Dwight A. Powell
North American blastomycosis is a pulmonary or disseminated fungal infection caused by Blastomyces dermatitidis.1,2 Although rare in children,3 the infection is often difficult to detect unless considered in the differential diagnosis. Blastomyces dermatitidis is acquired through the inhalation of spores that transform to yeast in the lungs. The vast majority of cases have occurred in the Ohio and Mississippi river basins and the southeastern United States. The highest incidence of cases appears to occur in Wisconsin, Minnesota, Mississippi, Kentucky, Tennessee, and Arkansas. In endemic areas, the annual incidence of symptomatic infection is about 1 to 2 per 100,000 population. Pockets of hyperendemic regions exist where the annual incidence of symptomatic infection may approach 40 per 100,000 population.
As many as 50% of infected individuals are asymptomatic. Most cases of symptomatic blastomycosis occur sporadically, but there are occasional reports of small outbreaks in communities over a short period of time. Infections have been reported in all age groups, including newborns.4 Human-to-human transmission is rare. The incubation period from exposure to primary disease is 21 to 106 days (median 45 days). However, latency with eventual reactivation disease is probable with the finding of newly recognized infection in individuals with no exposure to endemic areas for 3 or more years. Human-to-human transmission is rare.
The lungs are the usual portal of entry for B dermatitidis conidia. Inhaled conidia elicit an inflammatory response characterized by polymorphonuclear leukocytes (PMNs). The few conidia that survive the initial PMN phagocytosis transform to yeast, which are more resistant to phagocytosis by PMNs and alveolar macrophages. Response to the replicating yeast cells results in a mononuclear infiltrate with some granulomatous component. Spread of yeast from the lungs, although rare, may seed any body organ. Development of cell-mediated immunity is believed to be the primary mechanism in prevention of progressive blastomycosis.
Pulmonary disease is the most common manifestation of symptomatic blastomycosis. Onset is insidious, resembling a mild respiratory infection accompanied by low-grade fever, chest pain, and nonproductive cough, often with rapid resolution. As the symptoms increase in severity, spiking fevers, productive cough, and pleuritic chest pain develop. Rarely, the illness may be fulminant and resemble adult respiratory distress syndrome. Mortality is such cases may be greater than 50%.
Radiographic studies of the chest vary widely; there is no characteristic feature of blastomycosis that allows easy differentiation from other pulmonary infections. In acute cases, parenchymal infiltrates, lobar or segmental consolidation, and interstitial infiltrates have all been described. Large lobar infiltrates may mimic tumor. In the more chronic form of disease, airspace and interstitial infiltrates are seen, but masslike infiltrates, pulmonary nodules, and cavitation are more common.
Disseminated blastomycosis most frequently involves cutaneous and subcutaneous tissues as well as bone, the central nervous system, and the urogenital tract (only in adults). Cutaneous lesions, the most common extra-pulmonary manifestation, initially appear as benign, papulopustular verrucous nodules with a raised irregular border often with crusting and some drainage from an underlying abscess.5 Lytic bone lesions are the second most common extrapulmonary infection. Granulomatous lesions of the liver and spleen are found in more than 40% of patients with disseminated disease, and the kidneys, prostate, epididymis, bladder, and testes may be involved, causing dysuria, pyuria, and hematuria. Central nervous system involvement may be meningitis, but is more commonly epidural or cranial abscesses. Nearly every body organ has been reported including lymph nodes, eyes, and retropharyngeal soft tissue.
Mortality from blastomycosis is dependent upon age, immune state of the host, and the type of presenting illness. Adults older than age 65 years have a mortality rate from blastomycosis 10 times or more that of children. Although rare, blastomycosis has been documented to cause severe and often fatal infections in patients with many forms of immune deficiency.6
Although patients may present to a physician early in the course of infection, diagnosis is often delayed more than 30 days. Typically, patients receive one or more courses of antibiotics for bacterial pneumonia before blastomycosis is considered. Clinical diagnosis must be confirmed by laboratory studies, which include microscopic examination of smears, scrapings, aspirates, sputum, and bronchoscopic washings for the characteristic yeast along with fungal culture. In biopsy specimens, caseous necrosis usually is absent. Cutaneous lesions typically reveal pseudoepitheliomatous hyperplasia and budding yeasts in pyogranulomas that are characteristic and pathognomonic. Culture methods can confirm the diagnosis. Because of poor sensitivity and specificity, serologic methods such as complement fixation and enzyme-linked immunosorbent assay (EIA) usually are not helpful. Newer EIA tests, using more purified antigens, show sensitivities of 80% to 88% and specificities of 98% to 100% in proven blastomycosis. EIA titers greater than or equal to 1:32 support the diagnosis of blastomycosis. Cross-reactivity with antigens of other fungi, particularly Histoplasma, makes low titers of these serologic tests difficult to interpret. Until specific antigen or polymerase chain reaction methods are introduced, definitive diagnosis of blastomycosis will depend on finding the organism in tissue, body fluids, or culture.
The treatment of choice for life-threatening or severe disease is amphotericin B.7 Ketoconazole and itraconazole are very effective in less severe infections in adults, but the data on pharmacokinetics, safety, and efficacy of these drugs for children are insufficient. Itraconazole has fewer side effects than ketoconazole and is probably the preferred option. Small numbers of children have been treated for 6 months with a 5 to 10 mg/kg daily dose of itraconazole. However, until more data are available, patients treated with itraconazole who fail to respond within 2 to 4 weeks, who have inadequate serum concentrations, or who develop clinical deterioration, should be switched to amphotericin B.