Identification of new endemic tick-borne encephalitis foci in Poland – a pilot seroprevalence study in selected regions

In Poland, large-scale serologic surveys carried out in 1965–1972 revealed regions of Poland with particularly high prevalences of antibodies against tick-borne encephalitis virus (TBEV). The information provided by the routine surveillance of communicable diseases during 1970–2005 indicated, however, that the geographic distribution of the disease is limited to a few eastern and south-western regions of Poland (defined as endemic for the purpose of this study). In the present serologic survey, 1498 human serum samples collected in 1996–2005 were randomly selected from a serum bank, and 358 goat serum samples were collected from milk-producing farms in selected areas of Poland 2002–2006. Thirty-nine human samples were positive for anti-TBEV antibodies, with an overall seroprevalence of 2.6%. Seroprevalence in endemic provinces ranged from 0.8% to 4.3%, and seroprevalence in non-endemic provinces ranged from 1.9% to 4.3%. In endemic, compared to the non-endemic provinces, the highest seroprevalence was found in the age group >60 years (7% vs. 1%) and in inhabitants of villages (3.9% vs. 1.8%). In non-endemic, compared to endemic provinces, the highest seroprevalence was detected in the age groups 30–39 years (5% vs. 2%) and 40–49 (4% vs. 0%), and in inhabitants of large towns inhabited by >100,000 people (4.1% vs. 2.5%). Out of 358 goat samples, 17 (4.7%) were positive for anti-TBEV antibodies. Seroprevalence in goats reached 14/151 (9.3%) in endemic, and 3/207 (1.4%) in non-endemic provinces. The present study indicates the possible existence of endemic foci in north-western provinces of Poland, in which barely any cases were reported during 1970–2005. The socio-demographic profile of seropositive subjects in non-endemic regions suggests that they might have been exposed to TBEV during travels to known endemic regions. This would mean, however, that they were missed by the surveillance system. A thorough review of diagnostic protocols in non-endemic regions and work towards an improvement of the TBE surveillance sensitivity is necessary.