Elevating Patient Care with New Fecal Antigen Testing for Cystoisospora

For decades, one of the core screening tests for canines and felines has been the ova and parasite (O&P) test. While historically veterinarians have used this to identify common fecal parasites like Cystoisospora, guide treatment, create prevention strategies, and protect human family members from zoonotic disease, this test has a high rate of false negatives for various reasons, including human error.

More recently, science and technological advancements led to the creation of fecal antigen testing, a welcome addition to our small animal diagnostic repertoire for common parasites, including hookworms, roundworms, whipworms, and flea tapeworm. And soon, due to diagnostic advancements expected this year, we will be able to more accurately test for the common parasite, Cystoisospora in dogs and cats with fecal antigen testing.

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What Is Fecal Antigen Testing?

Fecal antigen testing is a newer, more accurate means to detect parasite proteins. This is a critical distinction from traditional fecal O&P, which requires team members to collect a sample of feces and mix it with a denser special solution which allows the eggs shed in the host feces to rise to the surface and be identified under a microscope. However, many infected dogs and cats can be asymptomatic, shed intermittently, have same-sex infections limiting egg shedding, have dense eggs that don't float, or have nonspecific clinical signs without a positive fecal O&P. Instead, the fecal antigen test looks for a protein secreted or excreted by the parasite into the host intestinal lumen. These unique proteins are then found by specific antibodies during testing, improving detection rates and accuracy. Fecal antigen testing currently tests for hookworm, roundworm, whipworm, and flea tapeworm.

The Importance of Adding Fecal Antigen Testing to Your Workflow

The Companion Animal Parasite Council (CAPC) recommends testing for fecal parasites at least four times in the first year of life for puppies and kittens and at least twice yearly in adults, depending on patient health and lifestyle factors. Fecal parasites, including Cystoisospora in dogs and cats, are more common in puppies and kittens. Suppose your practice relies solely on ova and parasite testing through fecal O&P. In that case, it's essential to be aware of the limitations of this testing and the many missed opportunities for early detection of parasitism, many of which could be zoonotic.

Fecal Antigen Testing for Cystoisospora

One opportunity ahead is the ability to use fecal antigen testing for Cystoisospora. Cystoisospora, formerly known as Isospora (sp) and commonly known as coccidia, is a protozoan parasite reproducing in the host's small intestine, producing unsporulated oocysts in the host's feces. Dogs and cats can acquire this parasite by ingesting the immature form (oocyst) from a contaminated environment like soil or feces.

We mostly think of puppies and kittens as presenting with this disease because they are commonly exposed to a contaminated environment with an immature immune system. But adult dogs and cats that experience the stress of a new move or the addition of a new family member, are immune suppressed, or deal with other chronic diseases can also be susceptible. These patients present with vague gastrointestinal signs such as diarrhea that's classically watery, bloody, and/or with mucus, weight loss, dehydration, and lethargy. Severe cases in puppies and kittens can also cause anorexia, vomiting, and even death.

Based on the presentation and clinical signs, veterinarians usually start their database with a fecal O&P, but soon we will be able to detect this disease more accurately through fecal antigen testing. Up until now, diagnosis of Cystoisospora in dogs and cats has been made by looking for large numbers of species-specific unsporulated oocysts.

Some dogs and cats with Cystoisospora can be asymptomatic, however, these infected pets can still shed in their feces and infect other puppies, kittens, adult dogs, or cats, especially knowing that the oocysts can remain infective in the environment for several months. This is why testing is critical not just for illness visits, but also for well exams. Dogs and cats of all ages should have fecal testing included in wellness screenings.


While Cystoisospora is not considered a zoonotic parasite, it is still a source of significant disease and communal transmission, making early and accurate detection significant. One of the factors that confound accurate diagnosis of parasitism, especially Cystoispora, is the presence and misdiagnosis of pseudoparasites. These are oocysts that dogs and cats eat, yet they pass through without being pathogenic. Eimeria spp. is a common "parasite" in the fecal O&P of healthy adult dogs, yet dogs don't host these parasites. With fecal O&P tests, these pseudoparasites can raise alarm, but with fecal antigen testing, it's simpler to rule out false positives. Antigen testing does not cross-react with other species of coccidia and therefore allows differentiation from Eimeria, a pseudoparasite often found in the feces of dogs. This avoids unnecessary treatment of the patient through misidentification.

While centrifugation using fecal O&P may still be the go-to test for some veterinarians, limitations suggest that fecal antigen testing should be combined with microscopic examination of feces for eggs to detect up to five times more intestinal parasite infections than just fecal O&P alone.

A key facet of veterinary medicine has always been preventive care—and continuing to educate clients on exams and diagnostic screening tests will help raise awareness of their importance. Adding fecal antigen testing for patients can help identify a large number of parasite infections earlier and with greater accuracy to best protect patients and their families.

Natalie L. Marks

Dr. Marks is a veterinarian, previous veterinary hospital owner, consultant, media expert, national and international educator, and angel investor with over 20 years experience. She is a passionate communicator within multiple media formats, such as industry magazines and national conferences. She has won many industry awards, including the Dr. Erwin Small First Decade Award, given to the veterinarian who has contributed the most to organized veterinary medicine in his or her first decade of practice. Other notable awards that she has received are Petplan’s nationally recognized Veterinarian of the Year (2012), America’s Favorite Veterinarian by the American Veterinary Medical Foundation (2015), and Nobivac’s Veterinarian of the Year for her work on canine influenza (2017). The views and opinions in this piece are the author's own and do not necessarily reflect the views of either The Vetiverse or IDEXX.

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