The fungus grows in soil and in the material which is contaminated with bird or bat dropping . the fungus has been found in poultry house litter, caves , areas harboring bats and bird roosts particularly those of starlings. The fungus is thermally dimorphic in the environment and it grows as a brownish mycelium and at body temperature at 37 degree in humans it morphs into the yeast. Histoplasmosis is not spread to others but it is contracted by inhalation of the spores from disturbed soil. The inoculum is represented principally by microconidia. These are inhaled and reach the alveoli. In the alveoli, macrophages ingest these microconidia. They survive inside the phagosome. As the fungus is thermally dimorphic, these microconidia are transformed into yeast.
They grow and multiply inside the phagosome. The macrophages travel in lymphatic circulation and can spread the disease to different organs. Within the phagosome, the fungus has an absolute requirement for thiamine. Cell mediated immunity for histoplasmosis develops within 2 weeks.
Within the phagosome, the fungus has an absolute requirement for thiamine Cell-mediated immunity for histoplasmosis develops within 2 weeks. If the patient has strong cellular immunity, macrophages, epithelial cells, and lymphocytes surround the organisms and contain them, and eventually calcify. In immunocompromised individuals, the organisms disseminate to different organs such as bone, spleen, liver, adrenal glands, and mucocutaneous membranes, resulting in progressive disseminated histoplasmosis. Chronic lung disease can manifest.