Morphologic Changes in the Lungs of Cats Experimentally Infected With Dirofilaria immitis

Abstract

The morphologic response of the pulmonary arteries and lungs in cats was studied after a five month heartworm infection produced by transplantation of four adult heartworms/cat. One group of seven heartworm infected cats was not treated, another group of seven cats was treated with 97.5 mg of aspirin given twice a week, and the third group of six cats was given aspirin at a sufficient dosage to block in vitro platelet aggregation throughout the study. A fourth group of eight noninfected cats served as controls. Five months after heartworm infection, the cats were euthanized and the lungs perfusion fixed for light microscopy and scanning electron microscopy of the pulmonary arterial surfaces.All cats in the three heartworm‐infected groups had live heartworms and the typical pulmonary arterial changes of heartworm disease at necropsy. The arterial surfaces, as viewed with scanning electron microscopy, had villus proliferations that were more numerous and exuberant than similar infections in dogs. Mean percentage of arterial surface involvement with villus proliferation of the nontreated heartworm infected cats was 67.3%; the aspirin treated cats, 73.8%; and the adjusted aspirin treated cats, 75.9%. The villi were myointimal proliferations in the small and medium‐sized arteries. The more elastic arteries had a predominance of fibromuscular proliferation. All heartworm infected cats had arterial muscular hypertrophy of the small arteries, in contrast to only three of eight of the nonheartworm infected cats. The caudal lobar arteries were frequently obstructed with either villus proliferation, thrombi, and/or dead heartworms. The muscular arteries had branches with marked dilation, a condition associated with pulmonary hypertension in man. However, only three cats, one in each group, had pulmonary hypertension. The adjusted aspirin group had less arteriosclerosis and thrombosis than the other two heartworm‐infected groups. There were no statistical differences between the heartworm groups with respect to arterial muscular hypertrophy, myointimal proliferation with and without eosinophilia, vein‐like dilations of the muscular arterial branches, and percentage of arterial surface with villus proliferation.Aspirin, when given at platelet inhibiting doses, reduced only a portion of the pulmonary response to heartworms in these cats. It was presumed that the inflammatory response to heartworms in the cat is so severe that trophic factors other than those released by platelets can also produce arterial disease. Based upon this study, aspirin is not recommended for heartworm disease in the cat.