PNEUMOCYSTIS CARINII

Pneumocystis cnrinii is generally believed to be a protozan occurring in two forms: a trophozoite of 1.5 m p diameter and cysts of about 7.0-mp diameter. Cysts are easily isolated from lung, but in the 60 years since the agent was described by Chagas, no one has been able to propagate the agent in vitro, and its exact nature remains to be delineated. Available evidence indicates that (1) Pneumocystis corinii is a pathogen of low virulence endemic throughout temperate and, probably, tropical climates, up to altitudes of 5,000 feet, (2) its only known habitat is the respiratory tract, particularly lungs of rodents, dogs, and subhuman and human primates, although congenital involvement of the lungs of newborn infants indicates that the maternal genital tract must ev ritually be excluded as a source harboring the agent, (3) while many subclinical involvements exist, the slow accumulation of organisms over a period of several weeks to fill the pulmonary air spaces may terminate in asphyxia1 death of the susceptible host, a state termed pneumocystis pneumonia, and (4) pneumocystic pneumonia tends to occur in microepidemics in circumscribed, often institutional populations, with a morbidity estimated as high as 80% .l These are characteristics of an opportunistic infection.