Zoonotic Sporotrichosis in Brazil Transmitted by Cats

The incidence of sporotrichosis transmitted from cats to humans has been increasing in Brazil over the past 20 years. In an article recently published in PLOS Pathogens, researchers from the Evandro Chagas National Institute of Infectious Diseases, Oswaldo Cruz Foundation (Rio de Janeiro) discuss disease pathogenesis, treatment challenges, and the potential for feline vaccine development.

Sporotrichosis is caused by fungi of the genus Sporothrix. The most common clinical manifestation is skin disease following exposure to fungal spores in the environment (usually from contaminated soil or plant matter); pulmonary and disseminated forms can occur as well. Sporotrichosis can also be transmitted through bites or scratches by infected cats. Outside Brazil, feline sporotrichosis is most often caused by Sporothrix schenckii; the cat-associated epidemic within Brazil is mostly associated with Sporothrix brasiliensis.

Although sporotrichosis is rare worldwide, an epidemic has been occurring in Brazil (particularly in Rio de Janeiro) since the late 1990s. From 1998 to 2011, the Oswaldo Cruz Foundation recorded 4188 human cases of sporotrichosis. In addition, 4703 cats were diagnosed from 1998 through 2015, and 244 dogs were diagnosed from 1998 through 2014. These numbers are from only one institution, say the authors, so the true incidence could be different.

“Due to the high incidence of feline sporotrichosis, Rio de Janeiro is presently considered hyperendemic for cat-associated sporotrichosis,” write the authors. Dogs have transmitted no known cases to humans, they say, even though the number of infected dogs is the highest documented to date. They attribute this to the low number of fungal organisms typically present in lesions in dogs.

Sporotrichosis is treated with long-term courses of antifungal drugs, usually itraconazole, amphotericin B, or potassium iodide. Genotyping has identified strains of Sporothrix resistant to itraconazole and amphotericin B, with the number of resistant strains increasing over time. “Identifying these epidemiological trends associated with the emergence of drug resistance is important to adjust antifungal therapy and to encourage the development of new drugs to treat sporotrichosis,” say the authors.

The pathogenesis of sporotrichosis in cats is still undetermined, say the researchers. The severity of the clinical presentation, which correlates with fungal load, may be associated with the cell-mediated immune response. Coinfections (with retroviruses and parasites, for instance) could affect cats’ susceptibility to Sporothrix infection, especially within Rio de Janeiro’s large stray cat population. Helminth infections induce cytokine shifts that may reduce cats’ ability to fight a fungal infection, so in theory helminth control could also improve fungal clearance.

Sporothrix organisms exist in two forms, depending on temperature: hyphae (the typical form in the environment) and yeast (more common in host tissue). Classic sporotrichosis is caused by inoculation of hyphae from the environment, say the authors, but zoonotic disease is more likely caused by the yeast form.

The researchers suggest that vaccinating cats against sporotrichosis would reduce the burden of disease in humans. Cats mount a specific humoral response to Sporothrix, and current research suggests that a Sporothrix cell wall protein could be a useful target for a feline vaccine. Results of tests using this protein with an aluminum hydroxide adjuvant have been promising, although the authors caution that a nonadjuvanted formulation would be preferable because it would reduce the risk of vaccine-associated sarcoma in cats.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.