Early Onset Pneumocystis jiroveci Pneumonia During the Periengraftment Phase in Haploidentical Haematopoietic Stem Cell Transplantation

Pneumocystis jiroveci pneumonia (PJP) remains a critical opportunistic infection in haematopoietic stem cell transplantation (HSCT) recipients, traditionally occurring late posttransplant. This report presents two rare cases of early onset PJP during the peri-engraftment phase in high-risk patients undergoing haploidentical HSCT one a paediatric case with inborn error of immunity due to dedicator of cytokinesis 8 ( DOCK8) deficiency and the other, an adult with Janus kinase 2 (JAK2) positive primary myelofibrosis. Both patients developed acute respiratory symptoms and characteristic radiographic findings within 10–22 days post-transplant. PJP was confirmed through bronchoalveolar lavage (BAL) and direct immunofluorescence assay. Prompt initiation of trimethoprim-sulfamethoxazole (TMP SMX) in therapeutic doses led to clinical improvement and successful haematological recovery. These cases underscore the potential for PJP to emerge earlier than expected, challenging existing guidelines that recommend prophylaxis primarily after engraftment. Earlier initiation of TMP SMX may be associated with a risk of delayed engraftment and graft failure. These cases highlight that the timing of prophylactic interventions, potentially initiating before or during the engraftment period, may enhance patient outcomes. Radiological imaging and early bronchoscopy remain essential for timely diagnosis. Further multicentric studies and registry analyses are warranted to inform evidence-based prophylaxis protocols and improve early detection strategies in this vulnerable patient population.