Background: Pythiosis is a chronic inflammatory disease that is caused by oomycete Pythium insidiosum. This illness affects several species including humans and horses. Equine is the most affected species, having no predisposition for breed, gender, or age. It is usually shown in cutaneous and subcutaneous forms, and the lesions, which grow quickly and are hard to treat, are located mainly in the extremities. The diagnosis is made via epidemiology, clinical signs, and macroscopic and microscopic aspects of the lesion. This study describes a case of cranioesophageal pythiosis in a horse, examining the epidemiological, clinical and pathological characteristics.Case: A 12-year-old male quarter horse, weighing 515 kg was taken to the Veterinary Hospital at the University Center of Espírito Santo (UNESC). The horse had an increase in volume in the cranioesophageal region, coughing, difficulty breathing, and a runny nose. On clinical examination, the horse showed an enlargement in the submandibular and retropharyngeal lymph nodes, subcutaneous edema in the larynx region, and a temperature of 38.2ºC. According to the owner, cough was recurrent and had lasted about 12 months even after treatment with different kinds of antimicrobials. On radiographic exam, there was a marked decrease in the tracheal lumen and increased soft tissue radiopacity in the region adjacent to the narrowing. The animal was taken to surgery to remove the mass, but he died because of complications during surgery. The animal’s owner did not allow necropsy, but a fragment of the mass in the cranioesophageal was removed and sent for histological examination. The fragment was fixed in 10% formalin and processed using routine histological analysis. Macroscopically, the mass was light yellowish and ulcerated, and it measured 7.0 × 5.0 × 5.0 cm. In the middle of the ulcerated areas, there were yellow and firm granular structures that were consistent with kunkers. Histologically, extending from the tracheal adventitia to the thyroid, there was a large number of lymphocytes, macrophages, neutrophils, eosinophils, and multinucleated cells (foreign body type) and a well-defined focus of coagulative necrosis, which was surrounded by a thin border of macrophages. Within the necrotic areas, there were negative images of tubuliform hyphae. Grocott’s silver methanamine staining showed hyphae that had irregular branches, rare septa, smooth and parallel walls, and was impregnated by silver. Histological sections of the mass were subjected to immunohistochemistry. Hyphae were positive for Pythium insidiosum.Discussion: The diagnosis of pythiosis was based on macroscopic, histological findings and positive immunostaining for Pythium insidiosum. This report shows the unusual location of the disease in the horse, which made the clinical diagnosis of the disease complex. Extracutaneous forms of pythiosis in horses are less frequent than cutaneous forms. The etiopathogenesis of these forms is still unclear, but it has been suggested that previous lesions in the intestinal mucosa caused by plant material or pathogens may be predisposing factors for the appearance of the enteric form of the disease. It was not possible to observe if the animal’s other organs were affected because a necropsy could not be performed. The agent probably penetrated the esophageal epithelium and spread throughout the trachea and thyroid, but its origin cannot be determined. The radiographic findings in this study are compatible with neoplasms. However, inflammatory processes such as those caused by pythiosis should be included in the differential diagnosis of horses with swelling in the cranial portion of the esophagus.
