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Background: : Veterinary knowledge regarding feline heartworm has been increasing significantly over the past two decades. Necropsy surveys of shelter cats have shown feline adult heartworm infection prevalence to be 5–20% of the rate in unprotected dogs; however, other studies have shown feline heartworm antibody prevalence up to 33%, reflecting higher exposure rates and potential immature adult infections. Thus, the true prevalence of feline heartworm infection is likely underestimated due to the limitations of current diagnostic techniques, inadequate testing protocols, and the high likelihood of cats exhibiting transient clinical signs or dying without confirmation of infection. Diagnosing Feline Heartworm Disease (FHWD), also referred to as Heartworm Associated Respiratory Disease (HARD), is one of the conundrums of veterinary medicine. The purpose of this study was to evaluate and characterize the occurrence of Heartworm Associated Respiratory Disease [HARD] in shelter cats, naturally-infected with D.immitis. Methods: Fifty shelter cats slated for euthanasia between December 2009 and June 2010 were investigated by gross necropsy, radiography, serology, and lung histopathology using techniques that have been established in experimental models of cat heartworm infection. The relationship between pulmonary vascular disease and serological markers for heartworm was also examined using correlations and statistical modeling. Serology included standard heartworm antigen test and a commonly used heartworm antibody test. Also included were heat-treated heartworm antigen test and two additional heartworm antibody tests previously evaluated on experimentally-infected cats. Results: None of the cats were heartworm antibody (HW Ab) positive on a commonly used HW Ab test used by many reference laboratories even though 20% of the study cats were heartworm antigen (HW Ag) positive on heat-treated samples. Two additional HW Ab test were positive on 26% and 22% of the study cats. The combination of heat-treated HW Ag, HW Ab tests, and histopathology indicated 34% of the study cats had HARD. Conclusions: Utilizing both, the above tests, and thoracic radiographs, enhanced the ability to predict vascular disease, possibly caused by infection with immature and adult heartworms and supported the premise that cats develop heartworm disease at the same rate as dogs. [ABSTRACT FROM AUTHOR]