Chapter 193. Histoplasmosis, Blastomycosis, Coccidioidomycosis, and Other Dimorphic Fungi

1. What are the dimorphic mycoses, and where are they endemic?

2. What are the clinical manifestations of symptomatic histoplasmosis?

3. What is the most common extrapulmonary manifestation of blastomycosis?

4. What patients are at greater risk for disseminated coccidioidomycosis?

5. What is the most common clinical presentation of sporotrichosis?

The dimorphic mycoses are fungi which exist in the environment as a mold, but adopt a yeast-like form in the warmer conditions of the infected host. They include blastomycosis, coccidioidomycosis, histoplasmosis, penicilliosis, paracoccidioidomycosis, and sporotrichosis. With the exception of sporotrichosis, these fungi are found only in certain geographic regions. However, they may stay dormant in the host, and reactivate when the host is no longer in an endemic area, making recognition of these infections more challenging.

Histoplasma capsulatum is endemic to river basins of the United States and Canada, particularly the Mississippi, Missouri, Ohio, and St. Lawrence River valleys, as well as southwestern Ontario. It is also prevalent in Central America and the Caribbean, Southeast Asia (especially Vietnam, Thailand, Indonesia, and Malaysia), and the Mediterranean basin. In the environment, H capsulatum is found in acidic, nitrogen-rich soil, conditions provided by bird or bat guano. Infections may occur when such soil is disrupted. Certain occupations, such as farming, landscaping, demolition, and construction, or recreational activities, such as spelunking, may lead to high-inoculum exposures and outbreaks.

Clinical Manifestations

In the vast majority of cases, persons infected with H capsulatum are asymptomatic or have only a very mild illness that is never recognized to be histoplasmosis. (These patients may be noted later in life to have pulmonary and splenic calcifications on imaging studies, consistent with remote histoplasmosis.)

A variety of clinical syndromes are seen in symptomatic patients. It is estimated that < 1% of infected patients develop symptoms, although the manifestations can be diverse (Table 193-1). It typically presents as a self-limited pneumonic illness, characterized by chest discomfort and dry cough, with fever, fatigue, headache, myalgias, and arthralgias. Radiographically, patchy lung infiltrates involving one or more lobes is seen, frequently accompanied by mediastinal and hilar lymphadenopathy. In most patients, improvement is prompt even without treatment, although fatigue may persist. Most patients who seek medical attention are initially misdiagnosed with community-acquired pneumonia syndrome due to atypical organisms such as Mycoplasma or Chlamydophila. These patients usually improve without antifungal treatment. A minority of patients with acute self-limited pulmonary histoplasmosis develop extrapulmonary manifestations, such as erythema nodosum, self-limited symmetrical polyarticular arthritis, or sterile exudative pleural or pericardial effusions. Severe acute pulmonary histoplasmosis may occur, especially in patients with impaired cell-mediated immunity, and occasionally in persons exposed to large inocula. It features the abrupt onset of fever, chills, cough, and dyspnea, with diffuse, bilateral reticulonodular or miliary infiltrates; intrathoracic lymphadenopathy may or may not be present. Acute respiratory distress syndrome may ensue.