Cytomegalovirus Infection in a Multiple Sclerosis Patient on Dimethyl Fumarate

Background The association between cytomegalovirus (CMV) infection and multiple sclerosis (MS) remains unclear. CMV infection has been reported in MS patients treated with alemtuzumab, rituximab, and ocrelizumab, but its occurrence in patients receiving dimethyl fumarate (DMF) is less studied. Objectives This case report explores the potential relationship between DMF therapy and CMV infection in a relapsing‐remitting MS (RRMS) patient, examining whether DMF‐induced immune modulation contributed to CMV reactivation or primary infection. Results A 29‐year‐old male with RRMS, on DMF for four years without lymphopenia, developed elevated liver enzymes and splenomegaly. CMV serology showed IgG (60.7 U/L) and IgM (140.0 U/L), with detectable viral DNA (524 IU/mL). Epstein–Barr virus (EBV) IgG was positive, but IgM was negative. Hepatitis A/B, herpes simplex virus (HSV), and varicella tests were negative. DMF was paused for 3 months, and antiviral therapy led to reduced liver enzymes and CMV DNA levels. MS remained stable without new MRI lesions. Conclusion DMF therapy may contribute to CMV infection despite the patient’s normal lymphocyte counts. It may be beneficial with routinely test with viral panel with patients with progression in MS. Further studies are needed to clarify the risk of CMV infection in DMF‐treated MS patients.