Heartworm screening is a familiar component of pet check-ups, but did you know that the most recent American Heartworm Society (AHS) heartworm testing guidance goes beyond antigen testing in dogs? The AHS Canine and Feline Heartworm Guidelines, updated in 2024, recommend microfilaria testing for dogs annually alongside antigen testing, provide instructions for interpreting discordant test results, and now urge routine heartworm screening for cats as well as dogs. To ensure early detection, the AHS recommends annual antigen and microfilaria tests to screen dogs for heartworm infection. Photo courtesy Paola Dominguez-Lopez, CVT, University of Florida, Small Animal Hospital, Gainesville, Fla. Heartworm testing in dogs Routine heartworm screening in dogs is performed to confirm a dog is not harboring adult heartworms before beginning a heartworm prophylaxis program and to ensure patients presently on preventatives have not become infected due to either a lapse in owner compliance, failure of the product to be consumed/absorbed or, rarely, product failure. Routine heartworm screening facilitates early detection of infection, allowing treatment to be initiated as early as possible in the disease process. Given the progressive nature of heartworm damage, this is a key factor in the patient’s outcome. Such screening also helps identify dogs harboring heartworm microfilariae and serving as potential reservoirs of infection for other pets. To meet these testing goals, the AHS recommends heartworm screening in dogs include antigen and microfilaria tests annually, or whenever a lapse in prevention occurs. Antigen tests Antigen tests detect small quantities of heartworm antigen, produced mainly by adult female heartworms. While highly sensitive, these tests—which include enzyme-linked immunosorbent assay (ELISA) and immunochromatographic and immunofluorescent antibody test systems—do not detect the presence of immature heartworms. Strict adherence to manufacturer instructions is vital to achieving reliable results on heartworm antigen tests. Nevertheless, false-positive and false-negative results are possible, with the latter being much more common than the former. Microfilaria tests Since 2016, the AHS has also recommended microfilaria tests, preferably using a modified Knott test, alongside antigen tests, to increase the sensitivity of annual heartworm screening. The presence of microfilaria identifies the dog as a potential source of infection to other pets and indicates the need for the dog to be treated with a macrocyclic lactone that is effective against microfilaria as part of treatment. Interpretation of results If a dog on year-round heartworm prevention tests negative on both tests (Ag-/Mf-), the result is deemed “no antigen detected,” and the dog can be prescribed a full 12 months of prevention. If the dog’s previous prevention history was unknown or inconsistent before testing, repeat antigen and microfilaria testing is recommended in six months to ensure an immature infection was not present at the time of the first screening. If the dog test positive on the antigen test but negative for microfilariae (Ag+/Mf-), the AHS recommends re-testing the dog with a new blood sample to confirm the diagnosis, using a different antigen test. If the second antigen test is positive, the diagnosis is confirmed, and heartworm treatment should be initiated at this point. It is relatively common for heartworm-positive dogs to test negative for microfilaria, especially in patients that received intermittent doses of heartworm preventative after becoming infected. Therefore, an Ag+/MF- test result should not dissuade practitioners from pursuing treatment after confirmation of the diagnosis. If a dog tests negative on the antigen test but positive for microfilaria (Ag-/Mf+), it is highly likely the dog is heartworm-positive.Immune complex formation is the most common reason for false-negative antigen test results in infected animals. In these situations, the AHS recommends two additional steps: repeating the antigen test with a heat-treated blood sample to dissociate the immune complex and free the heartworm antigen for detection and requesting identification of the species of microfilaria found to rule out Acanthocheilonema, A. reconditum or other potential species. Lastly, practitioners should avoid the temptation to interpret antigen test results based on the color intensity of a positive antigen test result. While it is true the amount of antigen in the blood can correlate to the number of female heartworms present to some degree, such interpretations are imprecise. They can be affected by other factors, including recent worm death, the age and reproductive activity of the worm, and the presence of antigen/antibody complexes. Teach your teams not to fall for the temptation to interpret a “light” color result as a “light” infection. The patient requires adulticide treatment. Without routine screening, heartworm infections can go undetected and lead to severe—even fatal—disease. Highly sensitive antigen tests detect the presence of adult heartworms living in the heart, lungs, and pulmonary vasculature. Photo courtesy Steven Jones, DVM, Lakeside Animal Hospital Feline heartworm testing Our understanding of feline heartworm disease and its impact has been plagued for years by a circular paradox: the true prevalence of feline heartworm disease is unknown because very few cats are tested, and cats are rarely tested because the disease prevalence is unknown. Further complicating the issue is the fact it is more difficult to detect feline heartworm than canine heartworm. Cases may be missed entirely or diagnosed as other conditions, such as asthma. While there are no FDA-approved treatments if a cat does test positive for heartworms, veterinarians can provide symptomatic therapy for these patients. Based on data collected by shelters in heartworm endemic areas, we now know the incidence of feline heartworm is roughly similar to that of dogs in those regions. It is important to look at this data carefully, to understand its implication for clinical practice. While cats are more resistant to adult infections than dogs (necropsy surveys of shelter cats have