Cutaneous alternariosis microscopically mimicking blastomycosis.

A 57-year-old man status post several myocardial infarcts and heart transplantation presented with a slowly growing violaceous plaque on his lateral left knee at the site of prior minor trauma. A biopsy revealed a suppurative dermatitis with associated pseudocarcinomatous epithelial hyperplasia. There were multiple non-pigmented eosinophilic organisms with clear cytoplasmic halos within the infiltrate. A methenamine silver stain showed round to ovoid organisms of slightly variable size. Rare uni-polar budding, some of which was broad based, was apparent. A few short hyphae with indeterminate septa were also noted. Fontana-Masson, mucicarmine, Alcian blue and Fite stains were all negative. These findings suggested a diagnosis of blastomycosis. However, a fungal culture grew colonies of Alternaria species. Alternariosis has been previously shown to possess morphologic characteristics that can simulate other fungal infections. To our knowledge, a striking similarity to blastomycosis, as seen in our case, has not been previously reported. Dermatopathologists should be aware that alternariosis may mimic blastomycosis, especially when hyphal forms are rare or absent in tissue specimens. Culture is necessary for definitive classification.