Rhinosporidiosis is a chronic, nonfatal, pyogranulomatous infection, primarily of the nasal mucosa and occasionally of the skin of horses, cattle, dogs, cats, and aquatic birds, caused by the fungus Rhinosporidium seeberi. Uncommon in North America, it is seen most often in India, Africa, and South America. The organism has not been cultured, and its natural habitat is unknown. Trauma may predispose to infection, which is not considered transmissible.
Infection of the nasal mucosa is characterized by polypoid growths that may be soft, pink, friable, lobulated with roughened surfaces, and large enough to occlude the nasal passages. The cutaneous lesions may be single or multiple, sessile or pedunculated. The nasal polyps and cutaneous lesions have a granulomatous, fibromyxoid inflammatory component and contain the fungal organism.
Rhinosporidiosis may be confused with other granulomatous lesions of the nasal mucosa and skin, including aspergillosis, entomophthoromycosis, “nasal granuloma,” and cryptococcosis. Microscopic demonstration of spherules (sporangia) of R seeberi in biopsy specimens confirms the diagnosis. The spherules may be numerous, vary in size (up to 300 μm), have thick walls that stain periodic acid-Schiff positive, and contain endospores 4–19 μm in diameter. Developing stages of varying size without spores are distributed throughout the lesion.