Histoplasmosis

Histoplasmosis (Darling’s Disease) is an endemic mycosis. The etiologic agent Histoplasma capsulatum (Teleomorph: Ajellomyces capsulatus) is a dimorphic fungus present in human and animal tissue as intracellular unipolar budding yeast cells. In the environment, the fungus is present as filamentous fungi producing typical echinulate macroconidia and microconidia. The current taxonomic position of the erstwhile H. capsulatum consists of five cryptic species: Histoplasma capsulatum sensu stricto, Histoplasma mississippiense, Histoplasma suramericanum, Histoplasma ohiense, and Histoplasma capsulatum var. duboisi and other phylogenetic lineages. Human histoplasmosis frequently occurs in river valleys of North America, Central America, parts of South America, including Brazil, and India. Amount of inhaled conidia, host immune competence, age, lung anatomy, and physiological status are factors determining the disease extent and pathology. Most persons who are infected remain asymptomatic or suffer from mild acute disease. Rarely, disseminated, chronic cavitary, or isolated organ-specific forms of the disease ensue. Complications include mediastinal granulomatosis, fibrosing mediastinitis, broncholithiasis, pericarditis, and pleural effusion. Histoplasmosis has been recorded in both wild and domestic animals. Infections are transmitted by inhalation, inoculation, or rarely ingestion of microconidia and are often associated with human activities causing disruption of soil and wooded areas contaminated with bat or bird guano. Diagnostic modalities include histopathology, culture, antigen detection, and serological testing. Modern molecular testing is making diagnosis rapid and more sensitive. Treatment is by antifungal agents such as amphotericin B and itraconazole.