Pneumocystosis versus pulmonary Pneumocystis carinii colonization in HIV-negative and HIV-positive patients

Pneumocystis carinii pneumonia (PCP) is now frequently considered to result from de novo acquisition of the parasite rather than from reactivation of latent infection. Indeed, P. carinii detection by polymerase chain reaction (PCR) on post-mortem lungs from immunocompetent persons failed to give results. However, low numbers of P. carinii organisms, which are undetectable by microscopy, can be detected using PCR on bronchoalveolar lavage (BAL) specimens from immunosuppressed patients without PCP. These low levels of parasites are usually considered to reflect pulmonary colonization, but their biological significance is still subject to debate. Moreover, colonization cases have been reported essentially amongst HIV-infected patients and there is little data concerning colonization amongst HIV-negative patients. In so far as HIV-negative patients are often more numerous in hospitals, the frequency of colonization in this patient population is worth assessing. Because the host-to-host transmission of P. carinii has been established in rodents and because it has been suggested that airborne interindividual transmission of the parasite takes place, the implication of colonization cases as well as cases of acute PCP in the circulation of P. carinii in the hospital environment has to be established.