Background
In 1894, Gilchrist first described blastomycosis in the United States. Blastomycosis is a pyogranulomatous fungal infection caused by Blastomyces dermatitidis. Serological studies that use enzyme-linked immunoassay indicate that different genotypes of B dermatitidis exist.
Blastomycosis is a common infection among dogs in endemic areas. It may serve as an indicator of human disease because of the shared environment. Blastomycosis is reported in other animals, including the horse, cow, cat, bat, and lion.
The following other eMedicine articles on blastomycosis may be helpful:
Pathophysiology
Infection occurs by inhalation of aerosolized conidial forms of the fungus from its natural soil habitat. The conidia are susceptible to phagocytosis and killing by neutrophils, monocytes, and alveolar macrophages. However, because B dermatitidis is a dimorphic fungus, the inhaled conidia transform at body temperature in lungs and tissues to the yeast phase (thermal dimorphism). This transformation provides a survival advantage to the infecting fungus as the thick cell wall of the yeasts provides resistance to phagocytosis and induces expression of an immune-modulating virulence factor (BAD1) on the cell surface. Then, the yeast forms multiply and may disseminate through the blood and lymphatics to other organs. The evoked pyogranulomatous inflammatory response has an initial influx of neutrophils, followed by macrophage and granuloma formation.[1]
Pulmonary infection may be asymptomatic in nearly 50% of patients. In others, the median incubation period, from inhalation of the fungus to manifestations of symptoms, is 45 days (range 21-106 d). Pulmonary symptoms of varying severity are common, and they most often occur without any symptoms of dissemination to other organs. Extrapulmonary dissemination more often occurs in patients with chronic pulmonary illness and in those who are immunocompromised.
Pulmonary involvement, as expected, is present in almost all cases. Rarely, an extrapulmonary site (eg, skin, bone) may be the only presenting clinical manifestation. In earlier reported case series, extrapulmonary involvement was noted in the majority of cases. However, in present times, with earlier recognition and effective treatment, the extrapulmonary manifestations are seen in only about 20% of cases.
Skin is the most common site of extrapulmonary blastomycosis and is involved in about 20% of the cases. Other areas affected, in order of frequency, are the bones (approximately 5%), prostate and other genitourinary organs (approximately 2%), and the meninges and brain (approximately 1%).[2] In rare instances, any organ can be affected, including the breast, eye, larynx, trachea, and ear. Reactivation of blastomycosis may occur after a pulmonary infection that resolved with, or without, treatment. An extrapulmonary site only rarely is a site of reactivation (eg, skin, bone, brain).
Epidemiology
Frequency
United States
Most cases of blastomycosis occur in the United States. It is endemic in the central and southeastern parts of the country (eg, near the Mississippi River, Ohio River, Great Lakes). True incidence and prevalence are unknown, because there are no reliable antigen markers for skin testing. Based on confirmed cases, the annual incidence is less than 1 case per 100,000 people in Mississippi, Kentucky, Arkansas, and Wisconsin. Within the endemic areas (eg, Vilas County, Wisconsin), infection by hyperendemic foci is reported at an annual incidence rate of 40 per 100,000.[3]
Blastomycosis should be considered in the differential diagnosis of an atypical pulmonary infection in the Great Lakes/Ohio River Valley area, even in urban areas. Significant construction, such as interstate road expansion, can release blastomycosis from the soil habitat.[4]
International
Blastomycosis is distributed throughout the world. Most reported cases outside of the United States are from Canada (Ontario, Manitoba) and the African continent . Cases have also been reported from disparate regions — Mexico, South America, Middle East, India
Mortality/Morbidity
Retrospective reports in the 1960s indicate a mortality rate of 42% in untreated blastomycosis and 5% in treated cases. Reports in the 1990s indicate a mortality rate of 0-2% in treated cases among immunocompetent patients. In contrast, the mortality rate in immunocompromised patients is 29%[5] and in the subgroup of patients with AIDS is 40%.[6] The reported mortality rate of patients presenting as adult respiratory distress syndrome (ARDS) is 68%.[7] Complications generally do not occur in immunocompetent patients, and patients can expect a full recovery.
Race
No known racial predilection exists.
Sex
Blastomycosis was thought to occur more often in males than females; however, analysis of outbreak cases from a common source and recent reports do not indicate a significant sex difference. A report from Wisconsin revealed that 55 of the 120 cases of blastomycosis were in women (nearly 1:1 male-to-female ratio).
Age
The mean age at diagnosis is approximately 45 years. Most patients are aged 30-69 years; however, persons of any age can acquire the disease. Blastomycosis also occurs in infants and in very elderly persons.[8]
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Chapman SW, Lin AC, Hendricks KA, et al. Endemic blastomycosis in Mississippi: epidemiological and clinical studies. Semin Respir Infect. Sep 1997;12(3):219-28. [Medline].
Klein BS, Vergeront JM, Weeks RJ, Kumar UN, Mathai G, Varkey B, et al. Isolation of Blastomyces dermatitidis in soil associated with a large outbreak of blastomycosis in Wisconsin. N Engl J Med. Feb 27 1986;314(9):529-34. [Medline].
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Chapman SW, Dismukes WE, Proia LA, et al. Clinical practice guidelines for the management of blastomycosis: 2008 update by the Infectious Diseases Society of America. Clin Infect Dis. Jun 15 2008;46(12):1801-12. [Medline].
Lentnek AL, Lentek IA. Successful management of Blastomyces dematitidis meningitis. Infect Med. 2006;23:39.
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