EPIDEMIOLOGY OF PNEUMOCYSTIS-CARINII INFECTION - APPLICATION OF MOLECULAR-GENETIC METHODS

Pneumocystis carinii causes a life threatening, diffuse, extracellular infection of the pulmonary alveoli in 20-70% of immunosuppressed subjects with AIDS, organ transplantation or malignancy. Although animal model experiments favour air borne transmission of infection, failure to culture P. carinii has impeded efforts to clarify its epidemiology. A high prevalence of positive serology to P. carinii in normal individuals has suggested, by analogy with tuberculosis, that opportunistic pneumonia in the immunosuppressed individual results from recrudescence of a dormant pulmonary focus of organisms, the legacy of childhood infection. We have cloned a series of mitochondrial genes from P. carinii as a basis for i) conducting comparative DNA sequence studies and ii) providing phylogenetic information for the development of a highly specific and sensitive DNA amplification technique for recognizing the parasite in different samples. We have not found carriage of P. carinii in normal lungs and have shown that P. carinii organisms infecting different mammalian hosts are genetically distinct. Comparative DNA studies based on DNA amplification products using P. carinii specific primers at reduced annealing temperatures in the polymerase chain reaction, suggest that P. carinii is most closely related to the ustomycetous red yeast fungi. These organisms are found freely in the environment and produce widely disseminated air-borne spores. These data suggest that opportunistic Pneumocystis pneumonia results from fresh infection, acquired from an environmental source.