Blastomyces dermatitidis is a dimorphic fungus capable of infecting canines and humans. This organism is endemic to the eastern third of the United States. In the fungal phase (saprophytic phase) this organism is found in moist, low pH soils, animal wastes and decaying vegetation where its hyphae produce 2 to 10 micron smooth-walled conidia. Humidity promotes the release of conidia. Virulent strains of blastomyces express a protein adhesin factor which adheres to respiratory epithelium.Inhalation of these conidia results in colonization of the terminal bronchiole where the thick-walled yeast conidia are transformed into 8 to 15 microns that reproduce by single, broad-based buds. These organisms elicit a localized pyogranulomatous inflammatory reaction followed by systemic dissemination to lymph nodes, skin, eyes, mammary gland, testes, bone, brain, and viscera. In these locations, nodular tubercle-like lesions contain low numbers of organisms composed of mononuclear cells and neutrophils. Clinical symptoms in the animal include skin lesions, respiratory distress with fever, depression, anorexia, and weight loss. Ocular lesions are common in dogs and may be the reason for initial presentation. Prognosis depends on the severity of respiratory disease and which organ systems are involved at the time of treatment initiation. Animals with multiple organ systems or brain involvement typically die within a few months. Fifty percent of dogs with severe lung disease die during the first seven days of treatment. Others exhibit worsening of symptoms within the first two to three days of treatment, secondary to the inflammation elicited by dying fungal organisms. Animals with mild posterior segment involvement and incomplete retinal detachment have a good chance of retaining eyesight. Animals with severe eye involvement will commonly develop glaucoma which will require enucleation. The fungal disease can be transmitted from animals to humans via bite wounds or contamination of an open wound, but this is uncommon, of greater concern is exposure and via inhalation of conidia from fungal cultures, from the point of source which the animal acquired its infection from the environment, or from brushing an animal with conidia in its coat. The canine has a shorter prepatent period than humans, thus dogs serve as sentinels for environmental Blastomyces dermatitidis reservoirs. These reservoirs commonly are close to waterways or excavation sites. Currently there is no preventive for blastomycosis. Consult with your local veterinarian to develop a treatment plan.