Evidence of horizontal transmission of Pneumocystis carinii pneumonia in simian immunodeficiency virus-infected rhesus macaques.

This retrospective study used sensitive immunohistochemical methods to detect Pneumocystis carinii infections in fixed lung tissues collected from 107 simian immunodeficiency virus (SIV)-infected and 10 noninfected rhesus macaques during a 4-year period. P. carinii were detected in 51% of 85 terminally ill SIV-infected macaques but in only 2 of 22 macaques killed at earlier stages of SIV infection. P. carinii were not detected in any SIV-infected macaques held in isolators or in uninfected controls. Infection rates varied significantly between rooms, and the percentage of clinically important P. carinii infections increased from 0 in the first 2 years to > 50% during the final year. Infections were centered on terminal airways in 59% of infected animals. These results challenge the assumption that P. carinii pneumonia (PCP) results primarily from reactivated latent infections and suggest instead that horizontal spread of infection is important in the epidemiology of PCP in immunosuppressed macaques.