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TICK-BORNE ENCEPHALITIS IN ALTAI KRAI: CHARACTERISTICS OF THE EPIDEMIC PROCESS AND PREVENTIVE MEASURES
Altai State Medical University, Barnaul
S.V. Shirokostup, A.V. Timonin, N.V. Lukyanenko
The article presents a comparative assessment of the dynamics of tick-borne encephalitis incidence in Russia, the Siberian Federal District and Altai Krai, which is endemic in this area. A high proportion of the rural population (45.7%) in the region determines the development of the epidemic process, taking into account the activity of natural and anthropurgic foci of infection, the frequency of contact with the local population. The analysis of the factors directly influencing the dynamics of tick-borne encephalitis incidence is presented, namely, the virus and the number of mites, measures of primary and secondary prevention of the disease. Key words: tick-borne encephalitis, epidemic process, prevention, vaccination.
Tick-borne encephalitis is one of the most common natural focal infections in Western Siberia and the Far East. The high frequency of contacts of the population of Siberian regions with active foci of this infection makes a significant contribution to the formation of incidence rates of tick-borne encephalitis in Russia. In recent decades, under the influence of man's economic activity, natural foci near the boundaries of populated areas have been transformed into anthropogenic foci characterized by high activity. The increase in the number of anthropogenic foci has a direct impact on the epidemic process of tick-borne encephalitis, causing an increase in the proportion of urban population in the overall structure of the incidence of this infection. Changes in the structure of morbidity entail the need for operational management of primary and secondary prevention of tick-borne infections by the epidemiological service. In this regard, the adoption of timely management decisions to reduce the incidence of tick-borne encephalitis against the background of increased activity of foci should be based on an objective scientifically grounded epidemiological analysis of the epidemic process of the infection.
Material and methods
Altai Krai is chosen as the model territory for the study, because 57 out of 65 administrative districts of the region are endemic for tick-borne encephalitis, characterized by the presence of active natural and anthropurgic foci of this infection. Reports from the Office of the Federal Service for Supervision of Consumer Rights Protection and Human Welfare in Altai Krai "On the Sanitary and Epidemiological Situation in Altai Krai" for 2001-2016, forms of state statistical reporting No. 2 "Information on infectious and parasitic diseases" for 2000-2016 in Altai Krai, Form No. 5 "Information on preventive vaccinations" for 20002016 in Altai Krai were considered as materials for the retrospective epidemiological analysis.
Data processing in the present study was carried out using the following statistical analysis methods: calculation of intensive and extensive indicators, mean values (X), coverage error (m), Fisher's statistical significance criteria and Spearman's correlation estimation. For computer calculations, computer programs "Excel 2010" and "Statistica 8.0" were used.
The purpose of the study was to identify the specific features of the epidemic process of tick-borne encephalitis in Altai Krai and to assess the impact of preventive measures on it.
Results and discussion
During the study period from 2000 to 2016, in the Russian Federation, the epidemic process of tick-borne encephalitis was characterized by a tendency to decrease of incidence rates by 60.8% from 4.1±0.03 to 1.6±0.03 0/0000, respectively (p <0.01). The average long-term morbidity rate for this period was 2.6±0.030/0000. The rate of decline in the incidence averaged 3.6% per year with a minimum indicator of the period 1,4±0,030/0000 in 2014. The downward trend in the incidence of tick-borne encephalitis in Russia during this period of time is largely due to a combination of long-term natural cyclic activity of natural foci and an increase in the volume of preventive measures.
The average long-term incidence rate of tickborne encephalitis in the Siberian Federal District (SFO) in 2000-2016 (9,9±0,35 0/0000) exceeded the similar indicator for the RF (2,6±0,03 0/0000) by 3.9 times (p <0.01). The current situation is caused by the endemic character of the SFO regions for tick-borne encephalitis, the presence of a large number of active natural and anthropurgic foci and the high frequency of population contacts with foci of infection. During the study period, the SFO was characterized by a tendency for the decrease of the incidence rate by 62.5% from 15,7±0,28 to 5,9±0,17 0/0000 (p <0.01). The rate of decline in morbidity averaged 3.7% per year with a minimum index 4,8±0,16 0/0000 in 2014.
The Altai Krai territory in 2000-2016 was characterized by the tendency to decreasing incidence of tick-borne encephalitis by 41.7% from 5,2±0,44 to 3,0±0,35 0/0000 (p <0.01). The rate of decline in morbidity averaged 2.5% per year with a minimum of this period in 2013 (1,4±0,190/0000) and 2014 (1,4±0,160/0000). The average long-term morbidity rate for the region was 3,0±0,18 0/0000, which is 3.3 times lower than the same index for the Siberian Federal District (9,9±0,35 0/0000, p <0.01) and 1,
2 times higher than the RF index (2,6±0,030/0000, p <0.01). 0000
A characteristic feature of Altai Krai is the high proportion of rural residents (45.7%) in the total population structure of the region in 2000-2016. A comparative assessment of the incidence of tickborne encephalitis in the urban and rural population made it possible to determine the significance of natural and anthropurgic foci of infection in the formation of morbidity indicators in the region. Based on the data of the Office of the Federal Service for Supervision of Consumer Rights Protection and Human Welfare in Altai Krai for 20002016, the urban population was attacked by mites mainly by contact with anthropurgic foci of infection within or near settlements. The rural population was attacked by mites, mainly by contact with natural foci of infection during the performance of professional or domestic work.
