Answer: Cyrptococcosis

Histology: Sections of lung show a granulomatous infiltrate with histiocytes filled with small, rounded organisms, with a background of chronic inflammation. The organisms are highlighted on GMS and FM stains.

Discussion: Infection by Cryptococcus mostly occurs in immune compromised patients but cases in immunocompetent patients can also occur. The majority of the infections consist of a meningitis or meningoencephalitis. Pulmonary cryptococcal lung disease occurs in 25-55% of cryptococcal infections in patients with AIDS, mainly when the CD4 count is < 100 cells/µl. The infection is contracted by inhalation and human-to-human transmission does not occur. The response consists of granulomatous inflammation in the immunocompetent host, and can be variably severe in the immune compromised (infiltrate, acute injury, etc). Symptoms and natural history are variable depending on the immune status of the patient. The diagnosis can be made by culture (2-7 days). Presumptive diagnosis can also be made on tissue. On GMS or PAS stain the microorganism is oval shaped, variable in size and has a narrow-based budding. Mucicarmine can stain the mucoid capsule but can be negative in cases that lack a capsule. Fontana Masson can be used to stain the melanin pigment present in the cell wall of the microorganism. The latter two stains can help in the differential diagnosis with other microorganisms such as Blastomyces and Histoplasma.