Incidence and mortality of herpes simplex encephalitis in Denmark: A nationwide registry-based cohort study

- The incidence of HSE was 4.64 cases per million population per year.
- The incidence was higher than reported by previous nationwide studies.
- Mortality rates following HSE remain high.
- Elderly and comorbid patients had remarkably higher mortality rates.

SummaryObjectivesWe aimed to investigate the incidence and mortality of herpes simplex encephalitis (HSE) in a nationwide cohort.MethodsFrom the Danish National Patient Registry, we identified all adults hospitalised with a first-time diagnosis of HSE in Denmark during 2004-2014. The HSE diagnoses were verified using medical records and microbiological data. Patients were followed for mortality through the Danish Civil Registry System. We estimated age-standardised incidence rates of HSE and 30-day, 60-day, and 1-year cumulative mortality. Furthermore, we assessed whether calendar year, age, gender, level of comorbidity, virus type, and department type was associated with HSE mortality.ResultsWe identified a total of 230 cases of HSE. Median age was 60.7 years (interquartile range: 49.3-71.6). The overall incidence rate was 4.64 cases per million population per year (95% confidence interval: 4.06-5.28). The cumulative mortality within 30 days, 60 days, and 1 year of the HSE admission was 8.3%, 11.3%, and 18.6%, respectively. Advanced age and presence of comorbidity were associated with increased 60-day and 1-year mortality.ConclusionsThis nationwide study of verified HSE found a higher incidence than reported in previous nationwide studies. Presence of comorbidity was identified as a novel adverse prognostic factor. Mortality rates following HSE remain high.