Tick-borne encephalitis (TBE) is a widespread neuroviral zoonotic infection in Northern Europe. In Estonia, several hundred TBE cases are notified each year. The disease is most prevalent in children and young people, who compose about 30% of the annually registered TBE cases. Over the last decade, a sharp rise in incidence in the middle of the 90s, a sharp fall in 2002, and a new rise in 2003 have characterized the epidemiology of TBE. The risk of human infection by TBE virus depends on many ??environmental factors and their interactions, influencing the ecology of TBE virus as well as vectors and reservoir hosts (1,2). Therefore, the epidemiology of TBE during 19942004 was analysed in more detail, taking into account the endemic areas of TBE virus, the two prevalent species of Ixodes ticks, genotypes of TBE virus isolated in Estonia, climatic and social factors.
Patients and methods
Patients with suspected TBE were hospitalised in different hospitals in four large cities of Estonia (Tallinn, Tartu, Prnu, Rakvere). The diagnosis of TBE was based on the detection of specific antibodies (IgM and IgG) to the TBE virus by ELISA test. The diagnoses were based on clinical presentations, epidemiological data, and results from serological investigations. The TBE diagnosis was serologically confirmed in all registered cases. Analysis of the epidemiological situation was carried out taking into account the characteristics of the tick vectors of the TBE virus, their ecology and social factors.
Up to the early 1990s, the TBE incidence in Estonia was characterized by cyclic changes, of on average 35 years, and the number of TBE cases did not exceed 100 cases per year (giving an annual incidence of 4.36.4 cases per 100 000 inhabitants). Since 1991, the highest incidence rates of TBE were registered in 1997 and 1998 (27.8 and 26.6 per 100,000, respectively ). After 1998, the incidence rate of TBE has decreased, but (with the exception of 2002) remained on higher levels than those seen prior to 1997 (Figure 1). The changes in incidence (increase or decrease) have been noted also in Latvia and Lithuania.
Age and sex distribution
TBE cases were noted in all age groups in 19942004. During all years, most TBE cases were diagnosed in persons above 60 years of age (Figure 2). Most of them were retired people. The number of TBE cases among children and younger adults, composed about 30% of the total number of registered cases per year. The TBE incidence rate in males was almost the same as in females; men constituted 4555 % of the annual TBE cases. The TBE incidence rate among the rural population was about 1.8 times higher than that among the urban population.
Seasonal and geographical distribution
The incidence of TBE in Estonia shows a strict seasonal pattern, which depends on the spread of the two prevalent species of Ixodes (I.) ticks: I. ricinus and I. persulcatus. I .ricinus is spread over the whole territory of Estonia, while the area of I .persulcatus overlaps the area of I. ricinus in eight of 15 counties, located in the southern and south-eastern parts of the country.
The epidemiс season of TBE coincides with the time of biological activity of the two species of ticks. It lasts for seven months from April to November (depending on weather), including the seasons of both tick species. The biological activity of I. persulcatus is characterized by one peak of seasonal activity in the spring, while two peaks; one in the spring and one at the end of the summer characterize the period of biological activity of I. ricinus. Subsequently, the seasonal distribution of TBE cases is also characterized by two peaks; one in the spring (end of May to the beginning of June) and one in the late summer/early autumn (end of August to the beginning of September) (Figure 3). The summer peak of TBE incidence coincides with the activity peak of both types of ticks, while the autumn peak coincides with the second activity peak of I .ricinus. Most TBE cases during the study period were notified in June, August and September, when people more frequently visit the forests to gather mushrooms and berries.
The geographical distribution of TBE in Estonia was studied more in detail. TBE could be found in 13 of the total 15 counties, but it was also shown that the geographical distribution of TBE cases was not uniform. The highest incidence rates were observed in western Estonia (in two counties and the island Saaremaa), in eastern Estonia (in one county) and in the southeastern Estonia (in two counties). A majority of the annual TBE cases were notified from these six counties.
As in the other Latvia and Lithuania, about 30% of the patients develop febrile forms of TBE, about 65% meningoencephalitis and about 7% a focal form of the disease. The disease mostly ends with complete recovery without sequelae. Lethal outcomes are rare events. Since 1990, most of TBE cases (9597 % ) have been laboratory-confirmed. Approximately 2/3 of TBE patients report a tick bite.
The analysis of the epidemiology of TBE for the period 19942003, taking into consideration the territorial distribution of the TBE-virus vector, climatic factors and some social factors, indicate various reasons for the variations in TBE incidence.
There seem to be three reasons for the sharp peak in TBE-incidence in 1997 and 1998. Firstly, there was a rise in the number of both the epidemiologically important ticks (I.persulcatus and I.ricinus) in the nature, and as a consequence an increase in the number of TBE virus-carrying ticks. This rise was due to a rise in the number of small rodents (host of ticks), especially in 1996 and 1997. There was also an introduction of I. ricinus to new areas due to a reorganization of agriculture during the study period. Secondly, more cases of TBE were detected, due to improved laboratory diagnostics. Thirdly, the socio-economic changes in Estonia during the study period led to the reorganisations in agriculture, and a subsequent rise in unemployment, resulting in an increasing number of visits to the forests by poor people collecting mushrooms and berries. The impact of vaccination on the epidemiology could not be assessed due to lack of detailed vaccination data.
In Estonia the simultaneous circulation of all in Europe and Asia known genotypes of TBE virus was found, i.e. the Western, Siberian and Far-Eastern subtypes of TBE virus (3). This unique situation is conditioned on the simultaneous spread of the two species of ticks I .ricinus and I .persulcatus, which are reservoirs and vectors of different TBE virus subtypes.
A high number of ticks in the nature, a presence of TBE virus in the wild population of ticks, a genetic variety of the virus, frequent contacts with the ticks, a low immunity in the population, and insufficient vaccination, are all permanent risk-factors for TBE in Estonia. All these factors maintain the incidence of TBE at a relatively high level. The morbidity of TBE has been on the same level for over 30 years in several counties (Prnumaa, Tartumaa, Plvamaa). This suggest that the factors responsible for the high incidence of TBE in these specific counties have been stable.
Better health-information to the population about preventive measures against TBE and a change to a strategy with free vaccination of children against TBE are deemed necessary on the basis of the detailed analysis of the epidemiological situation in this report.
In conclusion, the results of the present investigation show that TBE is a serious health problem in Estonia. Further, ecological, climatic and social studies need be carried out together with virological and epidemiological investigations to further identify the exact reasons for the high incidence of TBE in the country, especially among young adults, and to develop effective TBE vaccination strategies.
1. Sss J. Epidemiology and ecology of TBE relevant to the production of effective vaccines. Vaccine 2003; 21(Suppl 1):S1935.
2. Lindgren E, Gustafson R. Tick-borne encephalitis in Sweden and climatic change. Lancet 2001; 358:168.
3. Golovljova I, Vene S, Sjlander KB, Vasilenko V, Plyusnin A, Lundkvist . Characterization of Tick-borne encephalitis virus from Estonia. J Med Virol 2004; 74:5808.