Morbidity and Epidemiology of Tick-borne Encephalitis in the Russian Federation

  Published: 06.03.06 Updated: 09.03.2006 13:33:56

M. S. Vorobjeva, T. V. Vorontsova, E. A. Arumova, M. N. Paschshepkina

L. A. Tarasevich State Federal Institute for Standardisation and Control of Medical-Biological Preparations, Federal Centre of State Sanitary and Epidemiological Surveillance and Control

The rise in TBE morbidity in the 1990s may be explained by several factors including the lack of attention paid to studying the natural nidus of infection in the respective territories, and the insufficient vaccination coverage of the population. This primarily concerns city populations who constitute a significant part of those who contract TBE. Taking into account The rise in TBE morbidity in the 1990s may be explained by several factors including experiences from abroad (Austria, Germany and others), a strategy of specific and unspecific prophylaxis against TBE and improved diagnostics will help to reduce TBE morbidity.

A. G. Pavlov presented the first report of a serious neuro-infectious disease bearing the characteristics of spring-summer epidemic outbreaks in 1934. Viral TBE was isolated from Ixodes ticks for the first time in Russia in 1937 by L. A. Zilber in the taiga-regions of Khabarovsk and Primorsk. The same illness with similar clinical symptoms had also been observed earlier in the European parts of the country, in the Urals and in Siberia. After the detection in 1937 of the TBE-virus, all neuro-viral diseases were regarded as “Tick-borne spring-summer encephalitis”.

From the beginning of the 1950s, the morbidity of TBE in Russia had a cyclic character and an uneven distribution. Hardest hit was Western Siberia (46% of the total morbidity in Russia) and the Urals (34.5%). The morbidity in Eastern Siberia and the Far East constituted 6.3% and 5.9%, respectively, of the morbidity in the country.

In the 1990s some changes occurred. Following the mass reduction in the use of pesticides to control the natural nidus of TBE due to the ban on DTT, morbidity rose sharply everywhere (figure). In the period from 1995 to1999, the number of people contracting TBE increased 5.7 times in comparison with the period from 1971 to1987. The rise in morbidity of TBE in Russia over the last years is not only explained by the discontinuation of acaricidic cultivation (acaricides are substances that kill ticks and mites), but also by the ecological situation in the country.  Urbanisation of new territories, increased contact between the population and ticks in the natural nidus of TBE through visits to the forests for relaxation, berry and mushroom picking, tourism, etc. At the same time, the role of specific prophylaxis has sharply decreased.

Based on analysis of data from 1992 to 1999, a sharp increase in TBE morbidity was detected in the Russian Federation (from 6,310 to 10,298 cases). In addition to an overall increase, the number of lethal cases also rose (table 1).

Table 1.TBE incidence in Russia

The indicator of morbidity per 100,000 is higher for the rural population (table 2).

Table 2. TBE urban and rural incidence in Russia

An alarming TBE morbidity is also observed among children (table 3).

Table 3. TBE incidence in Russia in age group up to 14 years

Children up to 14 years of age constitute 1/5 to 1/3 of all reported cases. From 1995 to 1999, 27 children under the age of 14 died of TBE in Russia.

An analysis of TBE morbidity among individual subjects of the Russian Federation from 1993 to 1999 indicates that the Ural-, West Siberia- and East Siberia-regions are most affected (table 4). In the Ural-region (Udmurtia Respublic, Sverdlovsk and Primorsk oblasts), the indicator for morbidity of TBE has been 8 to 15 times higher than the Russian average for the last 7 years. In Western Siberia, the highest incidence figures (32.2-64.2 per 100,000) were seen in Tomsk oblast. In Eastern Siberia, the highest incidence of TBE was registered in Krasnojarsk krai (19. 2-52.8 per 100,000).

In the European parts of Russia, an unfavourable situation concerning TBE has developed in Kirovsk, Vologosonsk, Novgorod, Jaroslav and Kostroma oblasts and the Republic of Karelia. In Povolozhsk region, an increase in morbidity was registered in the Republic of Tatarstan, where 27 to 79 people fell ill every year, even if the incidence here is lower than the Russian average.

Fig. TBE incidence in Russia, 1939-1999

Table 4. TBE incidence in different regions of Russia