Tick-borne encephalitis: A 43-year summary of epidemiological and clinical data from Latvia (1973 to 2016)

Abstract

Background: The incidence of tick-borne encephalitis (TBE) varies significantly over time. To better understand the annual incidence of all TBE cases in Latvia we investigated the disease burden in the country from 1973-2016 using several available sources and case definitions.

Methods: We identified cases of TBE from an electronic database (maintained by the Centre for Disease Prevention and Control of Latvia [CDPC]) by the use of ICD-10 diagnosis codes for TBE (A84; A84.0; A84.1; A84.8; A84.9). In addition, previously unreported TBE cases were found by review of TBE diagnoses according to ICD-10 codes in four hospital databases.

Results: From 1973 to 2016 a total of 15,193 TBE cases were reported to the CDPC, 2,819 of which were reported from January 2007 through December 2016, additionally for this time period, 104 cases were identified via hospital survey. From all 2,923 reported cases (2007-2016), 1,973 met TBE case definition criteria and were included in the TBE study analysis. The highest average 10 year incidence was observed from 1990-1999 (27.9 cases per 100,000; range 4.6-53.0), however, the average 10-year incidence from 2007-2016 using officially adopted TBE case definition was 9.6 cases per 100,000 (range 5.8-14.6). For this 10-year time period most cases were adults (95.1%) and male (52.2%). The most common clinical form of TBE was meningitis (90.6%). A tick bite prior to TBE onset was reported in 60.6% of TBE cases and 98.2% of cases were not vaccinated against TBE.

Conclusion: The data demonstrate that the incidence of TBE varies by about one third based on the case definition used. TBE occurs almost entirely in the unvaccinated population. Regular TBE awareness campaigns could encourage the population in Latvia to use protective measures to further control TBE in the country, either via vaccination or tick avoidance.

Fig 1. Flow chart of TBE patients selected to the study.

Fig 2. TBE incidence rates in whole population; 1973–2016; vaccine uptake gradually increased from 39% (2009) to 52% in 2015.

Fig 3. TBE incidences in children (<18 years) and adults, 2007–2016, using the E-CDC case definition.

Fig 4. TBE average incidence rate per 100 000 inhabitants by regions of Latvia (period 2007–2016).

Fig 5. Average TBE seasonality by months (period 2007–2016;n = 1929).

Fig 6. Number of TBE cases per age group and gender.

Fig 7. TBE clinical forms by age groups.

Fig 8. Vaccine outcome in previously vaccinated TBE cases.