Delayed Diagnosis of West Nile Meningoencephalitis in a Patient Receiving Rituximab for Rheumatoid Arthritis

West Nile virus (WNV) neuroinvasive disease is associated with substantial morbidity and mortality. Clinical suspicion is usually confirmed with cerebrospinal (CSF) immunoglobulin M (IgM) detection using enzyme-linked immunoassay (ELISA) techniques. CSF polymerase chain reaction (PCR) is rarely used to confirm the disease and is not widely available. We present a detailed report of false-negative WNV IgM in a patient receiving rituximab therapy for rheumatoid arthritis. She was exposed to the virus during peak immunosuppression and strong clinical suspicion was confirmed with WNV PCR, illustrating the importance of such consideration with the recent incremental use of rituximab therapy. Despite the lack of specific anti-viral treatment for WNV, delayed consideration and diagnosis of WNV in those who are immunosuppressed would expose them to a wide panel of testing, with a subsequent increase in the cost of medical care.