Analysis of tick-borne encephalitis (TBE) and Lyme disease (LD) incidence among the population of Novgorod oblast during the last decade and the results of entomological monitoring showed that the incidence of the diseases increased every three years (in 2000, 2003, 2006, and 2009) and that the cycles of incidence growth and decline coincided with cycles of ticks' activity. The efficiency of the current prevention measures was assessed. Major goals for TBE and LD prevention and control activities to be implemented by the specialists of health institutions and sanitary and epidemiological surveillance facilities were identified.
Tick-borne encephalitis (TBE) and Lyme disease (LD) are endemic in the territory of Novgorod oblast. The first cases of TBE in this area were registered during World War II among soldiers of the Volkhov Front. Currently TBE and LD still pose a significant risk for the population of the region and thus prevention remains a major goal for healthcare workers. This paper describes the TBE and LD epidemic situation in Novgorod oblast. A retrospective analysis of epidemiological situation for TBE and LD was performed and the efficiency of comprehensive preventive measures taken against the tick-borne infections was assessed.
Materials and Methods
According to statistics as of January 1, 2010, the population of Novgorod oblast is 640,613: 446,876 people live in cities and 193,737 people live in rural areas. The climate is moderately continental with high humidity; winters are comparatively warm and summers are cool. The average temperature in January is –10оС; the average temperature in July may range from +16.5о to +18оС. The mean precipitation rate is from 500 to 650 mm per year. The maximum precipitation occurs during the summer (38%) and autumn (27%). The warm temperature and poor evaporation result in the formation of great bogs. Over 50% of the area is covered by coniferous, deciduous and mixed woods.
We have analyzed data on the incidence of TBE and LD, and the results of entomological investigations of ixodic ticks within their habitat and on the number of patients who visited out-patient clinics after being bitten by ticks during the period from 2000 to 2009. To track temporal changes in the natural focus and to forecast the future epizootological situation, a forest plot presenting a typical natural TBE and LD focus was monitored. The number of ixodic ticks was determined by the stationary flag method (ticks per flag/km). Ticks were counted every ten days from the beginning of April to the middle of September.
TBE virus antigen was detected by the enzyme immunoassay (EIA) method using test systems by “Vector Best” (St. Petersburg). Borrelias were found by dark-field microscopy using dyes by the Romanovsky-Gimza method. Serum samples were investigated by EIA for immunoglobulin M and G antibodies against TBE virus using test systems by “Vector Best” (St. Petersburg), and for borrelias using test systems by “Omnix” (St. Petersburg). The polymerase chain reaction (PCR) for molecular diagnostics has been used since 2010; the “AmpliSens® Borrelia burgdorferi sensu lato-FL” test system has been used for LD and “AmpliSens® ТВЕ-FL” test system by “Inter LabService”, Moscow, for TBE. The data on the number of registered TBE and LD cases is collected at the regional and federal level every month.
Over the last decade, the annual average number of TBE cases varied from 3 to 50, with the TBE incidence ranging from 1.0 to 7.4 per 100,000 population. Eighteen to 80 LD cases were registered every year, yielding an incidence of 6.1–18.4 per 100,000 population. The average TBE and LD incidence in the oblast were usually higher than the corresponding rates in the country as a whole (Figures 1, 2).
Fig. 1. Tick-borne encephalitis incidence in Novgorod oblast per 100,000 population, 2000-2009
Fig. 2. Lyme disease incidence in Novgorod oblast per 100,000 population, 2000-2009
TBE and LD incidences show a cyclic increase every three years (in 2000, 2003, 2006, and 2009) (2). In the reporting period, the share of children under the age of 14 among TBE patients was 8% and among LD patients 10% (Figure 3). The TBE incidence was especially high in Novgorod, Malaya Vishera, Chudovo, Lyubytino, Borovichi, Kresttsy, Pestovo, Staraya Russa and Khvoynaya districts. The LD incidence was high in Novgorod, Borovichi, Soltsy, Staraya Russa, Chudovo and Shimsk districts.
