Nasal leiomyosarcoma in a Quarter Horse

We described the clinical and anatomopathological findings observed in a case of nasal leiomyosarcoma in a five-year-old male Quarter Mile horse, whose main complaints were decreased sports performance and bilateral purulent nasal discharge. The nodule was observed in the nasal cavity, obstructing the left nostril and associated with purulent drainage. The nodule was of irregular shape and yellow color, measuring 19.4 cm x 6.9 cm x 4.3 cm in size, with coalescent multifocal areas that were brownish, friable, opaque, and fetid. When cut, the surface was compact, grayish-white, and smooth with yellow, friable, irregular multifocal areas, measuring 1-3.2 cm in diameter. Histopathological examination showed spindle-shaped neoplastic cells, which was negative on Masson's trichromic stain. A diagnosis of leiomyosarcoma was established based on the morphotintorial aspects of neoplastic cells and confirmed through immunohistochemistry, with positive immunostaining for antibodies 1A4, HHF35, desmin, and S100. Leiomyosarcoma primarily affects the nasal cavity of horses and should be included in the differential diagnosis of diseases that affect the nasal cavity and cause nasal obstruction associated with dyspnea.