Radiographic diagnosis of feline heartworm disease and correlation to other clinical criteria: results of a multicenter clinical case study.

Two-hundred fifteen cats with clinical signs consistent with feline heartworm disease (FHD) were entered into this clinical case study. In addition to physical examination, CBC, and heartworm antibody (Ab) and antigen (Ag) tests, thoracic radiographs were taken of 212 cats at initial examination. For cats that had a positive Ab or Ag test, or radiographic changes that could be associated with FHD, follow-up radiographs were taken at approximately 60 to 90 days after initial examination whenever possible. Each radiographic examination included VD, DV, and left lateral views, and each was read by the same board-certified radiologist who was blinded to heartworm serological results until after radiographic evaluation was completed. Criteria evaluated included heart size and shape, pulmonary artery enlargement, pulmonary parenchymal involvement, hyperinflation of lungs, tenting of the diaphragm, and pleural fluid accumulation. Summary interpretations and heartworm score were recorded. The heartworm score reflected the degree of suspicion of FHD based on radiographic signs: no radiographic signs of FHD; bronchointerstitial lung pattern only (consistent with but not specific for FHD); or pulmonary artery enlargement (with or without pulmonary or cardiac changes) mildly, moderately, or strongly indicative of FHD. Of 212 cats for which radiographs were taken at the initial examination, 38 (18%) had enlarged caudal lobar arteries indicative of FHD and 90 (42%) had bronchointerstitial pulmonary disease consistent with heartworms and/or enlarged caudal lobar arteries. Radiographic changes consistent with or indicative of FHD were evident in 9/22 cats (41%) presenting with gastrointestinal signs, 39/78 cats (50%) presenting with respiratory signs, and 41/80 cats (51%) presenting with both respiratory and gastrointestinal signs. Some cats presenting with only gastrointestinal signs had thoracic radiographic changes suggestive of FHD. Eleven cats tested DiroCHEK Ag positive, and radiographs were taken of 10. Of the 10 cats, 5 had radiographic changes indicative of FHD; one had changes consistent with FHD; and 4 cats showed no radiographic signs of FHD. Follow-up radiographs were taken of 6 of these 11 cats: 1/6 worsened in radiographic score, 3/6 improved in radiographic score, and 2/6 still demonstrated no radiographic signs of heartworm. Ninety-two cats tested Ab positive at initial examination, and radiographs were available for 91 of these cats. In 22/91 (24%), enlarged caudal lobar arteries indicated FHD. In 39/91 cats (43%) cats, radiographic signs consistent with or indicative of FHD were seen. Of the 38 cats that had radiographic signs indicative of FHD, 22 (58%) were Ab positive on one or both tests. Seven (32%) had positive DiroCHEK Ag tests at initial or recheck examination. At recheck examinations, 2 cats that initially had radiographic signs indicative of FHD converted from DiroCHEK Ag negative to positive, confirming their infection with at least 1 sexually mature female heartworm. While many affected cats show pulmonary arterial enlargement and/or a bronchointerstitial pattern typical of FHD, some cats with serologic evidence of heartworms do not demonstrate any thoracic radiographic changes. Conversely, some cats show radiographic signs suggesting heartworm disease when concurrent serology is negative. These discrepancies may be caused by time between infection and examination, by differences in individual cats' immune response to the presence of larval or adult stages of heartworm, or by aberrant migrations.