This article analyses 10-year incidence data of tick-borne encephalitis and Lyme borreliosis (over a 10-year period), as well as the results of entomological research to establish habitats of ixodid ticks in the Republic of Belarus. Information regarding ixodid tick population dynamics, the results of tick laboratory tests for Borrelia infection and the number of people seeking medical care in outpatient polyclinic institutions with complaints of tick bites is provided. Short-run tasks were determined for prevention specialists, treatment facilities and sanitary and epidemiological organisations for the prevention and control of natural focal tick-borne infections.
Tick-borne encephalitis (TBE) or Spring Summer encephalitis is an acute viral vector-borne natural focal disease characterised by general intoxication syndrome and nervous system impairment. In the Republic of Belarus, a Central European (Western) subtype of TBE is found. The peculiarity of this TBE subtype is a two-phase (rarely, single-phase) course of acute infection followed by the development of moderate neurological disorders mainly during the second phase. Lyme disease, also known as Lyme borreliosis or ixodid tick-borne borreliosis, is a bacterial vector-borne natural focal disease that primarily affects the skin, joints, nervous system, and heart.
Major efforts are made in the Republic of Belarus to detect and assess the degree of intensity of TBE and Lyme disease natural foci in woodlands that are most frequently visited by the population. Principal preventive measures for healthcare institutions and the need for disinfestations and, if necessary, extermination are based on these findings.
A molecular genetic analysis of TBE virus isolates indicated phylogenetic relationships with strains that circulate in countries neighbouring Belarus (Lithuania, Latvia and Russia). There are 2 dominant types of ixodid ticks in Belarus: Ixodus ricinus (synonyms: wood tick, dog tick, sheep tick) and Dermacentor reticulatus (bush tick).
The purpose of this article is to analyse the epidemiology of TBE and Lyme disease in the Republic of Belarus over a 10-year period and evaluate the intensity of natural foci in individual administrative territories.
Materials and methods
The Republic of Belarus is located in the centre of Europe and shares land boundaries with Latvia, Lithuania, Poland, the Russian Federation, and Ukraine. The population is 9,640,577. We analysed TBE and Lyme disease incidence data from regional sanitary and epidemiological institutions during the period from 1998 to 2007. The data were based on entomological research of ixodid tick habitats, population and infection with Borrelia, as well as numbers of people with tick bites seeking medical care at outpatient polyclinic institutions.
The criteria for reporting are registration of people with tick bites seeking medical advice and/or primary diagnosis of TBE or Lyme disease according to clinical signs and epidemiological anamnesis in the medical documentation of healthcare organisations. Information from healthcare organisations is promptly (within 12 hours by phone and within 3 days in writing) submitted to the district (municipal, territorial) centre of hygiene and epidemiology using Form 58/u “Expedited report of infectious disease, food poisoning, post-vaccine complication”. These centres report monthly aggregated data to the regional sanitary and epidemiological institutions and then to the Republican Centre of Hygiene, Epidemiology and Public Health and the Ministry of Health. If the diagnosis is cancelled or changed based on laboratory test results, the healthcare organisation will submit a subsequent report using Form 58/u, and the previously reported data will be corrected by sanitary and epidemiological institutions according to the revised report.
Departmental reporting of entomological vector monitoring (including tick’s bacterial infectivity studies) is based on data from primary entomological documentation provided by territorial sanitary and epidemiological institutions. Reports are submitted to superior bodies monthly (reports on tick populations at stationary points) and annually.
In order to analyse the intensity of tick infections foci in the republic there was developed and introduced in 2005 the natural foci intensity rate (NFIR) for both TBE and Lyme disease. The use of the rate (NFIRTBE and NFIRLD) allowed to level the wave-like pattern of annual vector population dynamics and their viral and bacterial infectivity.
A medium direct correlation was established (r = 0.7 with Р ≤ 0.05) between the incidence rate of tick-borne encephalitis and NFIRTBE. NFIRTBE is measured based on the number of infected vectors (I.ricinus tick species or morphologically similar species I.persulcatus referred to as I.ricinus) that a person can encounter when visiting a TBE- or Lyme disease natural focus. NFIR is measured in terms of the number of infected (viral and bacterial) vectors per flag kilometre. This measure is intended for completing operational tasks and for planning prophylactic and antiepidemic measures.
