Blastomycosis is a systemic fungal infection endemic in the south central, southeastern, and Midwestern United States, and the Canadian provinces bordering the Great Lakes.(1,2,3) In 1998, two cases of occupationally acquired blastomycosis occurred in Boulder, Colorado, suggesting that Blastomyces dermatitidis may be present elsewhere in the United States in low density.(4) Blastomyces infection has also been described in Africa.(5,6) Blastomycosis should be included in the differential diagnosis of acute pneumonia occurring in the setting of prolonged exposure of soil containing high nitrogen and organic content. After inhalation of the spores, which are taken up by bronchopulmonary macrophages, there is an approximate 30- to 45-day incubation period. In the Boulder outbreak, however, the incubation period was much shorter (between 13 and 18 days), suggesting that this may be related to the inoculum size.(4) The initial response in the infected site is suppurative and progresses to granuloma formation (also referred to as a pyogranulomatous response). B dermatitidis most commonly infects the lungs, followed by skin, bone, prostate, and the central nervous system (CNS).
Unlike many of the other systemic dimorphic mycoses, blastomycosis usually occurs in healthy hosts and is frequently associated with point-source exposure.(1,2) It occurs more commonly in men than women, which may reflect a larger number of male occupational exposures. However, in more recent reports, occupational exposure accounts for a smaller proportion of cases, presumably as recreational exposures increase. Immunosuppressed patients typically develop infection following exposure to the organism, but reactivation may also occur. Blastomycosis is uncommon among people infected with HIV and is not recognized as an AIDS-defining illness. In the setting of AIDS or other marked immune suppression, the disease is usually more severe with multiple-system involvement, including the CNS, and progresses rapidly to a fatal course.
In a review of 133 Canadian patients with blastomycosis, nearly half had underlying medical conditions, but not those typically associated with marked immunosuppression.(3) Of the 67 patients with an underlying medical condition, 10% had diabetes, 9% had hypertension, 8% had asthma, and 7% had heart disease. Only 2 of the 133 patients had HIV infection. One pregnant woman with blastomycosis did transmit the disease vertically to her fetus. The overall mortality was 6.3%, and mean duration of symptoms before diagnosis was significantly shorter (25.8 ± 10.5 days) among those who died compared with survivors (70.0 ± 83.2 days).
(Also refer to references 1,2,3)
Skin tests and serologic markers are useful epidemiologic tools but are of inadequate sensitivity and specificity to be diagnostic. Diagnosis depends on direct examination of tissue or the isolation of Blastomyces in culture. In the mold form, B dermatitidis has round to pyriform 4- to 5-µm conidia attached directly to the hyphae or on short stalks. Initially the colony appears yeastlike at room temperature and then develops hyphal projections eventually becoming a fluffy white mold. The spores are difficult to isolate from the soil or bird droppings but have been recovered from wet soil. The yeast form grows as a brown, wrinkled, folded colony at 37°C. Microscopically, the yeasts appear as round, budding, thick-walled cells with a daughter cell forming a single bud that has a broad base 5-15 µm in diameter and that may be found extracellularly or within in macrophages.
Therapy for blastomycosis is determined by the severity of the clinical presentation and consideration of the toxicities of the antifungal agent. There are no randomized, blinded trials comparing antifungal agents, and there are limited data available on the treatment of blastomycosis in patients infected with HIV.
Typically pneumonia is self-limited to 1-2 weeks and does not require therapy. If the pneumonia persists or the patient develops respiratory insufficiency or pleural disease, treatment with itraconazole 200-400 mg/day orally is recommended.(2) Amphotericin B, up to 2.5 g, is recommended in life-threatening systemic disease, CNS disease, and in patients with immune suppression, including AIDS. Ketoconazole or fluconazole 400-800 mg/day for at least 6 months is effective alternative therapy for non-life-threatening disease in immunocompetent individuals with blastomycosis.(2)
In a retrospective study of 326 patients with blastomycosis, those receiving amphotericin B had a cure rate of 86.5% with a relapse rate of 3.9%, and those receiving ketoconazole had a cure rate of 81.7% with a relapse rate of 14%.(10) A prospective, open-label, randomized trial of 65 patients reported a cure rate of 100% among patients receiving ketoconazole 800 mg daily for 6 months or more compared with 79% among those receiving 400 mg daily for more than 6 months.(11) Relapse rates are in the order of 10-14% in patients taking ketoconazole.(10,11) In a multicenter, nonrandomized, open-label trial of 48 patients with blastomycosis receiving itraconazole 200-400 mg daily, the overall cure rate for patients treated for more than 2 months was 95%.(12) Itraconazole, 200-400 mg/day orally is better absorbed, has enhanced antimycotic activity, and is better tolerated than ketoconazole; hence, itraconazole is the recommended first-line azole in non-life-threatening, non-CNS disease in non-immunocompromised patients.(2,12).
Fluconazole has excellent CNS penetration and may be considered as an alternative to amphotericin B for the treatment of CNS blastomycosis.(2) In a multicenter, randomized, open-label trial of 2 doses of fluconazole among 39 evaluable subjects, the success rates were 89% for the 400-mg dose and 85% for the 800-mg dose; however, subjects with CNS disease were excluded.(13) Therefore, the efficacy of fluconazole use in patients with life-threatening or CNS disease is unknown. Because neither ketoconazole nor itraconazole penetrates the blood-brain barrier, these drugs are not recommended for CNS involvement.
Although data is limited, chronic suppressive therapy is generally recommended in patients with HIV who have been treated for blastomycosis. Fluconazole, itraconazole, and ketoconazole are all used as chronic suppressive therapy; however, given the higher relapse rate observed with ketoconazole, itraconazole is preferred. Patients with CNS disease or intolerance to itraconazole should be treated with fluconazole for chronic suppression.
Although animal data suggests that voriconazole may have activity against blastomycosis, there are no human clinical studies demonstrating efficacy.(14) In addition, data on the use of liposomal amphotericin B for blastomycosis are limited to case reports, and its use should be based upon clinical judgment and expertise.
In summary, blastomycosis is a relatively uncommon fungal infection among individuals infected with HIV and is not considered an AIDS-defining illness. There are limited data on the occurrence of blastomycosis in this population, and the optimal treatment is unknown. The Infectious Disease Society of America (IDSA) guidelines recommend that all patients with HIV and blastomycosis be treated with 1.5-2.5 g of amphotericin B, with consideration of switching to itraconazole after 1 g of amphotericin B in those individuals without CNS involvement.(2) Despite treatment with amphotericin B, mortality remains high (30-40%) during the first few weeks, and relapse is more common than in the HIV-uninfected population. It is recommended that patients with HIV remain on chronic suppressive therapy with itraconazole or, in patients intolerant of itraconazole or with CNS involvement, possibly fluconazole. Unlike histoplasmosis or cryptococcosis, there are no data on the discontinuation of chronic suppressive therapy following improved immunologic responses on antiretroviral therapy; therefore, the decision to discontinue therapy should be made on an individual basis and only with expert guidance.