Q&A on internal parasite diagnostics: Tapeworms, heartworm, and the limits of in-clinic testing

Susan E. Little, DVM, PhD, DACVM (Parasitology), spent 30 years teaching veterinary parasitology at the University of Georgia and Oklahoma State University before retiring from academia to focus on parasitology consulting and veterinary communication. In this Q&A, she breaks down the strengths and limitations of common in-clinic diagnostic assays, the particular challenge of tapeworm detection, and why the most important diagnostic tool in any practice is still the veterinarian.

Editor's note: This dvm360 Q&A has been edited and consolidated from a verbal interview to better fit a written format while retaining the substance of the original conversation.

dvm360: When it comes to diagnosing internal parasites in dogs, where do you see the biggest gaps in clinical practice?

Little: We know parasitic infections when we see them, but we don't always have the diagnostic tools we would like in order to find those infections. There can be real challenges in diagnosing, particularly tapeworms, heartworm, and whipworm. Hookworms and roundworms absolutely matter, as do Giardia and Coccidia, but the diagnostic tests are better at identifying those infections. We have good diagnostic tests for all parasites, but they just all have limitations.

dvm360: What are the most important takeaways for veterinarians when it comes to parasite diagnostics?

Little: One key takeaway is that we don't have great in-clinic diagnostic tests for tapeworms. We just don't have good ways to recognize tapeworm infections when they occur. In the absence of a referral lab that we can send a sample to, we're really relying on the owner to tell us that they saw proglottids, and that's not an ideal way to diagnose anything in veterinary medicine, and certainly not tapeworm infections. There are newer assays available for some tapeworms, but the limitations in recognizing those infections when they occur remain significant.

A second takeaway is heartworm infection and how we recognize it. If the antigen test is positive or microfilariae are present, then we're confident in the diagnosis. But sometimes you get an infection without microfilaremia, or an infection with a false-negative antigen test. How do we recognize that and address it in a way that will best help the patient? That's something every practitioner needs to be thinking through.

dvm360: Can you walk through the key strengths and limitations of fecal flotation for parasite detection?

Little: Tapeworms are probably the most challenging internal parasite to diagnose with fecal flotation alone. Fecal flotation—direct microscopic examination of a prepared sample—works for hookworms and roundworms, is a little more challenging for whipworms, and for tapeworms, it's all but useless. Sensitivity values for Taenia tapeworms may be less than 50%, and for Dipylidium [caninum]—the flea tapeworm—the odds of actually detecting an infection by fecal flotation alone are less than 5%. They're just not helpful.

That's one of the reasons antigen tests have been developed for flea tapeworm. We've got better strategies now to find those infections, and we're certainly finding them. We're seeing that we were overlooking them before, which means the dog comes into the veterinary practice with a tapeworm infection and goes back home with a tapeworm infection if the right diagnostic test isn't done or if it's not anticipated that the infection is present.

dvm360: What is the single most important thing you want veterinarians to take away from this topic?

Little: The most important part of any diagnostic test is the veterinarian. We have great strategies, we have great resources, we have fantastic labs, but it's up to the veterinarian to put that information together in the context of the patient and the way the patient presents and come up with the best answer for that situation. We can't rely on just a blue spot, a red line, or a report from a laboratory. We really have to use our medical knowledge to best serve the patient.