Question

A 57-year-old man from Central America with HIV presented to a local clinic in Panama. He had previous history of Pneumocystis pneumonia, treated syphilis, and tuberculosis for which he received 1 year of treatment. The clinic admitted him to the hospital with fever, weight loss, oral lesions, and pancytopenia. A bone marrow aspiration found yeast. He was prescribed amphotericin B by IV, but left the hospital in Panama to return to family in the US. He presented to the US clinic weak and cachectic (appearing malnourished), but in no distress. He had oral thrush and edema to his lower extremities. The culture of his bone marrow from Panama was positive for a dimorphic fungus.

a. What are dimorphic fungi? With which of these organisms is he infected? Which of these have an increased rate of dissemination with AIDS patients?

b. Does the patient’s country of origin (Panama) give any additional clue? Are there particular environments in which this organism is more likely to be found?

c. How does this organism appear at body temperature? At room temperature?

d. How does disseminated disease with this organism occur in patients who have not recently been exposed to this organism?

e. Culture is a relatively slow method of diagnosing a disseminated infection with this organism. What other methodology is clinically useful in diagnosing patients suspected of having disseminated disease with this organism?

Answer 1

a. Dimorphic fungi are fungi that can exist in the form of both mould and yeast. The infection is caused by the inhalation of spores and when these spores reach the lower respiratory tract the conidia transform into the yeast phase. The dimorphic fungi Histoplasma capsulatum and Sporothrix schenckii are the most common pathogenic fungi in patients infected with HIV.

b. H. capsulatum infection is endemic in certain areas in Panama. Histoplasma capsulatum is found in soil, often associated with decaying bat guano or bird droppings.

c. This fungus is thermally dimorphic i.e surviving at two different temperatures. At body temperature (37 °C in humans) it morphs into a yeast. At the room temperature, it grows as a brownish mycelium.

d. This patient is suffering from AIDS and hence his immunity is already low. So he could have caught an infection.

e.  Since these fungi are primarily intracellular organisms, lysis-centrifugation methods should be used to release the yeasts from histiocytes.