Sporotrichosis is a sporadic, chronic, granulomatous disease of people and various domestic and laboratory animals caused by Sporothrix schenckii. The organism is dimorphic and forms mycelia on vegetation and in Sabouraud dextrose agar at 25°–30°C (77°–86°F) but is yeast-like in tissue and media at 37°C (98.6°F). It is ubiquitous in soil, vegetation, and timber; is distributed worldwide; and in the USA is most commonly found in coastal regions and river valleys. Infection usually results from direct inoculation of the organism into skin wounds via contact with plants or soil or penetrating foreign bodies. Disseminated disease caused by inhalation of spores is rare.
Sporotrichosis has been reported in dogs, cats, horses, cows, camels, dolphins, goats, mules, birds, pigs, rats, armadillos, and people. Zoonotic infections can occur. The cat may be the species with the greatest zoonotic potential, and transmission from cats to people has been reported without evidence of trauma. In contrast, transmission from other infected species appears to require inoculation of previously traumatized skin. The large number of organisms shed from the wound and in the feces of infected cats is believed to be responsible for the increased zoonotic potential of feline sporotrichosis. Epidemics of sporotrichosis have been reported in Brazil. Data from these studies support the importance of cats in the zoonotic transmission of the organism. Caretakers of infected cats were four times more likely to become infected than others living in the same household.
Sporotrichosis may be grouped into three forms: lymphocutaneous, cutaneous, and disseminated. The lymphocutaneous form is the most common. Small, firm dermal to subcutaneous nodules, 1–3 cm in diameter, develop at the site of inoculation. As infection ascends along the lymphatic vessels, cording and new nodules develop. Lesions ulcerate and discharge a serohemorrhagic exudate. In cats, lesions are most often seen on the head, especially on the bridge of the nose and pinnae. Although systemic illness is not seen initially, chronic illness may result in fever, listlessness, and depression. Respiratory signs may be apparent. The cutaneous form tends to remain localized to the site of inoculation, although lesions may be multicentric. Disseminated sporotrichosis is rare but potentially fatal and may develop with neglect of cutaneous and lymphocutaneous forms or if the animal is inappropriately treated with corticosteroids. Infection develops via hematogenous or tissue spread from the initial site of inoculation to the bone, lungs, liver, spleen, testes, GI tract, or CNS.
Diagnosis can be made by culture (samples obtained from unopened lesions) or microscopic examination of the exudate or biopsy specimens. In tissues and exudate, the organism is present as few to numerous, cigar-shaped, single cells within macrophages. The fungal cells are pleomorphic and small (2–10 × 1–3 μm); buds may be present and give the appearance of a ping-pong paddle. A fluorescent antibody technique has been used to identify the yeast-like cells in tissues. In species other than cats, Sporothrix organisms are often sparse in exudate and infected tissue, so that diagnosis usually requires culture. In cultures, a true mycelium is produced, with fine, branching, septate hyphae bearing pear-shaped conidia on slender conidiophores.
Itraconazole (10 mg/kg/day) is considered the treatment of choice for sporotrichosis. Treatment should be continued 3–4 wk beyond apparent clinical cure. Terbinafine has also been used successfully. Alternatively, a supersaturated solution of potassium iodide, administered PO, has been used with some success; therapy is continued 30 days beyond apparent clinical cure. During treatment, the animal should be monitored for signs of iodide toxicity: anorexia, vomiting, depression, muscle twitching, hypothermia, cardiomyopathy, cardiovascular collapse, and death. Cats are especially sensitive to iodides and the development of iodism.
Sporotrichosis is an important zoonosis, with animal-to-human transmission well documented. Strict hygiene must be observed when handling animals (especially cats) with suspected or diagnosed sporotrichosis. People in contact with infected animals should be informed of the contagious nature of the disease when therapeutic options are discussed.