Sporotrichosis
Basic Information 
Epidemiology
Associated Conditions and Disorders
• Sporadic infection affecting a number of susceptible hosts, including horses, mules, cattle, dogs, cats, rats, mice, domestic fowl, and humans.
• Similar to horses, the most common form of sporotrichosis in humans is cutaneolymphatic.
• In dogs, the most common forms are cutaneous and cutaneolymphatic; in cats, the disseminated form occurs in addition to the other two.
Clinical Presentation
Physical Exam Findings
• Primary lesions include alopecic nodules and plaques that ulcerate.
• Satellite lesions develop near the primary site.
• The infection spreads via lymphatic vessels, which become thickened, with formation of nodules and punctate ulcers along the lymphatics; pus (resembling rust) often drains from the ulcerated nodules.
• Regional lymph nodes rarely become enlarged or ulcerated.
• The primary cutaneous form, with no lymphatic involvement, is a much less common presentation.
Etiology and Pathophysiology
• Sporothrix schenckii is an ubiquitous dimorphic fungi.
• Infection is acquired from wound contamination with soil and organic debris from puncture by plant thorns or wood splinters.
• The mycelial form is inoculated into tissues. The fungus changes to the yeast form and slowly proliferates, extending along the lymphatic vessels and causing them to become thick and corded.
• Multiplication of the organism is associated with pyogranulomatous inflammation (although organisms are rarely seen inside neutrophils and macrophages in exudates of horses), leading to the formation of nodules associated with enlargement of lymphatic vessels. Nodules become ulcerated, resulting in a small amount of drainage of thick, red-brown to yellowish exudates or serosanguinous fluid.
• Most lesions develop on distal extremities and rarely affect the upper parts of the body, such as upper forelimb, chest, shoulder, hip, and perineum.
• Infection rarely spreads to regional lymph nodes.
• Dissemination through lymphatics into other organs is extremely rare in horses.
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