By Pablo David Jimenez Castro DVM, Ph.D. DACVM (Parasitology) and Kelly Mitchell BSc., DVM, DVSc., DACVIM (SAIM) • Mars Science & Diagnostics - Antech In November 2024, the American Heartworm Society (AHS) published new evidence-based guidelines for preventing, diagnosing, and managing feline heartworm infection. In these guidelines, the AHS aims to address the likely under-diagnosis of this parasite in felines, marking a significant shift in the official recommendations and a deliberate move from reactive testing to proactive screening. For decades, heartworm infection has been largely associated with dogs—and for good reason. Canids are the natural definitive host and main reservoir of Dirofilaria immitis. Infection can occur worldwide, with reports throughout the United States and Canada, with climatic and environmental changes expanding the parasite’s spread. The AHS guidelines state that wherever heartworm infection exists in dogs, cats are also at risk, regardless of whether the cat lives primarily an indoor or outdoor lifestyle. Cats are not the natural definitive host for D. immitis, resulting in several important differences as compared to dogs. Infection intensity in cats is often lower (usually less than 6 adult worms, often only 1-2) and single sex infections occur more commonly as compared to dogs. When considering their generally smaller body size, even a small number of adult worms represents a significant infection burden for a cat. Clinical signs of infection in cats are also different than dogs. While both species can present with asymptomatic infection, clinical signs in cats are varied, ranging from sudden death or sudden onset of severe respiratory or neurological signs to chronic and potentially intermittent signs such as cough, increased respiratory rate or effort, lethargy, and gastrointestinal signs. Less commonly, aberrant migration of adult worms in tissues such as eyes, body cavities, arteries or central nervous system can occur. PHOTO CREDIT: AMERICAN HEARTWORM SOCIETY1 Testing guidelines The current AHS feline guidelines recommend that all cats, regardless of indoor or outdoor lifestyle, are screened annually for heartworm infection by means of both a heat-treated antigen and a feline heartworm antibody test. This recommendation is different from dogs, where screening for infection generally relies on a combination of non-heat-treated antigen and microfilaria testing. The sensitivity of antigen testing is lower in cats compared to dogs, owing to their tendency towards lower worm burden and single sex infections. Routine heat treating of feline serum samples prior to antigen testing is recommended to release potential antigen-antibody complexes, thereby freeing antigen for detection. For feline heartworm antibody testing, the AHS guidelines specifically recommend the offering from Heska / Antech due to increased sensitivity compared to other assays. Antibodies are produced in response to infection with either sex and may be detected as early as 2 months post infection whereas antigenemia may not be detectable until 6-8 months post infection. While the presence of anti-heartworm antibodies confirm exposure or infection has occurred, it does not necessarily confirm current infection. Microfilaria testing (i.e., Modified Knotts) is not generally recommended in cats as circulating microfilariae are uncommon and only transiently present. The AHS guidelines also describe additional diagnostic options, including thoracic radiographs, echocardiogram and point of care ultrasound. Management and prevention Adulticidal therapy (i.e., melarsomine) is not recommended in cats due to potential toxicity and lack of evidence supporting its use. The treatment of heartworm infection in cats is therefore focused on supportive care and management of clinical signs (e.g., tapering course of corticosteroids, anti-leukotrienes in cats with respiratory signs, etc.). Treatment with doxycycline is recommended to target Wolbachia spp., an endosymbiont bacteria of D. immitis known to contribute to the pathophysiology of disease. Occasionally, surgical removal of adult worms may be indicated. Routine administration of an antiparasitic product labelled for prevention of D. immitis disease is imperative to target larvae from newly acquired infections. Specific monitoring recommendations will depend on the patient and their clinical signs. As per the AHS guidelines, monitoring should include both a heat-treated antigen and feline heartworm antibody test every 6 months. Antigenemia is expected to be resolved within 4-5 months of worm elimination (adult worms can live as long as 2-4 years). Antibody detection may persist longer or even indefinitely despite resolution of antigenemia. Given the expansive and increasing geographic distribution of D. immitis and the risk of heartworm infection to all cats, including those that live an indoor-only lifestyle, the AHS guidelines recommend the routine, year-round, administration of an antiparasitic product labelled for prevention of D. immitis disease for all cats. The guidelines also emphasize the importance of compliance with preventative care, vector control strategies, and environmental means of interrupting transmission. For more information about the updated Feline Heartworm guidelines, please visit https://www.heartwormsociety.org/. To learn more about Antech, please visit https://www.antechdiagnostics.com/;The American Heartworm Society recommends the combination of feline heartworm antibody—specifically the Heska-Antech feline heartworm antibody assay—and heat-treated antigen testing. These are available in combination (S14546) or individually (S14544 for antibody testing, S14544 or T613 for heat treated antigen testing). Dr. Jimenez Castro earned his D.V.M. degree in 2013 from the Universidad Nacional de Colombia. Following an internship in clinical trials involving parasiticides at a contract research organization, he transitioned to roles in regulatory affairs within various pharmaceutical companies. In 2016, he embarked on a Ph.D./Residency program at The University of Georgia, specializing in veterinary parasitology with a focus on hookworm resistance. He achieved board certification as a veterinary parasitologist in 2022 and presently holds the position of Director of Parasitology at Antech. Dr. Kelly Mitchell earned her D.V.M. degree in 2004 from the Western College of Veterinary Medicine. She completed an internship at the Atlantic Veterinary College, followed by a residency in small animal internal medicine at the Ontario Veterinary College, achieving board certification in 2008. She joined Antech in 2017 and presently holds the position of Director of Consultation Service (North America) at Antech Diagnostics. Reference American Heartworm Society. Heartworm in Cats. Accessed Feb. 7,2025. https://www.heartwormsociety.org/heartworms-in-cats.