shown adult infection rates are five to 15 percent that of dogs1), cats are susceptible to serious illness caused by immature stages of heartworm, which can only be detected by use of an antibody test. Feline heartworm antibody studies suggest the actual infection rate of cats is nearly the same as dogs.2 The AHS now strongly recommends routine heartworm testing in cats. Photo courtesy Angele Bice, DVM To begin to overcome this paradox, in its latest and comprehensive update of its Feline Heartworm Guidelines, the AHS made a pivotal decision designed to improve the veterinary profession’s understanding of feline heartworm disease: routine, annual screening for cats. Understanding detection is more difficult in cats, the AHS recommends a combination of antigen and antibody testing to maximize sensitivity. Microfilaria testing is not recommended, since microfilaremia in cats is uncommon and transient. The preferred diagnostic screening protocol now recommended by the AHS for cats includes the following: Antibody testing Heartworm antibody tests provide evidence of a cat’s immune response to past or present heartworm infections, including both male and female larval and adult stages of worms. It is important to note studies have revealed that different antibody tests vary widely in their sensitivity to each stage of heartworm larval development.2-5 Based on a comprehensive testing data analysis, the AHS has a recommended antibody test* due to its greater sensitivity. Antigen testing, using a heat-treated blood sample As noted above, heat treatment helps disassociate potential antibody-antigen complexes and can help the veterinarian obtain the most accurate result. While these complexes occur in both canine and feline blood samples, they occur more frequently in feline patients. Heat treatment of feline samples can increase their sensitivity by a factor of three to 12 times that of non-heat-treated samples and result in substantially more cats being diagnosed with heartworms, leading to a 4.5-fold increase in sensitivity.3, 6-9 With the adoption of routine heartworm screening in cats, veterinarians can expect the number of cats diagnosed in their practices to increase and a more accurate picture of heartworm prevalence in cats to emerge. This information, in turn, can supplement client education regarding feline heartworm disease and help support veterinary recommendations for heartworm prophylaxis in cats. To assist practices in implementing this new guidance, the AHS is working with reference laboratories to incorporate these two tests into standard annual screening packages for feline patients. Keeping heartworm screening costs manageable Financial constraints are a reality for veterinary clinics and clients. Fortunately, there are several ways veterinarians can make heartworm screening more affordable: Consider stocking a low-cost, in-house canine antigen test. It may be helpful to bundle the cost of heartworm testing into packages that include other services, such as annual exams and vaccines. Testing can even be bundled into the cost of 12 months of heartworm prevention. Subscription services are ubiquitous today, and veterinary medicine is no exception. Clients may appreciate having the ability to spread costs out month to month rather than face large bills once or twice a year. Veterinarians can also consider partnering with a veterinary care payment plan provider to lower barriers to comprehensive preventive care. *Veterinarians can access the Heska antibody test by calling 970-493-7272 and asking for #805514 or through the Antech test directory individually (S14544) or bundled with a heat-treated antigen test (S14546). Jenni Rizzo, DVM, graduated from the Kansas State University in 2012, and has since had the opportunity to work in many different facets of veterinary medicine, ranging from exotics, to shelters, to primary and specialty care, to education and outreach. She is the current president of the American Heartworm Society and the president and founder of IntroVet, an introvert-friendly continuing education community for DVMs and technicians. In her spare time, Dr. Rizzo still practices in Texas and Florida, and travels the U.S. with her two dogs, Pistol Pete and Biggie Smalls, educating on heartworm disease and exploring national parks. References Ryan WG, Newcomb KM. Prevalence of feline heartworm disease—a global review. In Proceedings of the Heartworm Symposium ’05, Auburn AL. American Heartworm Society, 1995, pp 79-86. Hays KM, Rodriguez JY, Little SE et al. Heartworm prevalence in dogs versus cats: Multiple diagnostic modalities provide new insights. Vet Parasitol X. 2020 Jun 12;4:100027. Little SE, Raymond MR, Thomas JE, et al. Heat treatment prior to testing allows detection of antigen of Dirofilaria immitis in feline serum. Parasites Vectors. 2014;7:1. Snyder PS, Levy JK, Salute ME, et al. Performance of serologic tests used to detect heartworm infection in cats. J Am Vet Med Assoc. 2000;216:693-700. Dillon AR, Brawner Jr WR, Robertson-Plouch CK, Guerrero J. Feline heartworm disease: correlations of clinical signs, serology, and other diagnostics-results of a multicenter study. In Recent Advances in Heartworm Disease: Symposium ’98, American Heartworm Society, Tampa, FL. 1998:153-158. Gruntmeir JM, Adolph CB, Thomas JE, et al. Increased detection of Dirofilaria immitis antigen in cats after heat pretreatment of samples. J Feline Med Surg. 2017; 19(10):1013-1016. Nelson CT, Johnson CM. Evaluation of feline heartworm disease based on gross necropsy, serology, pulmonary histopathology, and radiographic evidence in adult shelter cats in northeastern Alabama. Parasites Vectors. 2024;17(1):161. DiGangi BA, Dworkin C, Stull JW, O’Quin J, Elser M, Marsh AE, Groshong L, Wolfson W, Duhon B, Broaddus K, Gingrich EN, Swiniarski E, Berliner EA. Impact of heat treatment on Dirofilaria immitis antigen detection in shelter dogs. Parasites Vectors. 2017;10(Suppl 2):483. Barrantes Murillo DF, Moye A, Wang C. Heat Treatment Augments Antigen Detection of Dirofilaria immitis in Apparently Healthy Companion Dogs (3.8% to 7.3%): Insights from a Large-Scale Nationwide Survey across the United States. Pathogens. 2024 Jan 6;13(1):56.
