Prevalence of tick-borne-encephalitis virus antibodies in Lithuania
Introduction
Tick-borne encephalitis (TBE) is a flavivirus belonging to the family Flaviviridae. It is maintained in a cycle of Ixodes ticks feeding on small mammal hosts and the virus is transmitted to man by a tick-bite causing an infection characterized by biphasic febrile illness, often with neurological manifestations of meningoencephalitis. The disease is prevalent in certain areas of central, eastern and northern part of Europe. The disease foci seem to be quite stable and are characterized by heavy foliage and high moisture (Randolph et al., 2000). In endemic areas morbidity is high, in particular among people who for some reason must visit forests infested by virus carrying ticks. Although vaccination has effectively reduced the disease risk the disease has still a considerable public health impact. For instance in Lithuania several hundred cases are diagnosed annually. In about 10–30% of all infected persons the disease takes a more serious clinical course, 20–58% of them suffer from typical neurological infection, late complications such as neuro-psychiatric sequeleae or paresis and atrophies occur in 3–11% (Kaiser et al., 1994). Mortality is 1–2%.
An increase in the occurrence of TBE has been observed in all Baltic countries during the 1990s and in Sweden and Finland (Haglund et al., 1996, Gunther et al., 1997). In Lithuania, only a few cases were reported annually before the year 1990 but after that a significant increase in the incidence of the disease has been observed. This cannot be explained by increased availability of laboratory diagnostics (manuscript under preparation) and may be the result of climatic changes. Also, changes in living conditions, such as increasing outdoor activities during the 1990s could have contributed to the increase.
Seroepidemiological studies performed in various central European regions and Nordic countries have suggested that the disease is more prevalent among people who can be assumed to get exposed to tick bites frequently such as forest workers (Haglund et al., 1996, Cisak et al., 1998). The changing epidemiology might also bring along a change in the distribution of the disease and exposure of new groups of people. In order to map the current situation and possible risk factors, a population-based seroepidemiological study was conducted during the year 2000. The results indicate that the disease is prevalent, the recent observed increase in the disease incidence is true, and that the presumed risk factors do not well explain seropositivity.
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Materials and methods
Participants were invited to give blood for tick-borne encephalitis virus (TBEV) antibody assay and answer questions concerning behavior patterns that may relate to the risk for infection. People were recruited through local doctors of various specialities, in some sites by public campaigns, in some others by snowball-method. Also a group of military personnel (mostly recruits who had recently started their military service, N=611) was included. To avoid bias, people who had been vaccinated
Results
A total of 44 sera were positive (2.96%). When seropositivity was analyzed in different age groups we observed that seropositivity rate did not increase significantly with increasing age. Neither were the differences between the military versus the total study population or between the genders significantly different (Table 1).
None of the parameters describing outdoor activities correlated very strongly with the observed seropositivity. Among those who had a summer cottage the seropositivity
Discussion
Our study shows that approximately 3% of the population has been infected by TBEV during their lifetime in Lithuania. If we assume that the disease burden is even among different age groups and that the average life expectancy is 70 years, annually more than 1500 persons get the infection. Even though the exact ratio of subclinical to clinically manifested infections is not known, the figure is four times higher than the average 400 annual cases that are notified to the health authorities
Acknowledgements
This study was supported by the Nordic Council of Ministers. A. Juceviciene is a fellow of the project ‘Infectious Disease Control in the Barents and Baltic Sea Regions’. The excellent technical assistance of Hilppa Nykyri, Johanna Myllynen and Raija Leveelahti for performing the HI tests and Dr V. Bersenas for valuable help in collecting the sera from the military people, is acknowledged.
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