Genetic variation in Pneumocystis carinii isolates from different geographic regions: Implications for transmission

To study transmission patterns of Pneumocystis carinii pneumonia (PCP) in p ersons with AIDS, we evaluated P. carinii isolates from patients in five U. S, cities for variation at two independent genetic loci, the mitochondrial large subunit rRNA and dihydropteroate synthase. Fourteen unique multilocus genotypes were observed in 191 isolates that were examined at both loci. M ixed infections, accounting for 17.8% of cases, were associated with primar y PCP. Genotype frequency distribution patterns varied by patients' place o f diagnosis but not by place of birth. Genetic variation at the two loci su ggests three probable characteristics of transmission: that most cases of P CP do not result from infections acquired early in life, that infections ar e actively acquired from a relatively common source (humans or the environm ent), and that humans, white not necessarily involved in direct infection o f other humans, are nevertheless important in the transmission cycle of P. carinii f. sp, hominis.