Endemic Mycoses

Overview
  • Endemic Mycoses are caused by dimorphic fungal species that are ubiquitously present in the soil and environment of certain geographic regions. Consequently, infections tends to be an endemic feature of certain parts of the world. The primary culprit agents are Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides immitis.
Epidemiology
  • Overview:
    • Because culprit fungi tend to occupy certain geographic niches around the world, diseases tend to occur in particular regions.
  • Histoplasma capsulatum:
    • Histoplasmosis is found throughout the world especially in humid areas. Within the US, Histoplasmosis is most frequently found in the Ohio and Mississipi river valleys.
  • Blastomyces dermatitidis
    • Blastomycosis occurs in North America with a geographic distribution similar to Histoplamosis.
  • Coccidioides immitis
    • Coccidiomyocosis occurs largely within the Western Hemisphere. In the US, it tends to occur in the desert south west.
Immune Response
  • Culprit organisms generally grow as mycelia in the environment and release spores which deposit in an individual's lung. Alveolar macrophages rapidly phagocytose the spores which then germinate inside the cells and begin to replicate inside phagosomes. In most cases, alveolar macrophages are able to kill intracellular organisms following stimulation with Interferon-gamma from antigen-specific Th1 subtype CD4+ T-cells. Consequently, cell-mediated immunity is critical for proper control of endemic mycoses and in many ways resembles the immune response to Mycobacterium tuberculosis. When inoculating doses are large, granulomas are formed around foci of infection to contain the organism.
Clinical Consequences
  • Asymptomatic Infection
    • In the vast majority of individuals, host defense is capable of clearing any successfully germinated spores and so inoculation occurs without any further clinical consequences
  • Pulmonary Syndrome
    • In some healthy, immunocompetent patients who are exposed to large inoculating doses, a pneumonia accompanied by a flu-like syndrome may occur. Disease usually manifests with constitutional symptoms such as fever, headache, and myalgia together with a cough. Chest radiography often reveals nodular pulmonary infiltrates representing coalescent granulomatous inflammation together lymphadenopathy of the hilar lymph nodes. Following resolution, these granulomatous nodules may fibrose and calcify over years, remaining visible on chest radiography.
  • Disseminated Disease
    • Systemic dissemination of fungi largely occurs only in immunocompromised patients with reducedcell-mediated immunity such as the elderly or AIDS Patients. These organisms can seed a variety of organs and disease can be life-threatening.
Treatment