Aims: This study aimed to clarify the clinical characteristics of Pneumocystis jirovecii pneumonia (PJP) infection in patients with ulcerative colitis (UC) and to identify risk factors for PJP using a retrospective case–control study.

Methods: Of 4,525 patients with UC treated between 2007 and 2019, we identified those who satisfied the criteria for PJP. The Lichtiger clinical activity index (LCI) was compared between the initiation of immunosuppressive drug treatment and the onset of PJP. A retrospective case–control study was conducted using a PJP group and a non-PJP group.

Results: Nine patients experienced PJP, of whom two died. Since October 2014, there were no cases of PJP among UC patients aged ≥50 years who were prescribed three or more immunosuppressive agents given prophylactic sulfamethoxazole-trimethoprim (TPM-SMX). The median LCI (range) was 13 (8–17) at the initiation of treatment versus 2 (1–8) at PJP onset (p = 0.016). The median time to PJP onset was 83 days after treatment initiation. In the PJP group the median age was significantly greater (p = 0.022), three immunosuppressants were used significantly more frequently (p = 0.004), and the lymphocyte counts during treatment were significantly lower (p < 0.01) than in the non-PJP group. The cut-off lymphocyte count that distinguished PJP patients from non-PJP patients was 570/μL according to a receiver-operating curve analysis.

Conclusions: Prophylactic administration of TPM-SMX prevented further cases of PJP. The onset of PJP occurred at the same time as the symptoms of UC were stabilizing and the immunosuppressive drugs were being reduced. Greater age, lower lymphocyte count, and treatment with three immunosuppressive drugs were risk factors for PJP.


ulcerative colitis, Pneumocystis jirovecii pneumonia, inflammatory bowel diseases, case–control study