Histoplasmosis is a fatal infectious disease of tropical America resembling kala-azar of India. It is characterized clinically by splenomegaly, emaciation, irregular pyrexia, leucopenia and anemia. The pathological features are the invasion of endothelial cells in the smaller lymph and blood vessels and capillaries by enormous numbers of a small encapsulated microorganism (Histoplasma capsulatum) causing necroses of the liver with cirrhosis, splenomegaly, psudo-granulomata of the lungs, small and large intestines, with ulceration of the latter, and necrosis of lymph nodes draining the injected viscera.
The disease is caused by a small round or oval microörganism 1 to 4 µ in diameter possessing a polymorphous, chromatin nucleus, basophilic cytoplasm and achromatic spaces all enclosed within an achromatic refractile capsule.
The microorganism differs from the Leishman-Donovan body of kala-azar in the form and arrangement of its chromatin nucleus, and in not possessing a chromatin rod.