Prevalence of feline heartworm infections among cats with respiratory and gastrointestinal signs: results of a multicenter study.

Although heartworm infection in cats was first described in 1921, the diagnosis of the infection remains elusive in many cases. This is due to nonspecific clinical signs of feline heartworm disease, typically low worm burdens, unique pathophysiology in the cat, and the limitations of currently available heartworm tests. Consequently, knowledge about this disease is still limited. An objective of this study was to survey the occurrence and clinical presentation of feline heartworm infection among cats presenting with clinical signs consistent with heartworm disease. Two-hundred fifteen cases were submitted from 15 private practices in Florida, South Carolina, Tennessee, and Texas. Cats entered in the study were at least 6 months of age and presented with one or more of the following clinical signs: respiratory signs, including tachypnea, dyspnea, or coughing; gastrointestinal signs, including a pattern of intermittent vomiting unrelated to eating; or sudden death of uncertain etiology, particularly associated with respiratory distress prior to death. Data collected included: history and indoor/ outdoor lifestyle; physical examination findings; thoracic radiography evaluations; Knott or DIFIL test results, DiroCHEK antigen test results, and antibody test results (Animal Diagnostics, Inc. and Heska Corporation); and CBC results. Recheck examinations were scheduled for any cat with positive heartworm serological test results and for cats with radiographic signs consistent with or suggestive of feline heartworm disease. Data from 215 cases were collected and analyzed: 94/215 (44%) tested antibody positive for one or both antibody tests that were performed; 18/94 (19%) of the antibody-positive cats were reported as living 100% indoors by their owners; (12%) of the antibody-positive cats spent less than or equal to 10% of their time outdoors. Eleven of 215 cats (5%) were DiroCHEK antigen positive on initial examination. One cat was both DiroCHEK and microfilariae positive, but negative for both antibody tests. Radiographs were obtained for 10 of these cats and 6/10 had radiographic signs consistent with or suggestive of feline heartworm infection. At necropsy, heartworms were found in one other cat from which blood was not obtained. Additionally, two cats that had positive radiographic signs of heartworm infection converted from antigen-negative to antigen-positive status at recheck examination for a total of 13/215 (6%) DiroCHEK antigen-positive cats. One case that was submitted after acute death was DiroCHEK antigen positive, but interestingly, had relatively low antibody levels for both antibody tests. Initial radiographs were available on 212 of the 215 cases. In 90/212 (42%) cases, initial thoracic radiographs showed signs consistent with or suggestive of heartworm disease. Follow-up radiographs showed varying progression with radiographic signs worsening, improving, or staying the same on individual cats. Feline heartworm disease should be among the primary differential diagnoses in cats with respiratory disease, vomiting, or acute death. Radiography, antibody testing, and antigen testing are all useful tools to aid in making the diagnosis. A confirmed diagnosis may require doing multiple tests and clinical reevaluation. Cats classified by their owners as indoor only cats were found to be heartworm infected, thus lifestyle of the cat cannot rule out the disease. Actual antibody levels (high or low) may not correspond to severity of disease.