Fig. 3. Age structure of patients in Novgorod oblast in %, 2000 – 2009
The number of patients who visited out-patient clinics after being bitten by ticks changed with the same periodicity as the incidence (Figure 4).
Fig. 4. Number of patients who visited medical facilities after being bitten by ticks in Novgorod oblast, 2000-2009
The same tendency was revealed through analysis of the number of ticks per flag/km in different years (Figure 5).
Fig. 5. Average seasonal number of ticks collected by the flag method from plants in Novgorod oblast, 2000-2009
In Novgorod oblast the following species of ticks can be found: I.persulcatus (especially active) and I.ricinus (fewer in number). Annually 480 I.persulcatus ticks and 20 I.ricinus ticks are checked for TBE virus antigen and for borrelias. TBE virus is found in 0.7 – 9.5% cases while the Lyme disease pathogen is found in 15.9 – 30.6% cases (Figure 6).
Fig. 6. Viral and bacterial infectivity of ticks in Novgorod oblast in %, 2000 – 2009
TBE vaccination of the population is a traditional preventive measure and up to 6 thousand people are immunized every year. People at highest risk of infection are vaccinated first. The following professions are considered to have the maximum risk: road and railway constructors, oil and gas pipeline builders, topographers and forestry workers. Representatives of risk groups are immunized with the culture-purified concentrated inactivated vaccine “EnceVir” manufactured by the Federal State Unitary Enterprise “Microgen Scientific Production Association”, Tomsk, at the Ministry of Healthcare in the Russian Federation.
Human anti-tick immunoglobulin against TBE is used for emergency prevention in accordance with the sanitary rules “On Prevention of Tick-Borne Viral Encephalitis” approved by Resolution No.19 (dated 07.03.2008) of the Chief State Sanitary Doctor of the Russian Federation (4). The immunoglobulin is administered in emergency situations during the first three days after a patient has been bitten by a tick and is primarily used for preventive treatment among children. If it is possible to check the tick for TBE virus antigen, the decision to provide human immunoglobulin in each particular case is based on the results of rapid diagnostics of ticks. In 2009, 1374 ticks were tested; the TBE virus antigen was found in 67 of them (4.9%). In addition, acaricide treatment is provided in recreational areas where children and adults spend their vacations. In 2009, 182 hectares underwent this kind of treatment.
Among other preventive measures, non-specific measures using various acaricides and repellents for individual protection against ticks in natural foci are used including solid acaricide “Preticks”, “Bitex”, “Deta”, and aerosol “Reftamid”.
Unlike many other parts of Russia where the TBE incidence has declined steadily since 2000, the Northwestern region of the country still sees some increase in the disease incidence. In 2003, 2006 and 2009 the average incidence in the region was higher than in the country as a whole. In 2000-2009 the maximum number of TBE cases was registered in St. Petersburg, Vologda oblast and the Republic of Karelia. The joint share of TBE patients in all these oblasts was 60.5% while in Novgorod oblast the joint share of these patients was 7.1% (1).
The LD incidence in the Northwestern region and Novgorod oblast is increasing. In Novgorod oblast there are some persistent coupled foci of TBE and LD. The three-year activity cycles of the disease may be explained by biological factors regulating the activity of ticks.
The total number of ticks in nature and the level of their viral infectivity influence the incidence of TBE (3). Comprehensive preventive measures against TBE and LD have been implemented in the region. Anti-tick immunoglobulin is used for emergency prevention in accordance with the federal sanitary regulations.
Laboratory diagnosis of TBE and LD and the rapid analysis of ticks found on people are effective. Mass media pays much attention to issues addressing TBE prevention. Medical facilities have a sufficient supply of immunoglobulin for preventive treatment purposes.
The major goals of medical workers and sanitary epidemiological surveillance specialists include the following: extension of the patient cohorts covered by vaccination; acaricide treatment of recreational areas; entomological monitoring of ixodic ticks; community outreach on individual and public measures to be taken against the tick-borne infections.
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