Fig. 5

Histopathology of feline lung with alpha smooth muscle actin stain 18 months after L3 D. immitis infection. a Cats in Group A were treated starting 28 days PI with selamectin. Cats in Group A were generally normal but a wide diversity of abnormal myofibrocyte proliferation was noted between cats and even within the same slide with a subtle increase in isolated area of smooth muscle proliferation, which was interstitial and associated with alveolar struts. In four of the ten cats, isolated pulmonary arterial smooth muscle proliferation was also observed. b Cats in Group B had abbreviated immature adult heartworm infections (HARDs) and 18 months PI, some cats had normal areas of lung, but five of the ten cats had multiple areas of smooth muscle proliferation in the interstitium and pulmonary arteries. c, d, and e, Cats in Group C were infected (untreated) and at 18 months PI had consistent areas of smooth muscle proliferation of the pulmonary arteries and interstitium (c). Although present in all groups of cats, the alveolar struts, small pulmonary arterioles, and interstitium proliferation (d) was more severe in Group C based on SMA positive myofibrocytes and often in areas not associated with the major pulmonary arteries (e). At 18 months PI, cats in Group A and B could not be differentiated by histopathologic examination