The Perils of Parasites in Veterinary Practice: The Risk of False Negatives With Fecal Testing

Recent innovations using fecal antigens to detect intestinal parasites have significantly improved veterinarians' ability to diagnose some of the most common pet infections. This is welcome news, since the fecal flotation test has mostly stayed the same since 1918, when "Rapid Method for Detection of Ova of Intestinal Parasites in Human Stools" was published in the Journal of the American Medical Association. Since then, some improvements to the test have been made, but false negatives continue to affect veterinarians' ability to provide quality care and an effective course of treatment.

It's important to understand the many reasons why false negatives occur, especially if you are still performing this labor-intensive test within your practice. Here are some of the causes to guard against and reasons why you may want to consider having a reference lab perform this test going forward.

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Not Centrifuging the Sample

To improve the accuracy of results, it's recommended to centrifuge stool samples prior to the flotation, as it pulls large debris to the bottom and better allows parasite ova to float to the top. However, based on survey data collected by IDEXX, approximately 70% of practices who perform their own fecal floats are not centrifuging the stool sample first, increasing the risk for false negatives and denying that patient necessary treatment.

In the article "Comparison of Common Fecal Flotation Techniques for the Recovery of Parasite Eggs and Oocysts," Dr. Michael Dryden reported that including a five-minute centrifugation step prior to the fecal float significantly increased the number of parasite eggs recovered. For example, almost 300 roundworm eggs were recovered following centrifugation, compared with only 17 to 22 eggs with passive flotation after five to 20 minutes standing time. Similar discrepancies were also seen with hookworm and whipworm egg recovery. If you're performing your own fecal floats but not centrifuging stool samples, just think what you could be missing. Sending the sample out to your reference lab removes the task of centrifugation from your team and ensures that it occurs every time.

Little or No Training

Additionally, the expertise of the individual performing the microscopic exam is critical to accurate results. Parasite ova can be easily missed if the coverslip is not examined thoroughly, and false positive results can occur because other substances—pollen, algae, insect ova, and plant roots—can look incredibly similar to parasites and their ova. Consider offering professional development opportunities to keep your veterinary team well informed and up to date on current practices. Or better yet, having a reference lab perform this time-consuming test means a well-trained technician will be the one to evaluate the sample, every time.

Ova Gestation and Shedding

False negatives could occur if the testing time period isn't ideal. For example, during the prepatent period, an animal may show clinical signs from its parasite infection, yet no ova will be recovered because the parasite is not yet shedding eggs. Furthermore, single-sex parasite infections may manifest in clinical signs, but again, no ova will be found on fecal flotation. Intermittent egg shedding by the parasite and variability in egg density are additional reasons for these incorrect negative results. Even if eggs are there, they may be more dense than the flotation solution and will not float to the top to be detected. These are perhaps the strongest arguments yet in favor of combining innovative fecal antigen testing with centrifugal fecal flotation performed at a reference lab.

The Serious Risks of False Negative Fecal Flotation Results

A false negative can cause many significant impacts on everyone involved—pets, owners, team members, and the practice itself. Here are some of the major ones to consider.

Pets Remain Infected

After a false negative, an untreated infected animal will continue to shed and contaminate their surroundings, like the backyard or inside the house, which prolongs its infection. If an infected animal is asymptomatic, it could easily progress to a symptomatic infection, like a puppy with hookworms eventually becoming anemic. A symptomatic animal will continue to experience clinical signs, which could be significant, perhaps even fatal. Accurate fecal test results, especially when combined with fecal antigen testing, help to avoid these serious consequences.

Zoonotic Risks Increase

Many of the most common parasite infections, such as roundworms and hookworms, may have serious zoonotic risks. Certain clinical conditions, such as visceral, ocular, and neural larva migrans as well as severe abdominal pain may result from the ingestion of Toxocara eggs. Cutaneous larva migrans can develop in people after direct contact with skin from hookworm larvae in contaminated soil. These risks are exacerbated by false negatives, allowing an untreated infected animal to further contaminate the environment shared with its owner. Further, a false negative could miss other important zoonotic parasites, such as Toxoplasma gondii, Strongyloides stercoralis, Cryptosporidium spp., Echinococcus spp., and Baylisascaris procyonis on fecal floats performed without centrifugation.

Practices May Face Litigation

Finally, given the current litigious environment, financial consequences are a risk, as well. Malpractice suits and claims of negligence for not following the professional standard of care are always a possibility.

New Advances Mean Greater Accuracy

However small or even microscopic parasites can be, their impact on health can be massive—and even fatal—if not diagnosed and treated promptly. Following Companion Animal Parasite Council recommendations to combine centrifugal fecal flotation with fecal antigen tests "ensures the widest breadth of detection of intestinal parasites" and would be the best strategy to avoid false negatives. Improved accuracy means earlier and more effective treatment. Utilizing the most accurate testing, incorporating proper procedures and training, and learning to read Companion Animal Parasite Council maps can make a huge difference, fostering health for the patient, client, and practice.

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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