Heartworm infection in cats is likely to occur anywhere it occurs in dogs. Necropsies of cats and dogs at animal shelters reveal worms in cats at 5% to 20% of the rate found in dogs at the same location. Heartworms in cats survive for a relatively shorter period than in dogs (2 to 3 years), and a lower percentage of larvae survive into adulthood. Cats are also less likely to have circulating microfilariae. The lung is the most common organ affected by heartworm in cats, with common clinical signs including dyspnea, vomiting, coughing, and sudden death. A recent evaluation performed at the University of Florida (UF) suggested that even transient infections in cats may cause long-lasting pathologic pulmonary conditions. The true prevalence of infection as well as incidence of clinical signs is difficult to determine due to lack of highly reliable serologic tests. A wide disparity among tests was found in the UF evaluation. Using presence of adult heartworms at necropsy as the gold standard, the author and colleagues determined that 6 different antigen tests had sensitivities of 68% to 86% and specificities of 98% to 99%, whereas 6 different antibody tests had sensitivities of 32% to 90% and specificities of 78% to 99%. Thus, they use both antigen and antibody testing as well as a chest radiograph for initial screening of heartworm disease in cats. If heartworm disease is highly suspected, echocardiography is also done. Heartworm preventive is recommended for all feline patients. As Wolbachia infection can play a role in the inflammatory side effects of heartworm disease, doxycycline therapy might also be helpful for cats, eliminating this inflammatory organism in cases in which adulticide therapy for the heartworm infection is not feasible.

COMMENTARY: The author discusses how she and her colleagues approach the diagnosis of heartworm disease in cats and why they take this approach. Surprising findings include the results of comparisons between different heartworm antigen and antibody tests against a gold standard, as well as evidence of pulmonary lesions in 50% of cats with heartworm antibodies, but no adult heartworms. In addition, with a 30% mortality expected from adulticide therapy and subsequent pulmonary thromboembolism in heartworm-infected cats, the idea of treating these patients with doxycycline to help them live with heartworm infection is enticing and deserves further study.

Why heartworm prevention should be given year-round everywhere. Bowman DD. Proc NAVC 2007 p 980.