Eur J Clin Microbiol Infect Dis (2000) 19 : 671–678 Q Springer-Verlag 2000 Article Transmission of Pneumocystis carinii Disease from Immunocompetent Contacts of Infected Hosts to Susceptible Hosts A. Dumoulin, E. Mazars, N. Seguy, D. Gargallo-Viola, S. Vargas, J.C. Cailliez, E.M. Aliouat, A.E. Wakefield, E. Dei-Cas Abstract Pneumocystis carinii organisms constitute a large group of heterogeneous atypical microscopic fungi that are able to infect immunocompromised mammals by an airborne route and to proliferate in their lungs, inducing Pneumocystis carinii pneumonia. This pneumonia remains a crucial epidemiological challenge, since neither the source of Pneumocystis carinii infection in humans nor the process by which humans become infected has been clearly established. Polymerase chain reac- tion (PCR) assays have shown that profoundly immunosuppressed patients without pneumocystosis can be subclinically infected with Pneumocystis. Other PCR-based studies have suggested that healthy immunocompetent hosts are not latent carriers of the parasite. However, recent reports have indicated that Pneumocystis carinii can persist for limited periods in the lungs of convalescent rats after recovery from corti- costeroid-induced pneumocystosis, and also that immunocompetent mammals can be transiently parasitized by Pneumocystis carinii after close contact with hosts with Pneumocystis carinii pneumonia. Can transiently parasitized hosts be a source of infection for immunosuppressed hosts? In order to investigate this important clinical question, the ability of immunocompetent BALB/c mice, which were carrying subclinical levels of Pneumocystis carinii, to transmit the infection by the airborne route to highly susceptible, uninfected mice with severe combined immunodeficiency was studied. The results indicated that the immunocompetent mice, transiently para- sitized by Pneumocystis carinii organisms after close contact with Pneumocystis carinii-infected mice, were able to transmit the infection to Pneumocystis carinii-free mice with severe combined immunodeficiency. A. Dumoulin, E. Mazars, N. Seguy, J.C. Cailliez, E.M. Aliouat, E. Dei-Cas (Y) Department of Microbiology of Ecosystems, Pasteur Institute of Lille, BP245, 1 rue du Prof-Calmette, 59019 Lille, France e-mail: eduardo.dei-cas6pasteur-lille.fr E. Mazars Microbiology Laboratory, General Hospital Centre, 59300 Valenciennes, France J.C. Cailliez Faculté Libre des Sciences, Université Catholique, 59046 Lille, France D. Gargallo-Viola GlaxoWellcome S.A., 28760 Tres Cantos, Madrid, Spain S. Vargas Biomedical Sciences Institute, University of Chile School of Medicine, Santiago, Chile E.M. Aliouat Parasitology Service, Lille-2 University Pharmaceutical Sciences Faculty, 59006 Lille, France A.E. Wakefield Molecular Infectious Diseases Group, Department of Paediatrics, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DS, UK E. Dei-Cas Parasitology-Mycology Service, Faculty of Medicine, Lille-2 University Hospital Center, 1 place Verdun, 59045 Lille, France