Genetic variation among Pneumocystis carinii hominis isolates in recurrent pneumocystosis.

Pneumocystis carinii hominis is a ubiquitous organism that causes pneumonia in immunocompromised persons. Paired P. carinii hominis isolates from human immunodeficiency virus-infected persons who had two episodes of pneumocystosis were examined for genetic heterogeneity. Genetic variation was detected by sequence comparison of a portion of the mitochondrial ribosomal RNA gene. In 5 of 10 patients experiencing two episodes of pneumocystosis, genetically distinct isolates were associated with each episode. These included 4 of 6 patients whose second episode of pneumocytosis occurred > 6 months after their initial bout. The genetic data support the hypothesis that some recurrent episodes of P. carinii hominis pneumonia are caused by reinfection rather than by reactivation of latent infection.