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

Penile and preputial pythiosis in horses is a rare but serious fungal infection that causes ulcerative and granulomatous lesions on the external genitalia. This study analyzed clinical characteristics, diagnosis, and surgical treatment outcomes in affected Saudi Arabian horses.

Background and Purpose

• Pythiosis is a chronic granulomatous disease caused by the oomycete Pythium insidiosum.
• It typically causes invasive cutaneous lesions but rarely affects the external genitalia in horses.
• The study aimed to characterize penile and preputial pythiosis in horses, focusing on epidemiology, clinical and pathological features, diagnostic methods, and treatment success.

Study Design and Methods

• Retrospective analysis of 123 horses with ulcerative granulomatous skin lesions presented to a veterinary hospital between October 2022 and September 2025.
• Seventeen male horses with confirmed genital involvement (penis or prepuce) were included.
• Diagnostic confirmation involved:
• Histopathological examination showing eosinophilic granulomas with necrotic centers and hyphal elements.
• Immunohistochemistry confirming presence of Pythium insidiosum antigens.
• Mycological culture exhibiting characteristic pathogen growth at 37°C.
• Scanning electron microscopy identifying specific structures like sporangia and oogonia.
• All cases underwent complete surgical excision of lesions.
• Clinical data, lesion morphology, size, locations, and outcomes were recorded.

Results

• The affected horses had an average age of 6.5 years.
• Most affected horses were gray-colored (approximately 59%).
• Lesion anatomical distribution:
• Prepuce – 53%
• Urethral orifice – 29%
• Glans penis – 12%
• Penile body – 6%
• Lesions were mostly elliptical (59%) or circular (35%), with median sizes around 25 cm² (ranging widely from 9 to 117 cm²).
• Clinically, lesions appeared as ulcerative granulomatous masses with multiple draining tracts and bloody-serous discharge.
• Histopathology and immunohistochemistry confirmed Pythium insidiosum infection.
• Mycological culture and electron microscopy supported the diagnosis by identifying characteristic pathogen features.

Treatment Outcomes

• Complete surgical excision led to full recovery in 82.35% of horses.
• Recurrence occurred in 17.65% of cases, mostly related to:
• Larger lesion surface areas.
• Multiple lesions localized on the prepuce.
• Early and accurate diagnosis combined with thorough excision minimized recurrences.

Conclusions and Clinical Implications

• This study is among the most detailed characterizations of penile and preputial pythiosis in horses.
• It highlights the distinct epidemiological pattern, with a predilection for gray horses and certain genital locations.
• Combining histopathology, immunohistochemistry, culture, and electron microscopy provides confident diagnosis of Pythium infection.
• Complete surgical removal is an effective treatment, but larger or multiple lesions carry a higher risk of recurrence.
• Veterinarians should consider early diagnosis and prompt surgical management to optimize outcomes in penile/preputial pythiosis cases.