This organism is a dimorphic fungus, existing as a mold in the cold (20C) and yeast in the heat (37C). It can be detected in the serum or urine of patients using both serum and urine antigen detection tests.
The yeast form is 2-5 um and found within macrophages in the body.
Histoplasma are intracellular pathogens. Therefore the yeast form can be found within macrophages in the body. A common mnemonic used to remember this fact is “histo hides” (in macrophages).
Histoplasma capsulatum is found in Starling bird feces and contracted by inhalation of the spores.
Histoplasma capsulatum is found in bat feces and contracted by inhalation of the spores.
Because this organism is contracted via inhalation of spores found in bat and bird feces, it is commonly associated with entrance into caves.
This disease is most prevalent in the United States in the Mississippi and Ohio River Valleys.
Symptoms of this infection vary but the disease primarily affects the lungs, causing pneumonia. Pneumonia is an inflammatory condition of the alveoli in the lungs, associated with fever and respiratory symptoms. Chest X-ray will reveal consolidations and often bilateral hilar lymphadenopathy.
Histoplasma infections produce granulomas as the body attempts to wall-off the infection. These granulomas can undergo caseating necrosis and can form large areas of consolidation.
Erythema nodosum is inflammation of the fat cells beneath the skin. It is characterized by red tender nodules or bumps on the anterior surface of the shins. Histoplasma can be associated with erythema nodosum. In disseminated disease, other findings can include hepatosplenomegaly, pancytopenia, or oral ulcers.
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