Reassessing the definition of heartworm infection in cats.

JAVMA, Vol 231, No. 9, November 1, 2007 R studies by Browne et al and Dillon et al have corroborated earlier findings of Selcer et al, who reported radiographic evidence of pulmonary disease in cats experimentally exposed to heartworm infective larvae, but which did not develop mature heartworm infection (dirofilariasis). Importantly, the two more recent aforementioned studies identified histologic lesions in the pulmonary arteries or in airways and pulmonary parenchyma, as well as pulmonary arteries in cats with natural or experimental infections, respectfully. While it remains to be proven that naturally infected cats (with presumably lesser L 3 exposure) develop pulmonary parenchymal and airway lesions, it seems clear that naturally exposed cats that resist mature infection develop, at the very least, pulmonary vascular disease. These three landmark works broaden our understanding of the pathophysiology of the respiratory tract signs associated with exposure to various stages of Dirofilaria immitis. Their findings also raise questions as to appropriately descriptive terminology, particularly regarding the use of the word exposure. In 2005, I was an investigator on a study of heartworm risk in nondomestic cats in the southeastern United States. In that article, we defined antibodypositive cats as those exposed to, but negative for, heartworm infection if they were heartworm antigen negative and if no heartworms were found on necropsy. During the peer review process, one reviewer challenged this definition conceptually, arguing that antibody-positive cats indeed were infected. While acknowledging that exposed cats might well have pulmonary lesions, we argued that a cat without mature (L 5 ) heartworms did not meet the requirements for a diagnosis of heartworm infection. Our arguments were convincing enough that the manuscript was published with this distinction intact. Perhaps, in light of recent work, this point should be reconsidered. Conceptually, this represents a thorny problem. In an ideal world, cats would either have heartworm infection or not, but there is now a recognized gray zone in which cats that have successfully rejected an infection (probably at the young L 5 stage) may develop lesions (ie, disease) induced by D immitis. New terminology is Reassessing the definition of heartworm infection in cats