Pneumocystis carinii: where are we now?

Pneumocystis carinii pneumonia (PCP) is still the commonest AIDS-defining condition in the UK, although absolute numbers have fallen since 1990 with the advent of prophylaxis and highly active antiretroviral therapy (HAART). Further decreases in its incidence are unlikely unless efforts are made to reduce late first presentations of HIV, improve retention of patients in care and improve adherence to P. carinii prophylaxis. The widespread development of key mutations in the P. carinii dihydropteroate synthase gene in the 1990's suggests that the organism is evolving under positive evolutionary pressure, possibly mediated by widespread use of co-trimoxazole and dapsone prophylaxis. It is not clear whether these mutants lead to treatment failure with co-trimoxazole, since most episodes are treated successfully and failure of prophylaxis is rare. Molecular diagnosis of PCP is restricted to research use, due to difficulties with sensitivity, specificity and turn-around times. Patients are infected with multiple P. carinii subtypes and re-infection from an environmental source is probably more important than re-activation of an existing infection. Person-to-person transmission does not appear to be a major source of infection in HIV-infected patients. Recent reports have highlighted the potential complications of initiating HAART during treatment of PCP.