Clinical Information

Blastomyces dermatitidis and Histoplasma capsulatum are dimorphic fungal agents with increasingly overlapping endemicity throughout the Midwestern, South Central, and Southeastern United States, particularly in regions around the Ohio and Mississippi River valleys, the Great Lakes, and the Saint Lawrence River. These agents are also found in regions of Canada.

These 2 fungi maintain a yeast form in the host at body temperature but are maintained as molds in the environment, which release spores that are inhaled by individuals leading to infection. Through phylogenetic analysis, B dermatitidis has been separated into 2 distinct species: B dermatitidis and Blastomyces gilchristii, both able to cause blastomycosis. B dermatitidis infections are frequently associated with dissemination, particularly in older patients, smokers, and immunocompromised hosts, while B gilchristii has primarily been associated with pulmonary and constitutional symptoms. Additional species of Blastomyces have recently been discovered and characterized; however, the performance characteristics of this assay for these species are unknown.

Approximately half of patients infected with Blastomyces will develop symptoms that are frequently nonspecific, including fever, cough, night sweats, myalgia or arthralgia, weight loss, dyspnea, chest pain, and fatigue. Symptoms may appear anywhere from 3 weeks to 3 months following infection.

For Histoplasma infections, clinical manifestations are largely dependent on the fungal burden at the time of exposure and the patient's underlying immune status. While the vast majority (>90%) of exposed individuals will remain asymptomatic, individuals seeking medical attention can present with a diverse set of symptoms ranging from a self-limited pulmonary illness to severe, disseminated disease. Individuals at risk for severe infection include those with impaired cellular immunity or have undergone organ transplantation, are HIV positive, or have a hematologic malignancy.

Diagnosis of blastomycosis and histoplasmosis relies on a combination of assays, including culture and molecular testing performed on appropriate specimens, and serologic evaluation for both antibodies to, and antigen released from, the organism. Although culture remains the gold standard method and is highly specific, these organisms can take several days to weeks to grow, and sensitivity is diminished in cases of acute or localized disease. Similarly, molecular testing offers high specificity and a rapid turnaround time, however sensitivity is imperfect. Detection of an antibody response to Blastomyces or Histoplasma offers high specificity; however, results may be falsely negative in patients who are acutely ill or are immunosuppressed.