Sections from 35 formalin-fixed, paraffin-embedded, canine gastrointestinal stromal tumors consisting of 14 leiomyomas (five stomach, three small intestine, two colon, four rectum), 18 leiomyosarcomas (one stomach, five small intestine, nine cecum, three rectum), two undifferentiated sarcomas (two stomach), and one neurofibrosarcoma (small intestine) were examined for the expression of vimentin, S-100 protein, alpha-smooth muscle actin, and desmin via immunoperoxidase methodology using an avidin-biotin complex technique. The leiomyomas were 4/14 (29%) vimentin-positive, 3/14 (21%) S-100 protein-positive, 10/14 (71%) alpha-smooth muscle actin-positive and 13/14 (93%) desmin-positive. Leiomyosarcomas were 18/18 (100%) vimentin-positive, 11/18 (61%) S-100 protein-positive, 9/18 (50%) a-smooth muscle actin-positive, and 15/18 (83%) desmin-positive. The undifferentiated sarcomas were 2/2 (100%) vimentin-positive, 2/2 (100%) S-100 protein-positive, 1/2 (50%) alpha-smooth muscle actin-positive, and 0/2 (0%) desmin-positive. The neurofibrosarcoma was vimentin and S-100 protein-positive and alpha-smooth muscle actin- and desmin-negative. Thirty-one of thirty-five (89%) of all neoplasms demonstrated reactivity for either desmin and/or alpha-smooth muscle actin. S-100 protein reactivity occurred in 17/35 (49%) of all specimens. Lack of desmin and alpha-smooth muscle actin reactivity occurred in 4/35 (11%) of all specimens, all of which were vimentin-positive. The immunohistochemical results indicate that the majority of canine gastrointestinal stromal tumors (GIST) with light microscopic features of smooth muscle cells have immunohistochemical staining patterns supporting smooth muscle differentiation. Vimentin reactivity correlated with a light microscopic diagnosis of malignancy. The lack of smooth muscle cell markers in some tumors and the high percentage of cases positive for S-100 protein may suggest a more complex histogenesis or differentiation for subgroups of these tumors.

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A 5-year-old Thoroughbred mare presented with a 4 week history of weight loss, fever and leukopenia. Rectally, a large active foetus, thickened spleen and an abdominal mass were palpated. Leukopenia, mild anaemia, marked thrombocytopenia and hyperfibrinogenaemia were found. Cytology and cytochemical staining of a bone marrow aspirate supported a diagnosis of acute myelogenous leukaemia. The mare deteriorated despite medical therapy and was humanely euthanased.