Sporotrichosis, Subcutaneous Mycosis

Sporotrichosis is a chronic, contagious disease caused by Sporothrix schenckii. The organism occurs in two forms: it grows as mold in soil and as yeast in skin. It is distributed worldwide and is particularly common in tropics, where high humidity and temperatures promote fungal growth. In the United States, this disease is seen most often in southern coastal regions, Missouri and Mississippi river valleys. Dogs and cats can spread the disease to humans. Infected individuals, for example, gardeners, may get infected through a cut or puncture wound.

Ways of Transmission of Sporotrichosis

S. schenckii is present in soil, wood, and plants. It grows particularly well in soils rich in organic material. In warm environments with high humidity, it can also grow in plants and tree bark. Most cases of sporotrichosis are acquired from the environment, as a result of contact between broken skin and fungal spores. Penetrating wounds from dead vegetation and other items such as wood splinters, sphagnum moss, thorns, or hay are most likely to become infected.

Bites, scratches, pecks, and stings from a variety of animals, birds, and insects can also inoculate the organism into wounds, via spores carried on the surface of the body. Rarely, inhalation can result in a pulmonary form of the disease.

Sporotrichosis can also be transmitted from infected animals. In cats, skin lesions contain large numbers of organisms, which can be transmitted through cuts and abrasions. S. schenckii from cats may also be able to enter the body through intact or minimally damaged skin; zoonotic cases have been reported in people with no history of trauma at the inoculation site. In addition, the organisms can be found in the mouth and nasal cavity and on the nails of infected cats, facilitating transmission in bites and scratches. They also occur in the feces. Species other than cats can also spread sporotrichosis; however, the organisms are very sparse and transmission is much less likely. One case of person-to-person transmission, which occurred after frequent, close contact between a mother and her child, has been reported.

The lesions contain organisms and all discharging lesions should be considered contagious.

Forms of Sporotrichosis

There are three forms of the infection:

Cutaneous Sporotrichosis

Infection is limited to the skin. Multiple nonpainful nodules or ulcers and crusts develop.

Cutaneous-Lymphatic Sporotrichosis

Firm round nodules form at the point of entry and progress to the tissues underneath the skin and lymph nodes.

Systemic Sporotrichosis

This form is rare. Bone, lung, liver, spleen, kidneys, testes, gastrointestinal tract and central nervous system can be affected. Disseminated sporotrichosis is usually fatal without treatment.

The cutaneous and cutaneous-lymphatic are the most common forms of the disease.

Signs of Sporotrichosis

In dogs, sporotrichosis is typically a multinodular disease; the nodules resemble those seen in other species. The lesions often occur on trunk and head but may also be present on the limbs. Cording of lymphatics may be seen. In some cases, the lesions are found in the bones, liver, or lungs rather than the skin. The nodules may drain or crust. In cats, lesions initially appear as wounds or abscesses, mimicking wounds associated with fighting; they are most commonly found on the head, lumbar region, and limbs.

Diagnosis of Sporotrichosis

In animals, sporotrichosis can be diagnosed by fungal culture or direct visual observation of the organisms in lesions or discharges.

Treatment of Sporotrichosis

Various antifungal drugs including ketoconazole, itraconazole, and amphotericin B have been used to treat sporotrichosis in animals. Supersaturated solution of potassium iodide (SSKI) can be used in the cutaneous or lymphocutaneous forms. The solution is given orally and then continued for at least one month after the nodules completely subside. Recurrence of infection is common when treatment has not been administered for long enough period of time. Antibiotics for secondary bacterial infections are a part of treatment plan.⁶

Sporothrix schenckii is susceptible to a variety of disinfectants including 1% sodium hypochlorite, glutaraldehyde, iodine, phenolics, and formaldehyde. Its susceptibility to 70% ethanol is questionable. In addition, this organism can be inactivated by moist heat (121°C for 15 minutes or longer). Sporothrix schenckii survives readily outside the host. It may be found for months or years in soil, vegetation, wood, and other objects in the environment.⁵

Prevention of Sporotrichosis

In animals, there is no practical way to prevent infections acquired from the environment. Infected animals, particularly cats, should be isolated to prevent the organism from spreading. Keeping cats indoors during feline epidemics can reduce the risk of infection; cat fights and other feline behaviors have been implicated in the spread of the disease. Among dogs, this disease is seen most often in hunting dogs or other dogs frequently exposed to thorns and splinters.

References

1. Scott, Miller, Griffin. Small Animal Dermatology
2. Greene CE. Infectious Diseases of the Dog and Cat
3. Rippon JW. Medical Mycology
4. Scott DW. Large Animal Dermatology
5. Animal Disease Information, Iowa State University
6. Canine Medicine and Disease Prevention. Animal Health Publications, 2004

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