We herein report a case of a 79-year-old Japanese woman who developed severe oral
stomatitis during
methotrexate (MTX) treatment for
dermatomyositis. She had been treated with MTX (12 mg/week) and
prednisolone (5 mg/day) for
dermatomyositis for 4 years. She developed painful
stomatitis,
fever, and
pancytopenia. Initially, her symptoms were suspected to be caused by mucosal toxicity of MTX. Therefore, the
drug was discontinued, and
leucovorin was administered. However, oral
stomatitis worsened in a few days, resulting in intolerance of oral ingestion due to severe
pain. Polymerase chain reaction revealed the presence of herpes simplex virus type 1 (HSV-1) in oral erosive lesions, and blood examination was positive and negative for anti-HSV
IgG and anti-HSV
IgM, respectively. Therefore, HSV-1 reactivation-induced oral
stomatitis was diagnosed, and
acyclovir treatment was started, which promptly improved oral
stomatitis. HSV-1 reactivation is usually asymptomatic or results in localized vesicular lesions at the mucocutaneous junction of the lips in immunocompetent individuals. Our case illustrates that HSV-1 reactivation induces severe
stomatitis in patients treated with low-dose MTX for
autoimmune diseases, not just in those with severe immunosuppressive conditions. Of note, HSV-1 reactivation-induced
stomatitis is a diagnostic challenge, especially during MTX treatment.