Transmission of Pneumocystis carinii disease from immunocompetent contactsof infected hosts to susceptible hosts

Pneumocystis carinii organisms constitute a large group of heterogeneous at ypical microscopic fungi that are able to infect immunocompromised mammals by an airborne route and to proliferate in their lungs, inducing Pneumocyst is carinii pneumonia. This pneumonia remains a crucial epidemiological chal lenge, since neither the source of Pneumocystis carinii infection in humans nor the process by which humans become infected has been clearly establish ed. Polymerase chain reaction (PCR) assays have shown that profoundly immun osuppressed patients without pneumocystosis can be subclinically infected w ith Pneumocystis. Other PCR-based studies have suggested that healthy immun ocompetent hosts are not latent carriers of the parasite. However, recent r eports have indicated that Pneumocystis carinii can persist for limited per iods in the lungs of convalescent rats after recovery from corticosteroid-i nduced pneumocystosis, and also that immunocompetent mammals can be transie ntly parasitized by Pneumocystis carinii after close contact with hosts wit h Pneumocystis carinii pneumonia. Can transiently parasitized hosts be a so urce of infection for immunosuppressed hosts? In order to investigate this important clinical question: the ability of immunocompetent BALB/c mice, wh ich were carrying subclinical levels of Pneumocystis carinii, to transmit t he infection by the airborne route to highly susceptible, uninfected mice w ith severe combined immunodeficiency was studied. The results indicated tha t the immunocompetent mice, transiently parasitized by Pneumocystis carinii organisms after close contact with Pneumocystis carinii-infected mice, wer e able to transmit the infection to Pneumocystis carinii-free mice with sev ere combined immunodeficiency.