Pulmonary Vascular Proliferative Lesions in Wild Korean Raccoon Dogs (Nyctereutes procyonoides): Description of 13 Cases

Pulmonary vascular proliferative lesions are rarely reported and poorly characterized in animals. In this study, we describe 13 cases identified in wild Korean raccoon dogs (<i>Nyctereutes procyonoides</i>), suggesting a higher-than-expected incidence in this species. Gross examination revealed villous projections within the lumina of pulmonary vessels, sometimes accompanied by pneumonia, hemorrhage, or <i>Dirofilaria immitis</i> (heartworm) infection. Most affected animals also presented with thick, dark gray cutaneous crusts associated with scabies infestation. Histopathologically, the lesions consisted of papillary proliferations within thickened vascular lumens. Special stains (Masson's trichrome and Elastic Verhoeff-Van Gieson) demonstrated a single layer of endothelial cells lining fibromuscular and collagenous thick cores. Immunohistochemistry confirmed endothelial origin and benign proliferative nature, with positive expression of CD31, collagen types I, III, and IV, and proliferating cell nuclear antigen (PCNA). To date, pulmonary vascular proliferative lesions have not been well documented in <i>N. procyonoides</i>, and baseline pathological data, including findings from special stains, are lacking. These findings indicate that pulmonary vascular proliferative lesions may be underrecognized in raccoon dogs and suggest a likely association with chronic vascular injury related to parasitic infections. Further studies are warranted to elucidate the underlying mechanisms and contributing factors.