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Peer reviewed hearTworm hoTline Question & Answer: Heartworm screening for Cats Clarke Atkins, DVM, Diplomate ACVIM (Internal Medicine & Cardiology) The Heartworm Hotline column is cosponsored by Today's Veterinary Practice and The American Heartworm Society (heartwormsociety.org). This series presents questions and answers on topics related to heartworm infection, prevention, diagnostics, and/or treatment. Feline Friendly Article Q Question My practice is in an area of moderate to high heartworm prevalence. we make a dedicated effort to ensure that all clients' dogs are receiving heartworm preventive and, based on the american heartworm Society guidelines (heartwormsociety.org; Veterinary resources), i am now promoting feline heartworm prevention. a long-time client and i discussed heartworm prevention for her 4 indoor/outdoor, adopted cats that are routinely vaccinated, but do not receive heartworm preventive. after explaining that cats can become infected with heartworms and there is no definitive therapy, she decided to pursue heartworm prevention. however, when faced with the fees for initial heartworm tests, preventive medication, and follow-up tests, the owner changed her mind, stating that her decision was economically based. This experience raised the following questions in my mind regarding feline heartworm. • Is it absolutely necessary to test cats for heartworms prior to starting preventive therapy? • Is yearly testing advisable/necessary in cats receiving heartworm preventive? 50 Today's Veterinary Practice May/June 2013 A Answer This is an excellent and somewhat difficult question, particularly when product labels on heartworm (hw) preventive products advise or sanction "testing" prior to administering the product. Medically speaking, I personally believe that neither testing prior to initiation of preventive therapy ( pretesting) nor yearly testing are necessary or even advisable in a case such as yours—in which the client declined preventive therapy due to the costs of initial and follow-up testing. However, this does not mean that pretesting in cats is without value— it is never incorrect to test cats for HW antigen, microfilariae, or antibodies as long as the owner: • Understands the cost:benefit ratio and limitations of testing with current technology • Is involved in the decision to test or not to test. HEARTWORM ANTIBODY TESTING Arguments For Testing Prior to Preventive Therapy The HW antibody test reacts to the protein(s) associated with the L4 larval stage. A positive antibody test indicates the: • Cat has certainly been exposed to HWs • Possibility (approximately 10%–20%) that the cat may have a mature heartworm infection (HWI)1 • Patient is at risk for HW-associated respiratory disease (HARD),2 which is related to immune-mediated or treatment-related death of immature adult worms, and mature Dirofilaria immitis infection. The owner can be offered the opportunity to pursue diagnosis of heartworm disease (HWD)/HWI, including an antigen test, complete blood count, thoracic radiographs, and echocardiogram. In addition, the owner can be advised that the cat could eventually develop severe, even fatal, cardiorespiratory disease.