Background: Pneumocystis jirovecii pneumonia (PCP) is a serious and possibly lethal opportunistic infection in immunocompromised patients. An outbreak of 29 PCP cases occurred among renal and liver transplant recipients at a single transplantation centre between 2007 and 2010, routine PCP prophylaxis was not utilized. To investigate the outbreak, we retrospectively conducted a matched genotype study and a case-control study.

Methods: P. jirovecii isolates from 22 transplant patients (cases), 2 colonized renal transplants and 19 Pneumocystis -controls diagnosed within the same period were genotyped by restriction fragment length polymorphism analysis and multi-locus sequence typing. Maps were used to investigate transmission. To asses risk factors associated with PCP, cases were matched 2:1 with non-PCP transplant patients (transplant-controls).

Results: Three unique and temporally related clusters of Pneumocystis genotypes were shared among 18 of the 22 genotyped case-samples, the same strains were found in a colonized renal transplant and in two Pneumocystis -controls. A majority of the PCP cases had concurrent outpatient visits and admissions, but several cases were not cross-exposed. Among the clinical parameters analyzed, only concurrent CMV viremia was associated with PCP in both populations (p = 0.01 and p = 0.002). Mycophenolic acid was associated with PCP risk only in the renal transplant population (p = 0.04).

Conclusion: This is the first evidence of cross-infection between liver and renal transplant recipients in PCP clusters. Furthermore, multiple Pneumocystis strains may contribute to temporally related outbreaks of PCP. Interhuman transmission were not able to explain all cases. Colonised transplant recipients may act as a reservoir for PCP. A six months PCP prophylaxis should be considered in all renal and liver transplant recipients.

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