Sporotrichosis is a mycotic disease caused by the dimorphic fungus Sporothrix schenckii. S. schenckii exists in a mycelial form at environmental temperatures (25°-30° C) and as a yeast form in body tissues (37° C). The organism is distributed worldwide and can be found preferentially in soils that are rich in decaying organic matter. It has also been isolated from barberry and rose bush thorns, spaghnum moss, tree bark, and mine timbers. The handling of stored hay bales has also been associated with outbreaks. The traditionally accepted method of acquiring sporotrichosis is via the inoculation of the infectious organism into tissues. The disease in horses is often associated with a traumatic puncture wound on the distal extremity from a thorn, wood splinter, or barbed wire. Although contamination of a puncture wound by organisms in the environment is considered an important mechanism in acquiring this disease in people as well as horses, contact exposure to cats (especially barn cats) infected with S. schenckii is now considered a significant means by which a zoonotic infection can be established. It has also been shown that the claw of the cat may be contaminated with S. schenckii, and therefore a direct puncture wound from a claw of a cat needs to be considered a potential source of infection.
Sporotrichosis: Clinical Signs
Sporotrichosis can occur in three clinical forms: cutaneous, cutaneolymphatic, and disseminated. In horses, the cutaneolymphatic form is the most common. In most cases, the infectious organism is first inoculated into the dermis or subcutaneous tissue via a traumatic injury. The distal limbs are the most common site for the development of a cutaneous nodule or several nodules. This process extends proximally up the limb and follows the lymphatics, thus resulting in the formation of additional nodules and a “cording” of the lymphatics. Several nodules often become ulcerated and drain a purulent to hemopurulent discharge. In the more chronic cases the ulcerated nodules can develop excessive granulation tissue and take on a “proud flesh” appearance. The proximal draining lymph node may be palpably enlarged and may subsequently ulcerate and drain. Less commonly, the primary cutaneous form of sporotrichosis may be single or multiple intact or draining nodules that develop on the trunk, shoulder, hip, perineum, or face. To this author’s knowledge, the disseminated form of sporotrichosis has not yet been reported in the horse.
Differential Diagnosis of Sporotrichosis
The initial differential diagnosis for the formation of a cutaneous nodule on the distal extremity should include both bacterial (especially Corynebacterium sp., and Staphylococcus sp.) and fungal (especially blastomycosis, coccidioidomycosis, cryptococcosis, and histoplasmosis) etiologies. These differentials will vary greatly by regional differences in the incidence of these infections. It is important to obtain the horse’s geographic history for at least the past year of travel. However, in the more advanced forms of these diseases, it is very rare for the lymphatics to become “corded” proximally with the formation of additional ascending individual nodules. When individual nodules on the distal extremity become ulcera-tive, the differential diagnosis should additionally include squamous cell carcinoma, cutaneous habronemiasis (“summer sores”), exuberant granulation tissue (“proud flesh”), and the fibroblastic sarcoid.
Diagnosis of Sporotrichosis
The most reliable method for confirming the diagnosis of sporotrichosis is to biopsy an intact and nonulcerated nodule and to submit a portion for both histopathologic examination and macerated tissue culture. An 8-mm biopsy punch will give a suitable sample, and the resulting core of tissue can be cut in half longitudinally. Half of the sample should be submitted for histopathologic examination and the other half for culture. It should be requested that the tissue be macerated and cultured for both bacterial and fungal organisms as confirmed cases of sporotrichosis may have a concurrent bacterial component. In instances where only ulcerated and draining nodules occur, the tissue should still be biopsied and sampled as above. This is because both culture and cytologic examination of stained exudate from draining lesions is often negative for the presence of Sporothrix organisms. Some cases of equine sporotrichosis are initially negative on macerated tissue culture. In such cases, the histopathology report indicates the presence of a deep pyogranulomatous inflammatory reaction without the presence of any infectious agent upon examination with special fungal stains. In instances in which the lesions continue to persist and do not respond to treatment for any of the bacterial pathogens that are isolated, a portion of the biopsied tissue from a nodule should be submitted to the Centers for Disease Control and Prevention (CDC) laboratory in Atlanta, Ga., for the fluorescent antibody testing (Sporothrix antigen-specific direct immunofluorescent antibody test). This procedure is considered the most sensitive test for determining the presence of Sporothrix organisms.
Treatment of Sporotrichosis
The treatment of choice for the cutaneolymphatic or primary cutaneous form of sporotrichosis is systemic iodide therapy. The organic iodides have proven superior in efficacy to the inorganic iodides in the treatment of equine sporotrichosis; ethylene diamine dihydroiodide (EDDI Equine) is the drug of choice. This product is in the form of a feed additive and can be mixed with a small amount of grain and administered at a dosage of 1 to 2 mg/kg of the active ingredient given once to twice daily for the first week. The dosage can then be reduced to 0.5 to 1.0 mg/kg once daily for the remainder of the treatment. In general, lesions will begin to regress during the first month of treatment, and treatment should be continued for at least 1 month beyond the complete resolution of all cutaneous nodules and the healing of any ulcerated lesions. Discontinuing therapy prematurely will invariably result in an unnecessary relapse of the disease.
During treatment, the horse should be closely observed for any evidence of iodide toxicity (iodism), which includes excess scaling and alopecia, serous ocular or nasal discharge, excess salivation, anorexia, depression, coughing, nervousness, or cardiovascular abnormalities. Should any of these signs develop, the treatment should be discontinued for 1 week, and the treatment should be resumed at 75% of the dosage at which the iodism was noted. In most instances, the treatment is subsequently well tolerated. In the rare instances in which iodism is a recurrent problem or the horse fails to respond to treatment with organic iodide, griseofulvin therapy may be used. Griseofulvin has been reported to be effective when administered at a dosage of 20 mg/kg given orally once daily for the first 2 weeks and is then followed by 10 mg/kg given orally once daily for 1 month beyond apparent clinical remission. Itraconazole has also been suggested as an alternative treatment for refractory cases of sporotrichosis. The recommended dosage is 3 mg/kg twice daily mixed with grain, but its expense would be a restriction to its use in most cases.
Public Health Significance
It is important to remember that the accidental inoculation or contamination of broken skin from any contaminated tissues or exudates may infect any person who comes in contact with an animal infected with Sporothrix organisms. Several reports have documented the transmission of sporotrichosis to people by contact with a contaminated wound or the exudate from an infected cat.
In some instances, infection has occurred after exposure to an infected cat, although no known preexisting injury or penetrating wound on the person was known before the disease presented. With these considerations in mind, it is advisable that people who handle horses suspected of having sporotrichosis wear disposable gloves. Afterward, they should remove the gloves carefully and wash their forearms, wrists, and hands with either a chlorhexidine or povidone-iodine scrub.