During the 10-year period, the incidence rate of TBE and Lyme disease had a significant tendency to increase annually with an intensity of 9.6% and 20.9%, respectively (Figure 1). The TBE incidence rates ranged from 0.2 (19 people) to 1.1 (108 people) and from 0 to 3.4 (48 people) throughout the republic and in the oblasts, respectively. The Lyme disease incidence rates ranged from 1.0 (114 people) to 9.1 (878 people) and from 0.2 (2 people) to 16.6 (233 people) throughout the republic and in the oblasts, respectively (Figure 1).
Children aged less than 14 years (Figure 2) accounted for 10-15% of the total number of cases. The outbreak incidence of TBE transmitted via an alimentary route (through raw goat milk) was less than 3% as determined during the epidemiological investigation of TBE foci in rural areas. Two outbreaks were reported, in 2006 and in 2007, with a total of 16 people infected.
Figure 1. Incidence dynamics of tick-borne encephalitis and Lyme disease in the Republic of Belarus, 1998-2007
Figure 2. Incidence dynamics of tick-borne encephalitis and Lyme disease in children aged 7-14 years in the Republic of Belarus, 1998-2007
The highest incidence of TBE is found in the Brest and Grodno oblasts that are located in the west of the republic. Infections in these oblasts account for an average of 40% each of the total number of reported cases. For Lyme borreliosis, the Minsk and Brest oblasts and the city of Minsk had the highest incidence (due to diagnosis of cases in the city of Minsk although the cases occurred in Minsk oblast, or more seldom - in other oblasts), and account for 15 to 27% of the total number of cases (Figure 3-4). At least 3 species of Borrelia are found to be causal agents of Lyme disease: B. afzelii, B. gаrinii and B. valaisiana (strain VS116).
Figure 3. Tick-borne encephalitis incidence in the Republic of Belarus by administrative territories, 1998-2007
Figure 4. Lyme borreliosis incidence in the Republic of Belarus by administrative territories, 1998-2007
According to data for 2007, the same oblasts had the highest numbers of people seeking medical care for tick bites: Grodno oblast (499.2 per 100,000 population), Minsk (421.8), Minsk oblast (350.5) and Brest oblast (345.6). The mean rate was 326.7 per 100,000 population for the republic. During the 10-year period, the change in the TBE incidence rate ranged from -24.5% (Mogilev oblast) to +15.0% (Grodno oblast), and the change in Lyme disease incidence rate ranged from +14.7% (Gomel oblast) to +30.0% (Minsk oblast) (Figure 3-4).
The increase in incidence rates is related to the greater number of seasonal vectors in the habitats (average increase by 2.1% per year) and an increase in TBE infectivity of vectors by an average of 9.7% (from 3.1% to 16.8%) and in Lyme disease infectivity of vectors by an average of 8.1% (from 6.4 to 38.6%) (Figures 5, 6).
Figure 5. Seasonal average numbers of ixodid ticks in the Republic of Belarus, 1998-2007 (in terms of specimens per flag kilometre)
Figure 6. Viral and bacterial infectivity of vectors in the Republic of Belarus, 1998-2007 (in terms of % of tested ticks)
The natural foci of TBE and Lyme disease were identified and mapped in the republic through joint efforts of experts from territorial sanitary and epidemiological organisations and the Scientific Research Institute of Epidemiology and Microbiology as part of two 5-year programmes for entomological surveillance in the Republic of Belarus (from 1996 to 2000 and from 2002 to 2006). Over the 10-year period, circulation of TBE and Lyme disease causal agents was detected in 13 and 84 districts, respectively. In the beginning of 2008, the total number of districts affected by TBE and Lyme disease causal agents was 96 (71.5%) and 120 (90.7%) of all administrative districts of the republic, respectively (Figures 7, 8).
Figure 7. Administrative territories of the Republic of Belarus where circulation of tick-borne encephalitis causal agents was identified, 1998-2007
Figure 8. Administrative territories of the Republic of Belarus where circulation of Lyme borreliosis causal agents was identified, 1998-2007
The greatest intensities of natural foci were detected in the western areas of Brest and Grodno oblasts for TBE and in the western areas of Brest oblast and central areas of Minsk oblast for Lyme disease.
