Feline heartworm disease: correlations of clinical signs, serology, and other diagnostics--results of a multicenter study.

In cooperation with 15 practices in Florida, South Carolina, Tennessee, and Texas, data were collected on 215 cats with signs consistent with feline heartworm disease (FHD). Cats included in the study were over 6 months of age and presented with primary complaints of coughing or dyspnea, vomiting unrelated to eating, or acute death. Detailed signalment, thoracic radiographs, CBC, Knott or DIFIL test, DiroCHEK antigen test (Ag), and antibody (Ab) tests performed by Animal Diagnostics (AD) and Heska Corp (HC) were collected on each cat. Any cat that had positive antibody or antigen tests, and any cat with radiographic signs suggestive of FHD was scheduled for recheck examinations at 30 to 45 days and/or 60 to 90 days after initial presentation. This study was designed to identify cats with concurrent or previous FHD, and to better characterize the presentation of this disease by following their progress. Of the 215 cats, 94 (44%) were Ab positive based on one or both tests. This indicated that the cat had been successfully infected with third-stage heartworm larvae and those larvae had developed to at least the fourth stage. Of the Ab-positive cats, 23/94 (24%) presented with vomiting; 39/94 (41%) presented with respiratory signs; and 27/94 (29%) had vomiting and respiratory signs. Discordant results between the AD and HC antibody tests occurred, with the AD test detecting a higher number of antibody-positive cats. When comparing results of these Ab tests, no correlation was seen between the intensity of Ab level measured by the two tests, suggesting that different Ab is detected. One cat that died acutely with signs associated with FHD had relatively low Ab detected on both tests but had a positive DiroCHEK antigen test. No correlation between the level of antibody and the severity of clinical signs or radiographic pattern was found. Eleven cars were DiroCHEK Ag positive on initial presentation. Of the Ag-positive cats, 2 were AD negative and 3 were HC negative. One cat was Ag positive and microfilaria positive but negative for Ab with both AD and HC tests. Although it has been presumed that most cats with FHD are Ag negative, it would appear that some individual cats with adult heartworms can also be Ab negative. Although eosinophilia and basophilia were more frequently associated with cats that were Ab positive, abnormal CBC values were observed in cats that were currently Ab negative. Radiographic lesions did not correlate with clinical signs or Ab levels. Some cats with no radiographic signs of FHD were Ag positive. Further, some cats with typical radiographic lesions of FHD were negative based on all serologic evaluations. These results demonstrate that successful transmission of heartworms to cats is more common than previously thought and is clinically associated with coughing, dyspnea, and vomiting. A cause and effect correlation cannot be proven between clinical signs and laboratory evaluation. Radiographic evidence of FHD in cats with repeated negative antibody results may be an indicator of residual damage from previous heartworm infections. Most experimental studies of FHD have been short-term (< 1 yr). However, in this study, many cats underwent repeated clinical evaluation over a relatively short time. This demonstrated that the clinical picture of spontaneous FHD is a constantly changing syndrome, highly dependent on the stage (immature L5, adult, adult death, residual damage) of the parasite in the cat and the individual cat's response. On evaluation of clinically affected cats, no one test proved to be definitive (without exceptions). The diagnosis of FHD continues to require a combination of clinical evaluation and a series of diagnostic tests, often requiring reevaluations over time.