The annual incidence of acute infectious encephalitis in adults is reported between 0.5-16 per 100,000 persons. West Nile virus (WNV) is among the most important causative agents of viral encephalitis.

We retrospectively characterize the clinical course of acute infectious encephalitis among adult patients admitted to the Tel-Aviv Medical Center between 2010 and 2020 (n=95), comparing major causative agents and focusing on WNV encephalitis (WNVE, n=31), varicella-zoster virus encephalitis (VZVE, n=30) and herpes simplex 1 virus encephalitis (HSV1E,  n=13).

Patient groups differed in terms of age, with WNVE (median age 73) and VZVE (76) patients significantly older compared with HSV1E patients (51). CSF characteristics differed between groups, with VZVE patients having significant mononuclear white blood cell (WBC) predominance (99%) and HSV1E patients having lower CSF protein (57 mg/dL). Blood inflammation markers also differed significantly, with WBC higher in WNVE and C-reactive protein lower in HSV1E.

Functional outcome was the worst in WNVE patients at discharge (median modified Rankin Scale [mRS] score 4) and last follow-up (median mRS score 4), reaching statistical significance when compared with VZVE patients. Differences in functional outcome prevailed when excluding patients with poor functional baseline (mRS score > 2 before presentation). Logistic regression found age as an independent variable associated with worse functional outcome, and age, baseline mRS and WNVE (compared to VZVE) as associated with mortality.

In odds with previous reports, WNVE and VZVE were far more common than HSV1E. Differences in laboratory characteristics could prove clinically useful early in the presentation of acute encephalitis. In some aspects, WNVE patients had a course resembling a bacterial, rather than viral, CNS infection.

The detrimental outcome of WNVE emphasizes the need to promote solutions to WNV infection such as vaccinations and specific drugs.