Listeria monocytogenes encephalitis mimicking Herpes Simplex virus encephalitis: the differential diagnostic importance of cerebrospinal fluid lactic acid levels

Listeria monocytogenes is a common cause of bacterial meningitis in elderly patients and in those with impaired cellular immunity. The most common central nervous system infection caused by L. monocytogenes is acute bacterial meningitis; meningoencephalitis is uncommon and encephalitis is rare. Early diagnosis of L. monocytogenes meningitis is difficult because only 50% of cerebrospinal fluid (CSF) Gram stains are negative. L. monocytogenes is one of the few central nervous system pathogens associated with red blood cells in the CSF. When L. monocytogenes presents as encephalitis with red blood cells in the CSF, the clinical presentation mimics most closely herpes simplex virus (HSV)-1 encephalitis. Because the therapies for L. monocytogenes and HSV-1 are different, early diagnostic differentiation is clinically important. The CSF lactic acid is the best way to rapidly differentiate between these two entities; the CSF lactic acid level is elevated in L. monocytogenes but is not elevated in HSV-1 encephalitis. The case presented is an elderly man with chronic lymphocytic leukemia who presented with encephalitis. Advanced age and chronic lymphocytic leukemia predispose him to a wide variety of pathogens, but the rapidity and severity of his clinical presentation made L. monocytogenes and HSV-1 encephalitis the most likely diagnostic possibilities. The CSF Gram stain was negative, but the elevated CSF lactic acid levels with encephalitis and red blood cells in the CSF indicated L. monocytogenes as the most likely pathogen. We present a case of L. monocytogenes encephalitis mimicking HSV-1 encephalitis. While receiving ampicillin therapy, the patient remained unresponsive for more than 1 week and then suddenly regained consciousness and recovered without neurologic sequelae.