Phaeohyphomycosis (also known as chromomycosis) in domestic animals is an uncommon, opportunistic infection caused by a variety of pigmented fungal species. Infections are most commonly found in the subcutaneous tissues; however cerebral and systemic infections are also described.
The pigment produced by the fungi is melanin and is thought to act as a potential virulence factor contributing to the development of infection. In some cases, the melanin production can be so intense that the lesion can be mistaken for a melanocytic neoplasm (e.g. melanocytoma or malignant melanoma).
The organisms that cause this disease (including Alternaria spp., Bipolaris spp., Cladosporium spp., Curvularia spp., Exophiala spp., Phialophora spp., and Wangiella spp.) are present worldwide and can be found in soil and vegetation with some also present on healthy skin of animals. Subcutaneous infection is typically thought to occur secondary to traumatic implantation of organic material or through contamination of pre-existing wounds.
Lesions in cats most commonly affect the face and paws; and in dogs, cattle and horses, lesions can be multifocally scattered over the body.
In most cases of subcutaneous phaeohyphomycosis there is no evidence of underlying immune compromise. Dissemination can occur in some cases and immune compromise is thought to contribute to this.
The image above is a single biopsy of a nasal mass from a 7-year-old mixed-breed dog, with histologic examination of the H&E sections confirming nasal phaeohyphomycosis.
References: Maxie, G. G. (2015). Jubb, Kennedy & Palmer's Pathology of Domestic Animals: Volume 1. Elsevier health sciences.
