Pythiosis is a granulomatous infectious disease caused by the oomycete Pythium insidiosum. In Brazil, there are a considerable number of studies on that disease in dogs mainly from the state of Rio Grande do Sul. Given the unfamiliarity of the affectation in dogs from other locations, such as the Northeast, its description in native animals from federal units of this latter region becomes essential. In this sense, this study aimed to register the first case of canine pythiosis found in the city of Mossoró, Rio Grande do Norte (RN), Brazil. A two-year-old mixed breed female dog was referred to the Veterinary Hospital of the Rural Federal University of the Semi-Arid (Mossoró, RN, Brazil). Physical examination showed swelling in the anal mucocutaneous junction. During rectal palpation, lumen stenosis was detected. Laboratory screening tests were requested, but due to nonspecific results, performing incisional biopsy of the anal lesion was decided. Fragments were sent for histopathological analysis, being used special histochemical staining. Accordingly, a case of pythiosis was found. Pharmacotherapy was applied with itraconazole and terbinafine hydrochloride prescription. After three months and twelve days of treatment, complete remission of the lesions occurred. One should consider pythiosis in native dogs from the city of Mossoró, RN. Adoption of diagnosis appropriate methods is essential for early disease identification, with a positive impact on therapeutic decisions.
