Listeria monocytogenes in a healthy young adult

To the Editor: Meningitis caused by Listeria monocytogenes is rare in immunocompetent adults.1 In the absence of risk factors for L. monocytogenes infection, Australian antibiotic guidelines recommend a third-generation cephalosporin alone for management of community-acquired meningitis, which has no activity against L. monocytogenes.2

A 22-year-old immunocompetent woman presented to our hospital with 3 days of fever, headache, photophobia and neck stiffness. This had been preceded by a 2-day prodrome of diarrhoea. Medications on admission were cephalexin for 1 day and a combined oral contraceptive. The patient had a history of coeliac disease, controlled on a gluten-free diet, and penicillin allergy. On examination, she was febrile (temperature, 38.7°C), with nuchal rigidity and positive Kernig’s sign, but with no rash. Heart rate (80 beats/min), respiratory rate (18 beats/min) and blood pressure (145/80 mmHg) were within normal range. White cell count (WCC) (16 × 109/L; reference interval [RI], 5–10 × 109/L) and C-reactive protein concentration (206 mg/L; RI, < 2 mg/L) were elevated. HIV serology and β-human chorionic gonadotropin results were negative. Computed tomography of the brain gave normal results. A lumbar puncture revealed clear cerebrospinal fluid (CSF) with opening pressure > 35 mmH2O (RI, 10–20 mmH2O), a glucose level of 1.5 mmol/L (RI, 2.0–3.9 mmol/L), a protein level of 0.96 g/L (RI, 0.15–0.45 g/L), and a WCC of 1000 (polymorphonuclear cells, 80; mononuclear cells, 920) × 106/L (RI, < 5 × 106/L). Cryptococcal antigen was not detected in her CSF.

The patient commenced taking ceftriaxone, combined trimethoprim and sulfamethoxazole (co-trimoxazole) and aciclovir. CSF culture was positive after 2 days, showing catalase-positive, β-haemolytic colonies with gram-positive bacilli exhibiting tumbling motility at room temperature. These were identified as L. monocytogenes (99% on Vitek MS v2.0 and Vitek 2 Compact systems, bioMérieux), susceptible to ampicillin and co-trimoxazole (Etest, bioMérieux). Antibiotics were rationalised to co-trimoxazole alone, and the Victorian Department of Health was notified. Treatment ceased on Day 19 of 21, due to co-trimoxazole-related myelosuppression (WCC, 1.0 × 109/L; neutrophils, 0.8 × 109/L). Four weeks later, the patient’s full blood count was normal, and she was well, with no complications or immunodeficiency.

L. monocytogenes meningitis accounts for 5%–10% of bacterial meningitis and has a mortality rate up to 62%.3,4 It mostly occurs in extremes of ages, pregnant women and immunocompromised patients.5 As a cause of lymphocytic meningitis, it can be confused with viral meningitis.3 In our patient, differentiating factors were her high CSF opening pressure, low glucose level, and degree of elevated lymphocyte count and protein concentration. Other differential diagnoses included partially treated bacterial, cryptococcal and tuberculous meningitis.3 The latter two were less likely, given the short clinical history, a negative cryptococcal antigen and absence of exposure to tuberculosis.

We recommend a low threshold for empiric prescription of benzylpenicillin (or co-trimoxazole if the patient is allergic to penicillin) in patients with lymphocytic meningitis, regardless of underlying risk factors for L. monocytogenes infection.