Acyclovir-resistant herpes simplex encephalitis in a patient treated with anti-tumor necrosis factor-α monoclonal antibodies

Herpes simplex virus is the most common cause of severe sporadic encephalitis. We report a case of herpes simplex type 1-encephalitis in a 50-year-old woman receiving anti-tumor necrosis factor-α monoclonal antibodies adalimumab. Although she was an acyclovir naïve patient, a mixed viral population (wild-type and acyclovir-resistant bearing a thymidine-kinase mutation) was identified in the cerebrospinal fluid. The virus in cerebrospinal fluid evolved and a second thymidine-kinase mutant virus emerged. Combined foscavir and acyclovir treatment resolved the herpes simplex encephalitis. To our knowledge, this is the first report of acyclovir-resistant herpes simplex encephalitis in a patient treated with adalimumab.