Fecal Antigen Testing: Diagnosing Pet Infections Earlier

Advances in science and technology like fecal antigen testing have strengthened parasite diagnostics, leading to significant improvements in the scope and accuracy of fecal testing results.

This is welcome news, given that the fecal flotation test is more than 100 years old and hasn't changed much during that time, often producing false negatives. Veterinary practices may also utilize direct smear or wet mount evaluation of stool, but these methods are typically even less accurate than fecal floats.

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What Is Fecal Antigen Testing?

Testing antigens offers a newer, more accurate means to detect parasite proteins, according to the Companion Animal Parasite Council (CAPC). The primary job of antibodies, made by the immune system, is to attack antigens. This test method uses antibodies to detect specific proteins that are unique to certain organisms. The sought-after protein is secreted or excreted by a parasite within the intestinal lumen of the animal infected with it, independent of egg production by that parasite. The fact that fecal antigen testing does not require the identification of eggs is a critical feature of its success as an accurate screening test for infected dogs or cats that are not symptomatic or may be showing a variety of clinical signs that could be associated with parasite infection.

Here are a few reasons why antigent testing is superior to other testing methods.

Identifies a Broad Range of Parasites

Veterinarians use fecal antigen tests to detect parasite proteins. There's a misconception that this method of testing is only used to detect Giardia; the list of parasites this test can detect is much broader in scope.

Unique proteins within a pet's fecal sample will identify roundworm, hookworm, and whipworm infections. Excluding protozoa and insects, those three parasites alone accounted for approximately 90% of positive parasite test results, based on data collected in 2016 from IDEXX Laboratories.

Even more welcome news is that soon this list of identifiable parasites will include flea tapeworms. Antigen tests provide the bandwidth necessary to identify a variety of parasites, not just Giardia.

Diagnoses More Infections Earlier

Unlike fecal flotation, which relies primarily on identifying eggs, fecal antigen tests can detect parasite infections during the prepatent period. This advancement means the eggs don't have to be released in order to detect the infection.

Some of these periods can be quite long. For example, the prepatent period for hookworms is between 14 to 21 days, and for roundworms, it lasts roughly 21 to 35 days. For whipworms, it's significantly longer at 74 to 90 days, according to CAPC. A more sensitive test than fecal flotation is critically important, especially considering that a symptomatic whipworm infection can mimic Addison's disease. It could even resemble similar clinical pathological abnormalities, such as hyperkalemia, hyponatremia, eosinophilia, and anemia. When parasites are detected earlier, veterinarians can provide immediate courses of treatment to minimize the infection. Stressing the importance of establishing a diagnostic baseline, including fecal testing, helps improve the chance of finding and treating infection earlier.

Avoids False Negatives

Single-sex parasite infections can occur in pets and cause clinical signs. However, when only one sex of the parasite is present within an animal, eggs will not be produced, and a fecal flotation would result in a false negative. Fecal antigen testing can detect these infections because key proteins are released by both male and female parasites, regardless of whether or not the opposite sex is present.

The test's success is due to the fact that it does not need to identify eggs, only antigens, which are secreted or excreted by a parasite within the intestinal lumen of the infected animal. It is a much more accurate test than a fecal float to screen asymptomatic infected pets or pets presenting a variety of clinical symptoms. Furthermore, parasites may undergo intermittent egg shedding, which is yet another scenario that could lead to false negative fecal floats—seeing as if no eggs are being shed, the infection cannot be detected.

Finally, fecal flotation tests can often miss dense eggs that are too heavy to float: typically whipworms and proglottid segments of flea tapeworms.

Communicating the importance of testing is critical to effective preventive care. Appropriate courses of treatment can limit clinical signs as well as the zoonotic risk associated with each parasite. Using fecal antigen testing to identify a spectrum of parasite infections earlier and with greater accuracy means pet owners will receive the most comprehensive evaluation of their pets and pets can get better care.

Jennifer Steele

Jennifer Steele is a board-certified small animal internal medicine specialist living in Madison, Wisconsin.  After a short time as a registered nurse in human medicine, she returned to school and received her DVM degree from the Western College of Veterinary Medicine in Canada, then completed an internship at the University of Minnesota and residency training at the University of Wisconsin.  Over her 20 years as a small animal internist, she's worked at the University of Minnesota College of Veterinary Medicine as an Assistant Clinical Professor, an Internal Medicine Consultant and Field Medical Specialist at IDEXX, and most recently, she was Associate Veterinarian in Small Animal Internal Medicine at a busy emergency and specialty referral practice in Wisconsin.  In her spare time, she enjoys spending time with her cat, gardening, and training for triathlons. Dr Steele has already completed an Ironman triathlon and fully intends to complete Ironman Wisconsin in September, 2023, while raising awareness and money for the organization Not One More Vet. 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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