CHAPTER 52 Sporotrichosis
Sporotrichosis is a chronic, progressive, lymphocutaneous infection of horses and other animals caused by Sporothrix schenckii. The disease in horses and mules was first described in 1909.1
ETIOLOGY
Sporothrix schenckii is a dimorphic fungus, growing in a yeast form in tissue and in culture at 37° C (98.6° F) but as a filamentous fungus at 30° C (86° F).2,3 The mycelial form has fine, septate, branching hyphae that carry ovoid roseate conidia. The organism converts to the yeast form at 37° C when grown on blood agar in 10% carbon dioxide. Yeast colonies are cream colored, moist, and smooth. S. schenckii is a common saprophyte of decaying plant material but also can live on sphagnum moss and other living plants.
EPIDEMIOLOGY AND PATHOGENESIS
Sporotrichosis is an uncommon disease of people, horses, and other animals.4,5 The organism and disease occur worldwide but are more common in tropical and subtropical climates.2 Disease occurs when the fungus is traumatically inoculated into the dermis. Although infection can theoretically be passed between animals, no case of horse-to-human transmission has been reported.2 After 3 to 5 weeks,2 the infection establishes in the skin and subcutis and can spread in lymphatics to other parts of the skin, with resultant lymphangitis. Proximal lymph nodes may become involved in this manner, and rarely the infection can disseminate to other organs. Virulence factors are not well understood but may correlate with thermotolerance in culture.5 Agglutinins to the yeast form occur during infection and are detected in various diagnostic tests.6
CLINICAL FINDINGS
Lesions most often begin on the distal limb, although the face, neck, or torso can also be the initial site of infection3,6–9 (Fig. 52-1). There are single to multiple, firm, well-demarcated (0.5-5 cm in diameter), nonpruritic, nonpainful cutaneous nodules. Lesions enlarge slowly and usually ulcerate and drain a creamy red-brown to yellow purulent discharge. The infection may stay localized or spread along lymphatics (see Fig. 52-1
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