The results of the retrospective epidemiolog-ical analysis showed that the average long-term incidence of tick-borne encephalitis in rural areas (3,3±0,53 0/0000) in 2000-2016 is by 16.5% higher than in urban residents (2,8±0,45 0/0000, p <0.01). The minimum incidence rate in this period among the urban population was registered in 2012 and amounted to ,2±0,30 0/0000, among the rural population - in 2013 and amounted to 1,3±0,35 0/0000 (p <0, 01). The rate of decline in incidence among urban residents was 37.2%, or an average of 2.2% per year, among residents of rural areas - 46.5%, or an average of 2.7% per year.
Age structure of the incidence of tick-borne encephalitis in 2000-2016 in Altai Krai was characterized by a 2.1-fold prevalence of cases of illness among the adult population (3,4±0,41 0/0000) compared with the age group under 18 (1,6±0,56 0/0000, p <0 , 01). The minimum incidence rate in the age group under 18 was registered in 2014 and amounted to 0,6±0,37 0/0000, among adults this indicator was registered in 2013 and amounted to 1,5±0,28 0/0000. Age groups were characterized by a pronounced tendency to reduce morbidity - by 58.2%, an average of 3.4% a year, in the adult age group and by 51.9%, an average of 3.1% per year, in the group of individuals up to 18 years.
During the period of 2000-2016, 18 cases of tickborne encephalitis with a lethal outcome among the adult population were registered, among children under 17 years of age, there were no cases
of lethal disease. The diagnosis in all cases was confirmed by laboratory methods of PCR and ELISA. 66.7% of all fatal cases were reported before 2004, when the highest incidence rates were recorded.
Circulation in natural and anthropurgic foci of highly virulent strains of tick-borne encephalitis is confirmed by the presence of severe forms of the disease in 45% of cases in patients in 20002016. All the diseased persons did not have preventive vaccinations against tick-borne encephalitis and did not apply to medical organizations for se-rological prevention.
The intra-annual dynamics of the incidence of tick-borne encephalitis in the Altai Krai, as well as in the whole territory of the Siberian Federal District, is characterized by pronounced spring-autumn seasonality. The appearance of the first cases of the disease coincides with the onset of mite activity and falls on April. During the study period from 2000 to 2016, the average long-term indicator of the number of population appeals to medical organizations regarding the sucking of ticks was 444,1±4,21 0/0000. In the age structure of bitten by mites, the adult population (70.2%) prevails. The index of treatment for bites of mites of the age group under 18 years (662,2±11,43 0/0000) is 1.7 times higher than that of adults (390,2±4,42 0/0000).
At the same time, the results of the correlation analysis of the dependence of the population's turnover rate on medical organizations on tick bites and tick-borne encephalitis incidence showed a weak positive correlation (r=0.18, p<0.01). The obtained data reflect the role of primary and secondary prevention in the development of the epidemic process of tick-borne encephalitis in the region.
Among the natural factors that affect the epidemic process of tick-borne encephalitis, it is also necessary to isolate the number of ticks per 1 km of pathway and virus-resistance. During the period of 2000-2016, the average number of ticks in Altai Krai was 417.0 ticks per 1 km of track. The analysis of correlation dependencies between this indicator and incidence of the population of the region with tick-borne encephalitis revealed a weak positive correlation (r=0.20, p<0.01).
In the Siberian Federal District, in 2000-2016, the average long-term value of virus tick resistance by Beklemishev was 4.0%, in Altai Krai - 1.4%. The active introduction of the PCR method into laboratory diagnostics of tick-borne encephalitis in the regions of the SFO provided an improvement in the quality of diagnostics and reliability of data on the spread of infected ticks. At the same time, during the evaluation of correlation dependencies between virus-resistance and the incidence of tickborne encephalitis, a strong positive correlation was noted in the whole in the SFO during the period from 2005 to 2016 (r=0.85, p <0.01). For Altai Krai in this period, there is a positive correlation of medium strength (r=0.43, p<0.01). The obtained
results reflect a significant effect of the tick-virus viral index on the epidemic process of tick-borne encephalitis.
Vaccination against tick-borne encephalitis is the main method of mass prophylaxis of this infection. During the period of 2000-2016, in Altai Krai, the vaccination rates of the population against tick-borne encephalitis decreased by 56.9% from 145,132 to 62,490 doses, respectively. Among children under the age of 14, the vaccination volumes decreased by 41.5% during the study period from 67,693 to 39,576 doses, respectively. On average, the share of the child population in the overall structure of the contingents to be vaccinated in 2000-2016 constituted 60.6%. Children under the age of 14 received the vaccine free of charge within the framework of the regional program "Vaccine prophylaxis". In 2016, the vaccination plan for children was fulfilled by 66%, which is due to the lack of a vaccine for preventive vaccinations for all contingents. The adult population living in endemic encephalitis territories was vaccinated at their own expense or at the expense of employers' funds.
