Prevention of Infection Due to Pneumocystis spp. in Human Immunodeficiency Virus-Negative Immunocompromised Patients

Pneumocystis was initially identified in the lungs of rats as a stage in the life cycle of Trypanosoma cruzi (in 1909 by Chagas and in 1910 by Carini). The first case of Pneumocystis infection in humans was described by van der Meer and Brug in 1942 (167); Jirovec has been credited with describing epidemic in- fection in humans in the 1950s (85). Pneumocystis carinii was thought to be a protozoan parasite based on morphologic appearance, proposed life cycle, and antimicrobial susceptibil- ities. Subsequent phylogenic analyses using rRNA sequences suggested that the organism was more closely related to the fungi despite the absence of ergosterol in the cell wall (34). Animal models of Pneumocystis pneumonia have been highly predictive of the clinical experience with human infection and are extensively used in studies of disease pathogenesis and therapy. However, some molecular and immunologic studies suggest that Pneumocystis associated with human disease is distinct from the strains found in animal models. This distinc- tion has resulted in a suggested reclassification of organisms isolated from humans as Pneumocystis jiroveci while P. carinii remains the designation of rodent-derived organisms (161). The use of this name remains controversial (72). Some back- ground information about the biology of infection is useful in considering strategies for the prevention or treatment of Pneu- mocystis pneumonia. This review refers to both P. carinii pneu- monia and P. jiroveci pneumonia as PCP (161).