Introduction

Blastomycosis is a systemic fungal infection caused by the dimorphic pathogen Blastomyces dermatitidis. B. dermatitidis grows as a mold in moist soil or vegetation that, under certain conditions, releases spores that can be inhaled by nearby animals or people. Once they enter the lungs, the spores transform into large thick walled broad-based budding yeast that multiply within the lung and other tissues of the infected animal. Thus, the dimorphic state of B. dermatitidis is defined by its growth as a mold in the environment and a yeast in tissue. If it continues to multiply in the lung, blastomycosis can cause significant pulmonary disease (pneumonia) in animals and people. The yeast also have a tendency to disseminate to other sites in the body, particularly to the skin, eyes, and joints in dogs, and sometimes to the skin in people.

Like other systemic dimorphic fungal pathogens (i.e., Histoplasma capsulatum and Coccidoides immitis), B. dermatitidis is found in specific endemic areas in North America. Although infection has been reported from a variety of states and regions, the greatest prevalence appears to be in the upper Midwest, and extends southward along the Mississippi and Ohio River basins.

An example of a location where B. dermatitidis was isolated.

The precise environmental conditions related to this distribution are not clear, but epidemiologic studies indicate that acquisition of blastomycosis often involves residence, or time spent vacationing near, a body of water. Within endemic areas in Wisconsin, one can find hot spots in which the incidence of infection appears to be far greater than in other areas in the state. This suggests that there may be microclimate conditions that are necessary for multiplication or sporulation of B. dermatitidis, or that facilitate its transmission to animals and people entering the area.

Unfortunately there is no convenient or reliable means of detecting B. dermatitidis in the soil. Not can it be eliminated from soil following diagnosis of infection in dogs or people. The spores are probably only present for a short period of time, thus accounting for the mysterious appearance of clinical cases of blastomycosis in dogs and people.

In people, it appears that at least some cases of blastomycosis can resolve spontaneously without therapy. This is similar to histoplasmosis and coccidioidomycosis, in which only a small proportion of those infected with the fungus go on to develop significant disease. The available data for blastomycosis are more limited, but are consistent with the hypothesis that only some proportion of humans that are infected with B. dermatitidis will develop severe disease that requires antifungal therapy. Immunodeficiency (such as HIV infection) can increase the severity of the disease and might lead to more severe complications, such as adult respiratory distress syndrome, in some individuals.

	A dog paw with a leison caused by B. dermatitidis

The domestic animal species most often afflicted by blastomycosis is the dog. Infection has been reported in cats, and other species, but it is rare. The reasons for this are not clear. Perhaps it relates to resistance of the host, exposure to the organism in the environment, or some physiological need of the fungus that is not satisfied by the tissues of these other mammalian species. Although one typically associates canine blastomycosis with large hunting and working breeds of dogs, all breeds of dogs are susceptible to blastomycosis. Evidence suggests that both male and female dogs are affected in roughly equal numbers in endemic areas. For more extensive information on the epidemiology of canine blastomycosis the reader is referred to excellent articles on the subject.

When blastomycosis disseminates in dogs or people, antifungal therapy is necessary. Otherwise, the disease will progress and can be fatal. Antifungal therapy presents problems to the human or veterinary clinician. There are a limited number of antifungal drugs, all of which require prolonged treatment, are very expensive, and pose significant toxicity problems. At present, the preferred treatment of canine blastomycosis at the University of Wisconsin-Madison Veterinary Medical Teaching Hospital is with the triazole drug itraconazole. Other clinicians have reported success using fluconazole (a different triazole), ketoconazole with amphotericin-B, or amphotericin-B alone. The latter drug presents risks because of its toxicity for the kidney. Therefore, renal function must be monitored carefully during amphotericin-B therapy. Amphotericin-B also presents problems because it cannot be given orally, is poorly soluble, and must be administered slowly by the intravenous route. Itraconazole, fluconazole, and ketoconazole can be given orally, and hence are much more readily administered, particularly to canine patients.