In 2003, healthcare organisations implemented a complex system for prevention of Lyme disease for people bitten by infected vectors. The system includes a bacteriological examination in sanitary and epidemiological institutions to test ixodid ticks removed from people, prompt notification of healthcare workers in territorial outpatient polyclinic institutions about laboratory-confirmed results, and prescription of appropriate prophylactic treatment for infected people. As a result, 10,307 cases of infection were prevented after bites by infected vectors over a 5-year period, more than 3 times higher that the number of Lyme disease cases reported during that period (Figure 9). People seeking medical attention for tick bites provided I. ricinus vectors that they removed from themselves to regional sanitary and epidemiological institutions. During examination, a certain number of ticks were found to be infected by Borrelia (blue columns on Figure 9). This information was promptly submitted to the territorial healthcare organisation and appropriate prophylactic treatment was administered during the incubation period. Thus was determined the number of prevented cases of Lyme borreliosis (Figure 9).
Figure 9. Ratio of Lyme borreliosis cases to the number of prevented cases in the Republic of Belarus, 2003-2007
Vaccination (up to 3000 people a year) is usually used as a measure of TBE prevention among citizens visiting endemic areas of Spring Summer encephalitis (Trans-Urals, Siberia, Far East) or regions of high-intensity natural foci of western TBE in Belarus (Belovezhskaya Pushcha National Park). For prevention of disease, people bitten by ticks receive anti-tick immunoglobulin (up to 2000 people a year) and remantadine as chemoprophylaxis.
In general, the current epidemiological situations of TBE and Lyme disease are considered to be unstable. The situation is affected by climate and ecological conditions, maintenance of the vector population, bacterial and viral infectivity, and how often the population visits natural tick habitats. It is predicted that the situation will remain unstable in the near future.
The highest-intensity natural foci of TBE were detected in the western areas of Brest and Grodno oblasts. These areas adjoin the Belovezhskaya Pushcha National Park where all anti-tick measures are prohibited. The highest-intensity natural foci of Lyme disease are found in western areas of Brest oblast and central areas of Minsk oblast. Brest oblast is popular among gardening societies and the forest areas along the commuter train lines are favourite locations for mushroom and berry pickers in Minsk and Minsk oblast. Minsk oblast usually ranks No. 2 or 3 in the tick population and bacterial infectivity (after Brest oblast and sometimes Gomel or Mogilev oblasts). The authors have no data on whether there are significant differences in climate between these oblasts and other oblasts of the republic.
The TBE and Lyme disease incidence rates per 100,000 population in neighbouring countries were much higher than in the Republic of Belarus in 2008 (see table):
The Republic of Belarus
The primary objectives of healthcare organisations include: seasonal training concerning protection against bloodsucking arthropods and prophylaxis of TBE outbreaks caused by dairy consumption, entomological and microbiological monitoring of population levels and bacterial infectivity of ixodid ticks for implementation of disinfestation measures, providing preventive medical care to people bitten by infected vectors, and census of occupational groups that are subject to planned immunisation against TBE.
- Информационно-аналитические бюллетени ГУ «Республиканский центр гигиены, эпидемиологии и общественного здоровья» Минздрава РБ «Гельминтозы, протозоозы, трансмиссивные зоонозные и заразные кожные заболевания в Республике Беларусь» за 1998 – 2007 годы.
- Информационно-аналитические бюллетени ГУ «Республиканский центр гигиены, эпидемиологии и общественного здоровья» Минздрава РБ «Энтомологический надзор за акаро-энтомофауной, имеющей медицинское значение, в Республике Беларусь» за 1998-2007 годы.
- Протас И.И. Вклад сотрудников клинического отделения в изучение актуальных для Беларуси нейроинфекций (1964-2004гг.). Материалы юбилейной конференции, посвященной 80-летию НИИЭМ, 27-28 октября 2004 г., Минск.
- Самойлова Т.И., Вотяков В.И., Титов Л.П. Достижения и перспективы изучения арбовирусных инфекций в Республике Беларусь. Материалы юбилейной конференции, посвященной 80-летию НИИЭМ, 27-28 октября 2004 г., Минск.
- Зинкевич Н.А., Самойлова Т.И., Михайлова А.А., Климко Н.К, Яшкова С.Е., Веденьков А.Л. Выявление антигена вируса клещевого энцефалита в иксодовых клещах, снятых с людей на территории Минской области непрямым методом иммунофлюоресценции. Материалы юбилейной конференции, посвященной 80-летию НИИЭМ, 27-28 октября 2004 г., Минск.