Annually in the region, emergency prevention of tick-borne encephalitis is carried out for persons who have applied to medical organizations for the sucking of ticks. In the period of 2000-2016, on the average, emergency prophylaxis with im-munoglobulin was carried out in 70% of those affected by tick bites. The results of the correlation analysis showed the presence of a negative correlation of the average strength between the volume of serological prophylaxis and the incidence of tickborne encephalitis in Altai krai (r=-0.40, p<0.01).
On the territory of Altai Krai, in 2000-2016, aca-ricidal treatments were performed as a measure of nonspecific prevention of tick infections. During this period, the volume of acaricidal treatments was increased by 4.3 times and in 2016 constituted 2290 hectares. The effectiveness of this preventive measure is due to the correct choice of the location and time of treatments during the tick activity season. The presence of a negative correlation of the average strength between the acaricidal treatment area index and the incidence of tickborne encephalitis in the region (r=-0.41, p<0.01) reflects the ability to manage the epidemic process by regulating the volume of non-specific prevention measures.
Conclusion
Altai Krai is endemic in tick-borne encephalitis and is characterized by a tendency to decrease of incidence rates by 41.7% from 5.2±0.44 in 2000 to 3.0±0.35 0/0000 in 2016 (p<0.01). A special feature of the region is a high proportion of rural residents in the overall structure of the region's population, accounting for 45.7%. At the same time, the incidence of rural population in the period under study
(3,3±0,53 0/0000) is 16.5% higher than that of urban residents (2,8±0,45 0/0000, p<0,01). The influence of natural and anthropogenic factors on the incidence of tick-borne encephalitis causes the current trends in the development of the epidemic process. The average long-term viral mite index in the region was 1.4%. At the same time, the circulation of highly virulent strains of the virus in the foci of infection confirms the presence of severe forms of the disease in 45% of cases in 2000-2016. A promising direction for improving preventive measures in relation to tick-borne encephalitis is an increase in the amount of vaccination of the population subject to preventive vaccinations. The expressed inverse correlation dependencies of the average strength between morbidity and serological prophylaxis r=-0,40; p<0,01), between incidence and acaricidal treatments (r=-0,41; p<0,01) are a reflection of the use of factorial analysis in the management of the epidemic process, rationalization of the use of funds in the planning of the volumes of preventive and antiepidemic measures.
References
1. Rudakov N.V., Saveliev D.A., Andaev E.I., Balakhonov S.V., Kriga A.S., Vaitovich M.A., Lety-ushev A.N., Shchuchinov L.V., Shchuchinova L.D., Scherbatov A.F., Kutkina N.V., Starchenkova T.E. Differentiation of endemic territories on the risk of infection of population by agents of tick-borne infections as the basis of the tactics of their prophylaxis. National priorities of Russia. 2017; 4 (26): 60-67.
2. Shchuchinova L.D., Shchuchinov L.V., Zlo-bin V.I. Analysis of the factors influencing the effectiveness of vaccination against tick-borne encephalitis. Epidemiology and Vaccine Prevention. 2016; 2 (87): 72-76.
3. Konkova-reidman A. B. Tick-borne neuroinfections: Clinical characteristics, immuno-pathogenesis, and new pharmacotherapeutic strategies. Therapeutic archive. 2016; 11: 55-61.
4. Veje, Malin, et al. Diagnosing tick-borne encephalitis: a re-evaluation of notified cases. European Journal of Clinical Microbiology & Infectious Diseases. 2017; 37: 339-344.
5. Kollaritsch, Herwig, and Ulrich Heininger. Tick-Borne Encephalitis Vaccines. Pediatric Vaccines and Vaccinations. Springer, Cham. 2017: 137-145.
Contacts
Corresponding author: Shirokostup Sergey Vasi-lyevich, Candidate of Medical Sciences, Associate Professor of the Department of Epidemiology, Microbiology and Virology of the Altai State Medical University, Barnaul. 656038, Barnaul, Lenina Prospekt, 40. Tel.: (3852) 566869. Email: shirokostup@yandex.ru
Timonin Andrey Viktorovich, Lecturer of the Department of Epidemiology, Microbiology and Virology of the Altai State Medical University, Barnaul.
656031, Barnaul, ul. Papanintsev, 126. Tel.: (3852) 566924. Email: sekttor@mail.ru
Lukyanenko Natalia Valentinovna, Doctor od Medical Sciences, Professor of the Department of Epidemiology, Microbiology and Virology of the Altai State Medical University, Barnaul. 656031, Barnaul, ul. Papanintsev, 126. Tel.: (3852) 566924. Email: natvalluk@mail.ru
