Nasal rhinosporidiosis in the dog.

Rhinosporidiosis was diagnosed in six dogs from the southeastern United States. All six dogs had unilateral nasal polyps with multiple small white sporangia visible beneath the surface. Microscopically, the polyps consisted of organisms and fibrovascular tissue with a surface of columnar or squamous epithelium. Juvenile sporangia were unilamellar, 15-75 microns in diameter, nucleated, and accounted for about 65% of sporangia seen. Approximately 5% of the sporangia were in intermediate stages of maturation, were bilamellar, 100-150 microns in diameter, and contained immature endospores. Mature sporangia comprised about 30% of the total, were usually unilamellar, 100-400 microns in diameter, and contained a mixture of immature and mature endospores. The inner layer of the wall of the intermediate sporangia and the single wall of the mature sporangia were argyrophilic and carminophilic. Ultrastructurally, the earliest stage contained a nucleus and many ribosomes, lipid droplets, and phagolysosomes. Maturing sporangia contained discrete membrane-bound, round clevage products. These structures subsequently matured to spores, each of which had a wall and contained a nucleus and many lipid droplets. The organism from one dog was cultured and grown in vitro for 7 months and is the first successful cultivation of Rhinosporidium seeberi.