A nationwide seroprevalence screening for tick-borne encephalitis virus in the population of Bulgaria
To assess local circulation and risk for human infections with TBEV, nationwide seroprevalence study was conducted in 2015 for the first time in Bulgaria.
Serum samples were prospectively collected from persons visiting laboratories for routine checkup in primary healthcare centers in all districts of Bulgaria: Blagoevgrad (n = 64), Gabrovo (n = 63), Vidin (n = 40), Montana (n = 78), Dobrich (n = 52), Plovdiv (n = 62), Targovishte (n = 42), and 50 samples from each of the rest 21 districts.
Information about age, sex, and area of residence for each sampled person was collected in the laboratories.Using ELISA kits (Euroimmun, Lubeck, Germany), all serum samples were tested for TBEV IgG antibodies. Then, all IgG-positive samples were tested for specific IgM antibodies by ELISA. In addition, avidity tests (from the same manufacturer) were applied to distinguish between acute and non-acute TBEV infections.
A total of 1451 residents of all districts in Bulgaria (population 7.2 million), 622 male and 829 female, the mean age ± standard deviation was 53.2 ± 18.8 years, were tested for TBEV-specific IgG antibodies.
Nine persons were found reactive by IgG ELISA; mean seroprevalence was 0.6%. IgM antibodies were not detected. IgG avidity index ranged from 33-85%, main 60%. The nine people were residents of six districts. The highest seroprevalence rate was found in districts of Gabrovo (4.8%) and Ruse (4%) (Figure 3). No significant association between age and TBEV infection was detected [27].
The first and nationwide seroprevalence survey on TBEV circulation in Bulgaria found overall seroprevalence of 0.6% for TBEV However, district analysis showed TBEV seroprevalence up to 4-4.8%. The great variability of IgG avidity indices is suggestive of recent and past infections.
TBE is endemic in Central and Northern European countries. By occasion, TBE has been detected in Southern Europe and the Balkan Peninsula in particular. Nevertheless, data obtained from the seroprevalence study indicates noticeable appearance of TBEV infections in Bulgaria.
Figure 3.
TBEV seroprevalence in Bulgaria, 2015.
The level of TBEV seroprevalence in Bulgaria showed that the infection seems to be more widespread in the country as has been described so far. It is evidence that some viral encephalitis or meningoencephalitis cases in the country are underdiagnosed and underreported.
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