Penile and preputial pythiosis in Saudi Arabian horses: clinicopathological features and surgical outcomes.

BACKGROUND: Pythiosis, caused by Pythium insidiosum, is a chronic granulomatous disease of horses characterized by invasive, ulcerative, and pyogranulomatous cutaneous lesions. Involvement of the external genitalia is uncommon and remains poorly characterized, particularly regarding environmental exposure and treatment outcomes. This study aimed to describe the epidemiological pattern, clinicopathological features, diagnostic findings, and surgical outcomes of penile and preputial genital pythiosis in a hospital-presented horse cohort. METHODS: A retrospective study was conducted on 123 horses presented with cutaneous ulcerative granulomatous lesions to the Horse Hospital, Salam Veterinary Group, between October 2022 and September 2025. Of these, 17 male horses with confirmed penile or preputial involvement were included. Clinical data, lesion characteristics, and outcomes were analyzed. Diagnosis was confirmed by histopathology, immunohistochemistry, and mycological culture, with ultrastructural identification by scanning electron microscopy. All cases were treated by complete surgical excision. RESULTS: The mean age of affected horses was 6.5 ± 1.8 years, with the majority of cases occurring in gray-colored horses (58.82%). Lesions were predominantly located on the prepuce (52.94%), followed by the urethral orifice (29.41%), glans penis (11.77%), and penile body (5.88). Morphologically, lesions were mainly elliptical (58.8%) or circular (35.3%), with a median surface area of 25.0 cm2 (range: 9.0-117.0). Clinically, lesions appeared as ulcerative granulomatous masses with multiple draining tracts and serosanguinous discharge. Histopathological examination revealed eosinophilic granulomas with necrotic centers and peripheral hyphal elements, while immunohistochemistry confirmed P. insidiosum infection. Mycological culture demonstrated characteristic growth at 37 °C, and scanning electron microscopy identified characteristic sporangia and oogonia. Surgical excision resulted in complete recovery in 82.35% of cases, while recurrence was observed in 17.65%, particularly in larger surface areas or in multiple preputial lesions. CONCLUSION: Early accurate diagnosis and complete excision are essential to minimize recurrence, especially in extensive preputial lesions. This study provides one of the most focused characterizations of genital pythiosis in horses and highlights key prognostic factors relevant to clinical management.