Tick-borne encephalitis (TBE) is the most common central nervous system infectious disease in Estonia. Estonia is a tick-borne encephalitis-endemic area as Ixodes ricinus and Ixodes persulcatus ticks, the vectors of TBE-virus, are spread throughout the country. Notification of TBE in Estonia started in 1949. While the incidence of TBE was low or moderate from the 1950s to the 1980s, a clear increase in the incidence has been observed since the 1990s. The highest incidence was noted in 1997 (27.8 cases per 100,000 population). By 2008, the incidence (6.7 cases per 100,000 population) returned to the level observed in the early 1990s (6.5 cases per 100,000 population).
Materials and methods
The analysis was performed on data from the Estonian National Infectious Disease Registry for 1999 – 2008 using descriptive and graphic methods. In Estonia, each report of TBE is sent using established procedures to the regional Health Protection Service institution where data are entered into the national registry. All laboratory-confirmed cases of TBE must be registered.
General trends in incidence
An analysis of data showed that since the 1970s, the incidence of TBE in Estonia had a steady upward trend (Fig. 1)
Fig. 1. Incidence of TBE in Estonia, 1970-2008 (number of cases per 100,000 population)
A peak incidence was noted in 1997 and 1998 (27.8 and 27.0 cases per 100,000 population, respectively). Since 1999, a downward trend in the incidence rates has been observed.
During the period from 1999 to 2008, no deaths from TBE were reported. The proportion of persons with TBE that were hospitalised ranged from 75.6% to 96.7%.
In the age structure of the TBE-infected population, the highest morbidity in Estonia from 1999 to 2008 was among people over the age of 40 (Fig. 2), 53.5% (lowest rate of 42.1% in 1999, highest rate of 66.5% in 2004) of the entire infected population.
Fig. 2. TBE cases by age in Estonia, 1999-2008 (%)
The proportion of children aged 0-14 years in the TBE-infected population was 16.0% (lowest rate of 8.8% in 2004, highest rate of 23.5% in 2000). Among children aged less than 1 year, 3 cases of TBE were reported (0.2% of total cases).
A seasonality analysis of the disease (Fig. 3) shows that the majority of cases (86.7%) occur from May to September.
Fig. 3. Seasonality of TBE incidence in Estonia, 1999-2008
In recent years, there is a trend towards a longer TBE incidence season; cases were reported in March (2 cases, in 2004 and 2008) and December (2 cases, in 2000 and 2002).
In Estonia, cases of infection are registered at the patient’s place of residence. Standard data entered into the Infectious Disease Registry also include information about the suspected area where infection occurred.
An analysis of data on suspected areas of infection from 2004 to 2008 showed that:
- in 79.5% of cases, the infection originated in Estonia;
- in 1.1% of cases, the infection originated outside Estonia (imported cases);
- in 19.4% of cases, the place of infection origination was unknown.
In 1999-2008, the highest number of infections in Estonia (Fig. 4) occurred in Ida-Viru County (North-Eastern Estonia; 21.5% of all cases), Harju Country (including Tallinn, Northern Estonia; 15.6%) and Saare County (a group of islands by the western coast of Estonia; 15.4%).
Fig. 4. TBE cases by place of infection in Estonia, 1999-2008
The lowest number of cases occurred in the counties of Central Estonia; Järva County (0.6%), Rapla County (1.1%) and Viljandi County (1.3%).
Routes of transmission
A distinctive feature of TBE epidemiology in Estonia in recent years is that the infection, in many cases, is believed to be transmitted via the alimentary route when consuming raw goat milk. Investigation of the outbreak in 2005 (37 cases) also included testing of goats, which revealed positive TBE–virus antibodies. The percentage of cases transmitted via an alimentary route was 2.2%, 29.3% and 8.8% of all reported cases in 2004, 2005 and 2006, respectively. In 2007 and 2008, no such cases were reported.
Infectiousness of ticks
According to the data from V. Vasilenko (unpublished data), ticks collected from 2006 to 2008 from different regions of Estonia were infected with TBE-virus with the following frequencies; 5.6% Laeva (Tartu County), 6.4% in Jarvselja (Tartu County), 3.6% in Oonurme (Ida-Viru County), 0.2% in Puhtu (Lääne County), 0.6% in Kilingi-Nõmme (Pärnu County) and 6.0% in Tudulinna (Ida-Viru County).
TBE immunisation is not included in the National Immunisation Schedule in Estonia. Vaccination is provided for a fee, but is free for individuals whose job is associated with a high risk of TBE infection (foresters, irrigators, military personnel). Vaccination of these individuals is covered by the employer. There is no information about TBE cases among vaccinated (or partially vaccinated) during the study period.
Fig. 5. Number of people immunised against TBE in Estonia, 1999-2008
The number of people immunised against TBE has steadily increased in recent years and currently is approximately 30,000 people per year (Fig. 5) while the total population of Estonia is 1,340,000. Revaccination rates range from 30.0% to 45.0% of the total number of immunisations. Children aged 0-14 years account for 23.1% of the immunised population. The overall TBE vaccination coverage in Estonia is 10%.
The existing natural focus of TBE in Estonia is stable. Virus circulation is maintained by populations of rodents, wild animals and ticks. The warming climate promotes the growth of vector populations and prolongs the active period of the vectors. In addition, the reduction of agricultural lands encourages an extension of tick habitat.
In recent years, there has been a downward trend of TBE incidence in Estonia, and the number of vaccinated people is growing. Unfortunately, the vaccination coverage is low and does not have a significant effect on the epidemiological situation in Estonia.
A summer-autumn seasonality of the disease is evident. People older than 40 years still remain the most commonly affected population. It is appropriate to recommend vaccination for tourists visiting Estonia during the summer especially if they plan to stay in non-urban areas.