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

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Abstract

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.

Introduction

Central European tick-borne encephalitis (TBE) is a viral disease of the central nervous system, which persists endemically in several Central-European countries (Dumpis et al., 1999). Although the main route of transmission of tick-borne encephalitis virus (TBEV) is through tick bites, TBE outbreaks were linked to the consumption of raw milk (Matuszczyk et al., 1997; Kerbo et al., 2005). TBE is a mandatory reportable disease in Poland since 1970. The administrative division of Poland and the geographic distribution of registered TBE incidence per 100,000 inhabitants in 1994–2005 are presented in Fig. 1. The routine surveillance data indicate clustering of TBE incidence in two north-eastern voivodships, Podlaskie and Warminsko-mazurskie, and only sporadic case reports from isolated districts in other voivodships (Fig. 1b). Before the introduction of the mandatory surveillance, large-scale serologic surveys were performed in 1965–1972 among more than 20,000 foresters and 17,000 other subjects representing the general population (Wroblewska et al., 1968, Wroblewska et al., 1973). Antibodies against TBEV detected using the hemagglutination inhibition test were found in 0.5–6.5% of the population and in 7.0–27.0% of the foresters in different regions. Regions with a particularly high prevalence of antibodies against TBEV have been identified, including areas in the northern and eastern parts of Poland (Fig. 2). Further evidence on the presence of active TBE foci was provided by virological studies of ticks in selected regions and regional seroprevalence studies (Prokopowicz et al., 1995; Cisak et al., 1998, Cisak et al., 2002). In these studies, evidence on the presence of active foci in the Pomorskie, Podlaskie, and Lubelskie voivodships was strengthened. In general, the extent of the disease incidence as notified within the surveillance system is not compatible with the seroprevalence distribution established in field studies.

The aim of the present study was to perform a serologic survey among humans assessing the prevalence of antibodies against TBEV in selected regions of Poland classified as endemic and non-endemic. Additional objective evidence of the endemic status of selected regions was obtained from a parallel serologic survey among goats.

Section snippets

Materials and methods

The present seroprevalence study among humans was based on serum samples (n=1498) selected from a serum bank collected by the Dept. of Virology, National Institute of Public Health, in 1996–2005. The samples were obtained from subjects referred to local health departments for testing against different pathogens not related to neuroinfections, including HBV, food-borne pathogens, etc. A stratified sample without replacement, weighted on age group, gender, and geographic location was selected

Results

Out of 1498 human samples tested, 39 were positive for anti-TBEV antibodies, giving an overall seroprevalence of 2.6%. Out of 38 re-tested samples, 35 were confirmed with TBEV-specific tests. However, only 7 out of the 10 re-tested sera from Podlaskie voivodship, 3/10 from Kujawsko-Pomorskie, 1/5 from Pomorskie, and merely 4 out of the 14 tested sera from the remaining voivodships (Lubelskie, Zachodniopomorskie, Wielkopolskie, Dolnośląskie) indicated earlier infection based on IIF and HI

Discussion

The present study explored the possible existence of TBE endemic foci in regions of Poland considered free of disease based on information provided by the routine communicable disease surveillance system. The results of this pilot survey indicate that altogether as many inhabitants of non-endemic regions as inhabitants of endemic regions were exposed to TBE virus. In the 4 north-western non-endemic voivodships, where the TBEV antibody prevalence was >1%, only sporadic cases were reported during

Acknowledgements

This work has been funded by Med-Vet-Net, Workpackage 31. Med-Vet-Net is an EU-funded Network of Excellence (FOOD-CT-2004-506122).

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