Babesia microti infection in a patient with multiple sclerosis treated with ocrelizumab.

Ocrelizumab is a humanized monoclonal anti-CD20 antibody approved for treatment of relapsing-remitting and primary progressive multiple sclerosis (MS). Rare parasitic infections have been reported in patients with lymphoproliferative disorders using rituximab, a chimeric anti-CD20 antibody used off-label for the treatment of MS. Here, we report a patient with MS on ocrelizumab with B-cell depletion who developed severe Babesia microti (B. microti) infection with neutropenia, hemolytic anemia, and thrombocytopenia. He recovered after prompt diagnosis and treatment. This case represents the first published occurrence of babesiosis in a patient with MS on ocrelizumab. It also adds to the accumulating evidence from databases of emergent severe or relapsing B. microti infection in patients receiving anti-CD20 antibodies. This presentation stresses the diagnostic vigilance required by MS neurologists in endemic areas to identify cases of babesiosis in patients on anti-CD20 therapy and to better counsel these individuals on their risks of B. microti infection.