Heartworm-associated respiratory disease (HARD) induced by immature adult Dirofilaria immitis in cats

BACKGROUND: A controlled, blind research study was conducted to define the initial inflammatory response and lung damage associated with the death of immature adult Dirofilaria immitis in cats as compared with cats developing adult heartworm infections and cats on preventive medication. METHODS: Three groups of cats were utilized, 10 per group. All cats were infected with 100 third-stage (L3) larvae by subcutaneous injection. Group A cats were treated topically with selamectin (Revolution®; Zoetis) per label directions at 28 days post infection (PI) and once monthly for 8 months. Group B cats were treated orally with ivermectin (Ivomec®; Merial) at 150 μg/kg at 70 days PI, then every 2 weeks for 5 months. Group C cats were untreated PI. At baseline (Day 0) and on Days 70, 110, 168, and 240 PI, peripheral blood, serum, bronchial lavage, and thoracic radiographic images were collected on all cats. Upon completion of the study (Day 245), cats were euthanized and necropsies were conducted. RESULTS: Results were analyzed statistically between groups by ANOVA and by paired sample T testing for changes within the group over time. The selamectin-treated cats (Group A) did not develop radiographically evident changes throughout the study and were free of adult heartworms or worm fragments at necropsy. The heartworm life cycle was abbreviated with oral doses of ivermectin (Group B), shown by the absence of adult heartworms or worm fragments at necropsy. The early stage of immature adult worm in Group B cats, however, did induce severe pulmonary airway, interstitial, and arterial lung lesions, revealing that the abbreviated infection is a significant cause of respiratory pathology in cats. Cats in Groups B and C could not be differentiated based on radiographic changes, serologic antibody titers, complete blood count, or bronchoalveolar lavage cytology at any time point throughout the study. Eighty percent of cats in Group A and 100% of cats in Groups B and C became heartworm antibody positive at some time point post infection. CONCLUSIONS: The clinical implications of this study are that cats that become infected with immature adult heartworms may not develop fully mature heartworms and are only transiently heartworm antibody positive, but do develop Heartworm-Associated Respiratory Disease (HARD).