Histoplasmosis

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Key points

  • Although histoplasmosis is highly endemic in certain regions of the Americas, disease may be seen globally and should not be overlooked in patients with unexplained pulmonary or systemic illnesses.

  • Most patients exhibit pulmonary signs and symptoms, accompanied by radiographic abnormalities, which often are mistaken for community-acquired pneumonia caused by bacterial or viral agents.

  • Once a diagnosis is considered, a panel of mycologic and non–culture-based assays is adequate to establish a

Epidemiology

Histoplasmosis is most commonly understood as intensely endemic in the Ohio and Mississippi River Valleys in the United States and much of Latin America (Fig. 1),3, 4 although it has been known for some time that other areas around the world saw cases as well.5 The HIV pandemic and the increasing use of other immunosuppressive medications, such as calcineurin and tumor necrosis factor inhibitors, has resulted in more cases of histoplasmosis and thus improved understanding of the distribution of

Pathogenesis

Infection with H capsulatum occurs by inhaling microconidia after disturbance of environmental sites containing the organism.32 Infection is usually asymptomatic in healthy individuals unless a large inoculum has been inhaled.25 In the absence of immunocompromising conditions, acute infection resolves with the development of cell-mediated immunity.33 As a consequence of production of T lymphocytes that recognize the organism, tumor necrosis factor α and interferon gamma are induced, activating

Acute Pulmonary Histoplasmosis

After inhalation of Histoplasma microconidia, a majority (90%) of patients develop subclinical, self-limited, and most often unrecognized disease (Table 1). Symptoms are more likely to manifest after high-inoculum exposures, in immunocompromised patients, in those at the extremes of age, and possibly with more intrinsically virulent strains.34, 35 The overt acute pulmonary syndrome occurs after a median incubation period of 14 days and is characterized by fever, chills, dyspnea, and cough.

Pathology

Rapid identification of H capsulatum var. capsulatum is facilitated by visualization of ovoid yeast cells measuring 2 μm to 4 μm in greatest dimension in tissue and/or body fluid specimens. The average size of H capsulatum var. duboisii (range, approximately 6–12 μm or greater), however, is often much larger than that of H capsulatum var. capsulatum, so presumptive identification should not rely solely on microscopic morphology. Budding yeast is connected at a narrow base, which helps

Treatment

Histoplasmosis resolves without treatment in most healthy individuals, in whom treatment is usually not recommended. The main exception is healthy individuals with recent exposure to a site contaminated with the organism, in whom early treatment is usually recommended to shorten the duration of illness and prevent the rare occurrence of PDH. In contrast, histoplasmosis is progressive in most immunocompromised patients, in whom PDH is highly likely and treatment is always recommended. Rare

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