Tick - borne encephalitis in Latvia

Incidence

TBE is a notifiable disease in Latvia. Every case is investigated by an epidemiologist of the local Environmental Health Centre, and a completed investigation form is submitted to National Environmental Health Centre for data analysis using a modified EpiInfo programme.

Since the beginning of the 1990s, especially since 1993, we have registered an increased incidence (table 1). In 1998, 1029 persons fell ill and ten of the patients died. During the ten first months of 1999, we have registered 312 cases, including two fatal cases. The epidemiological situation of TBE in 1999, in comparison with the last years has improved (table 1).

Table 1. Tick - borne encephalitis cases in Latvia, 1990 - 1999

Age and sex distribution

The TBE incidence rate among adults is about two times higher than among children. The highest incidence rates were registered in the age groups over 40 (table 2). Most of all the adult cases (2/3) were retired people and unemployed persons.

Table 2. Distribution of TBE cases by age groups in Latvia, 1990 - 1999

During the last decade, the proportion of children among the patients has slightly decreased and the number of middle-aged persons increased (especially older than 60).

The TBE incidence rate among males is 1.2 times higher than that among females. The incidence rate among the rural population is about 1.5 times higher than that among the urban one.

Seasonal and geographical distribution

The disease incidence shows a strict seasonal pattern, probably due to patterns in human visits to the forests and the tick activity. The alimentary route of tick-borne encephalitis remains an important problem in Latvia, accounting for at least 2% of cases annually during the last decade.

The geographical distribution of TBE is not uniform. The highest incidence rate has been observed in the central part of the country, the lowest - in the south-eastern part.

Clinical features

About 30% of patients developed a febrile form of the disease, 60% a meningeal form and and about 6-10 % a focal form. Most of the TBE patients recovered without sequelae, about 0.8% died. Since 1993, most of TBE cases (more than 95%) have been confirmed by ELISA test.

Two thirds of patients report a tick bite. Most people were infected mainly in the vicinity of their residences within the regional borders.

Discussion

The causes of the increased incidence of TBE in Latvia in the 1990s seem to be:

• An increase of both tick number and their feeder population. Entomological data show a significant increase in density and spread of ticks. Annual investigation of TBE natural foci indicates a considerable increase in the density and spread of both the epidemiological significant Ixodes species: Ixodes ricinus (prevalent in the whole territory of Latvia) and Ixodes persulcatus (prevalent only in the eastern part of Latvia). Since 1990, the average seasonal tick number per square kilometer (data from the investigation sites) of I. ricinus (imagos) has increased more than six times and of I. persulcatus (imagos) almost five times.
• More frequent visits to the forests, often by poor people harvesting mushrooms, berries and herbs.
• Insufficient level of active immunisation in the population.

The decreasing incidence in 1999 can be explained by specific weather conditions during the ticks' activity season. This has negatively influenced the number of ticks. The public's rising awareness about TBE preventive measures, and the increased number of vaccinated adults have also played a role.

Prevention

The National Environmental Health Centre is responsible for surveillance and development of the TBE prevention policy. Vaccination is considered as the most effective preventive measure. A strategy of free vaccination of children against TBE in the highly endemic communities has been developed on the basis of detailed analyses of the epidemiological situation. In the beginning of 1998, vaccination was organised in 21 administrative territories, and 5940 children received two doses of vaccine. In the autumn of 1998, the same children received a third dose of vaccine. During the autumn 1998 and spring 1999, two doses of vaccine was given to 19 420 children in 57 administrative territories. In the autumn - winter 1999/2000 those children will receive the third dose of vaccine.

Recently it is planned to start TBE vaccination of orphans. Furthermore, in the beginning of the year 2000 we plan to start vaccinating children in the 11 highly endemic administrative territories. Since the beginning of the vaccination campaign until 1999, 50 999 doses of vaccine have been purchased by government funds and 37 725 doses have been donated as humanitarian aid, including 33 990 doses from Baxter Medical AB, 2880 doses from Denmark and 855 doses from Malteser.

Following the mass vaccination campaign, only three TBE cases among children in the highly endemic territories were registered in 1999 as compared to the expected 46.6 cases. National Environmental Health Centre specialists are planning to continue this work.