Blastomycosis: A Case Study of a Dimorphic Fungal Disease

Clinical Laboratory Science, Summer 2004 by Boswell, Elizabeth, Aziz, Hassan

A 17-year-old male presented to a local hospital with symptoms of pneumonia and a hacking cough productive of yellow sputum. Direct examination of a bronchial lavage sample revealed organisms morphologically consistent with Blastomyces dermatitidis, which was confirmed by culture. The patient was placed on intravenous antifungal therapy and his condition improved dramatically.

OBJECTIVES: to describe the gross and microscopic morphologies of both yeast and mold forms of B. dermatitidis; to identify B. dermatitidis given patient history, and microscopic and colony morphology; to describe the symptoms of primary pulmonary infections caused by B. dermatitidis, and to name additional tissues typically affected by the systemic pathogen.

ABBREVIATIONS: AIDS = acquired immunodeficiency syndrome; ANA = antinuclear antibodies; HSV = herpes simplex virus; KOH = potassium hydroxide; RBC = red blood cell; WBC = white blood cell.

INDEX TERMS: blastomycosis; Blastomyces dermatitidis; dimorphic fungal disease; systemic fungi.

Clin Lab Sci 2004;17(3):145

A 17-year-old male presented to a large medical center with a three-week history of fever, malaise, and shortness of breath with a hacking cough productive of yellow sputum. He stated that he had been diagnosed with bacterial pneumonia by another local medical facility ten days previously. His symptoms persisted during this time despite a course of outpatient antibiotics. Laboratory results on admission are depicted in Table 1.

Two sputum collections from the patient failed to produce an acceptable specimen for culture. A bronchial lavage was performed when his clinical picture showed little improvement following a week of antibiotic therapy in the hospital. The resulting specimen yielded light growth of normal oropharyngeal flora and no acid-fast bacilli, Mycoplasma sp., Legionella sp., or viruses. Many broad-based budding yeast cells were seen on a fluorescent potassium hydroxide (KOH) combined with calcofluor white preparation of the bronchial lavage. After three days of proper incubation, fungal culture revealed heavy growth of a broad-based budding yeast identified as B. dermatitidis, confirming the disease etiology as pulmonary blastomycosis.

BLASTOMYCOSIS: A DIMORPHIC FUNGAL DISEASE

The disease blastomycosis was first characterized in the 1890s by Dr T Caspar Gilchrist.1 He described a patient with disfiguring skin lesions originally attributed to tuberculosis, though yeast and not tubercle bacilli were present in a biopsy. The organism grew as a mold in culture when incubated at 25 �C to 30 �C, while inoculation with affected tissue led to the development of similar yeast-containing lesions in animals. Dr Gilchrist morphologically differentiated the yeast form of Blastomyces dermatitidis from Candida sp. through its broad-based budding and large size.2

B. dermatitidis is categorized as one of the true systemic fungal pathogens. It is a dimorphic organism that grows as yeast or spherule forms in humans and culture at 37 �C, while producing a mold form in the external environment and culture at 25 �C to 30 �C. The fungus is capable of causing disseminated infection in immunocompetent hosts and is endemic to distinct geographic regions. In the United States, blastomycosis is mainly distributed throughout the Mississippi, Missouri, and Ohio River valleys in addition to North Carolina and the Great Lakes states. Canada, Africa, and the Middle East are also endemic zones for the fungus. Blastomyces sp. has been recovered from soil and other natural environments, but correlation between environmental exposure and development of disease is often difficult. Specific conditions support the organism's growth, including high temperatures and moist or wet soil enhanced by decaying vegetation. Other dimorphic fungal organisms include Hutoplasma capsulatum, Coccidioides immitis, and Paracoccidioides brasiliensis.

PATHOGENESIS

Humans typically contract B. dermatitidis by inhaling infectious conidia in the environment. Direct cutaneous inoculation is an infrequent source of disease and may result from an animal bite or improper handling of clinical samples or cultures that contain the organism. Like other systemic mycoses, the infection is most prevalent in middle-aged men. This finding presumably correlates to their greater occupational and recreational exposure to soil.3 Upon inhalation, Blastomyces conidia are transported to the lungs, where they convert to yeast forms and elicit an acute inflammatory reaction. During this primary phase, fungal dissemination may occur through the bloodstream and lymphatics. The cellular immune response typically produces granulomas in affected areas of the bronchopulmonary tree.4 Blastomycosis develops after an incubation period of 30 to 45 days, although it is speculated that at least 50% of infected individuals remain asymptomatic.5

In contrast to the clinical picture seen with the majority of fungal infections, most patients with blastomycosis are healthy with no history of immunosuppression. For this reason, Blastomyces is not considered an opportunistic pathogen, even though the infection appears more likely to disseminate and prove fatal in immunodeficient patients. In a study of 100 cases of pulmonary blastomycosis, four of 84 immunocompetent patients and three of 16 immunocompromised patients died of the disease.4