The most likely diagnosis in this patient is meningitis caused by herpes simplex virus (HSV) type 2. Viral meningitis is the most common cause of “aseptic” meningitis, in which cerebrospinal fluid (CSF) Gram stain and cultures are negative. Most patients have typical meningitis symptoms, such as fever, nuchal rigidity, headache, and photophobia. HSV meningitis syndromes can be related to primary infections, with central nervous system involvement as a secondary consequence, or reactivation of latent infection presenting as aseptic meningitis. HSV-2 is more commonly associated with meningitis and is the most common cause of recurrent meningitis (recurrent benign lymphocytic meningitis). HSV-1 is associated with encephalitis. HSV can cause meningitis year round. CSF findings resemble enteroviral meningitis, with lymphocytic pleocytosis, a normal glucose level, and a mildly elevated protein level as in this patient. CSF polymerase chain reaction studies may be used for diagnosing HSV and enterovirus meningitis.
Enteroviruses are the most common cause of viral meningitis, but they usually present between May and November in the Western Hemisphere, with symptoms including headache, fever, nuchal rigidity, photophobia, nausea, vomiting, myalgias, pharyngitis, maculopapular rash, and cough. This patient's presentation in the winter makes this an unlikely cause of her illness.