Do We Know the True Burden of Tick-Borne Encephalitis? A Cross-Sectional Study

Tick-borne encephalitis (TBE) is an acute human arboviral infection of the central nervous system caused by a virus that is transmitted to humans mainly by tick bites. TBE is endemic in Europe and has become an increasingly important public health concern in recent years. Cases of TBE in Italy have occurred mainly in the north-east and central parts of the country. Vaccination is recommended for people who live in or visit areas at higher risk of tick bites.

The aim of our study was to ascertain the burden of TBE in the Veneto Region (north-east Italy).

Cases of TBE occurring in the region from January 1, 2007, to December 31, 2018, were extracted from the database of the mandatory notification system (MNS) and from hospital discharge records (HDRs) of admissions relating to a diagnostic code 063 according to the International Classification of Diseases, Ninth Revision, Clinical Modification. Capture-recapture methods were used to estimate the completeness of each data source (as a percentage of cases). Records including diagnostic codes 322.9 (Meningitis, unspecified) and 323.9 (Unspecified cause of encephalitis, myelitis, and encephalomyelitis) were also extracted from the HDR database. Municipalities were grouped by location, based on their elevation above sea level, as "mountains," "hills," or "lowlands". After selecting only the municipalities where cases of TBE had occurred, the proportion of cases of TBE out of the total cases of encephalitis and meningitis identified was calculated and used to estimate the number of cases of TBE potentially occurring in the municipalities that reported none. Then the observed and adjusted TBE rates per 100,000 population were calculated.

During the 12 years considered, a total of 281 cases of TBE were identified; 155 emerged from the HDRs and the MNS, 89 only from the MNS database, and 37 only from the HDRs. The degree of completeness of the data was 93.0% for the MNS and HDRs combined, 80.8% for the MNS alone, and 63.6% for the HDRs alone. The observed annual rate of TBE in the Veneto in recent years was calculated at 0.48 per 100,000 population, with the highest standardized rate in the province of Belluno (5.95 per 100,000 population). A significant rising trend in the number of cases observed in the latest period (2015-2018) was apparent for the mountainous areas in the region (average annual percent changes: 43.7 [95% CI 19.0-73.5]). The number of cases peaked in the mountains in the month of June (0.89 per 100,000), in hilly areas in July (0.23 per 100,000) and in the lowlands in October (0.04 per 100,000).

Our findings seem to confirm an underreporting of the cases of TBE and inadequate TBE surveillance in the Veneto, despite the clinical severity of the disease and the fact that it is mandatory to report all cases. The routine integration of different databases is crucial to the successful implementation and assessment of targeted prevention strategies and fundamental to public health decision-making on this issue.