Background: Ruxolitinib is a selective Janus kinase inhibitor 1/2 used to treat myelofibrosis (MF) and polycythemia rubra vera (PV). Ruxilotinib treatment has been associated with both M. tuberculosis (M.tb) and non-tuberculous mycobacteria (NTM) infections (3% incidence in Lescuyer et al 2019 IJID 80;134-136).

Aim: To present 4 cases of mycobacterial infection in patients on ruxolitinib.

Results: N=4 of 75 patients taking ruxolitinib (5.3%). Two patients were female, 2 male. Patient 1 (white British, 69) received 3 months of ruxolitinib to treat MF, before presenting with fever and splenomegaly. Bone marrow and lymph node (LN) biopsies cultured M. intracellulare. She had ongoing, progressive radiological changes despite 4 months? treatment but subsequent biopsies were culture negative. Patient 2 (Sudanese, 71) received 2 years of ruxolitinib to treat PV then secondary MF before developing T10 spinal, iliac and mediastinal LN M. kansasii with a good outcome after 12 months? treatment. Patient 3 (white British, 77) received 15 months of ruxolitinib for MF before developing pulmonary M. xenopi. Despite treatment for 8 months, he has progressive pulmonary disease on CT. Patient 4 (Chinese, 74) received 3 months of ruxolitinib for MF before being diagnosed with disseminated M.tb (sputum and LN smear positive), and died despite TB treatment.

Conclusions: Atypical presentations of disseminated mycobacterial infection occur on ruxlotinib treatment. Despite antimycobacterial treatment and withholding ruxolitinib, there can be a variable radiological response that may reflect immune reconstitution.