressed, young, or otherwise immunocompromised. Therefore, I’m much more likely to “believe” a positive Giardia test for a puppy that has just been bought from a pet store than for an indoor-only, adult cat who has lived in the same house with no other animals for the last five years.

I also tailor treatment for giardiasis based on a pet’s history, clinical signs, and test results. When I’m as certain as I can be that Giardia and Giardia alone is causing a pet’s diarrhea, I prescribe fenbendazole. This medication only has to be given for three to five days and is extremely safe.

When I still have doubts as to a diagnosis of giardiasis, I’ll often hedge my bets and prescribe metronidazole for five to ten days. Metronidazole will kill Giardia as well as some of the bacterial causes of diarrhea in dogs and cats. It also has anti-inflammatory properties and will therefore improve some cases regardless of the underlying cause.

As with many things in veterinary medicine, diagnosing and treating suspected or known cases of giardiasis in dogs and cats is as much of an art as it is science.

Dr. Jennifer Coates

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