Mastitis by Pythium insidiosum in Mares

Background: Pythiosis is a chronic inflammatory disease caused by the oomycete Pythium insidiosum. This disease has been reported in several species, but most cases occur in horses. In this species, the cutaneous form is more prevalent, with occasional involvement of adjacent lymph nodes and bone tissue. The involvement of the mammary glands was not properly characterized. Therefore, the aim of this study is to describe cases of pythiosis with the involvement of the mammary glands in mares.Case: A retrospective study of all biopsy samples and necropsy examinations of horses, performed at the Animal Pathology Laboratory of the Federal University of Campina Grande from January 2000 to December 2018, was carried out. During the study period, 142 cases of cutaneous pythiosis were identified. Of these, in three the lesions extended to the mammary glands. Mares with 3-, 7- and 12-year-old, mongrel, from the backlands of the Paraiba were affected. On physical examination, the mares presented swollen mammary glands and multifocal wounds, which discharge serosanguinolent secretion. Clinical evolution ranged from 15 to 30 days. The animals had been medicated, but the wounds did not heal and mastectomies were performed. Macroscopically, the mammary glands were enlarged and with multifocal areas of ulceration measuring 1 to 3.5 cm in diameter. At the cut surface, they exhibited fistulous tracts containing kunkers. In the histopathological evaluation, was observed diffuse and marked inflammatory infiltrate of eosinophils extending through the deep dermis and interlobular mammary stroma; and multifocal areas, sometimes focally extensive, of eosinophilic necrosis with intralesional negative images of hyphae. In histological sections stained by periodic acid–Schiff, the walls of the hyphae were weakly stained in blue; but when impregnated by Grocott methenamine silver, they were strongly marked in black and were characterized by semi-parallel walls, rare septations and branching, and a diameter varying from 3-8μm. At immunohistochemistry, the hyphae showed strong cytoplasmic and wall immunostaining for the anti-P. insidiosum antibody.Discussion: The diagnosis of mastitis by P. insidiosum was established based on the epidemiological, clinical, anatomopathological and immunohistochemical findings. Lesions in horses are more frequently located at the extremities of the limbs and the ventral portion of the thoracoabdominal wall. In these cases, it is likely that the lesions h