Intramuscular desmoid tumor (musculoaponeurotic fibromatosis) in two horses

Intramuscular desmoid tumors (musculoaponeurotic fibromatosis) were discove red in two young adult horses. The tumor in one horse was in the lateral ce rvical musculature, and that in the second horse occurred in the pectoral m usculature. Histopathologic features were similar in both horses and includ ed proliferation of fibroblasts and cells expressing muscle actin (myofibro blasts), with extensive dissecting fibrosis within muscle. These features a re similar to those of desmoid tumors in humans, particularly those also kn own as musculoaponeurotic fibromatosis. Dissection of these lesions reveale d a single central (horse No. 1) or multiple central (horse No. 2) fluid-fi lled cavities with associated sterile inflammation. The presence of these c avities supports the hypothesis that equine desmoid tumors are traumatic in origin, possibly occurring at sites of injections or bursal rupture. Surgi cal excision of the tumor in horse No. 1 was apparently curative, but the e xtent of the tumor in horse No. 2 precluded surgical excision.