The current epidemiological situation regarding tick-borne viral encephalitis is serious and has recently acquired some new characteristics. As a result of anthropogenic influence, the range of ixodic ticks has extended, with urban population cohorts becoming involved in the epidemic process. Working-age people get infected most often. Due to various biotic and abiotic factors, the density of ixodic ticks and their hosts is currently increasing and the biology of ticks is changing. Simultaneously, the natural and anthropurgic foci of the infection are expanding. More sophisticated preventive and counter-epidemic measures in the foci of tick-borne infections are needed.
Tick-borne viral encephalitis (TBE) is a neuroviral zoonotic infection widely spread in European North, Siberia and the Far East. The endemic area for TBE is a region in which the virus circulates among ticks and their vertebrate hosts. The development of natural foci of TBE depends on the activity of infected ticks and their hosts, population density, susceptibility of hosts to the infection, share of the population that is not susceptible to infection, characteristics of a biotype, temperature, climate changes and social factors [6, 8]. The prevention of TBE is particularly important since at present there is no effective treatment for the disease.
The Republic of Komi is situated in the far north-east of European Russia. In the south and south-east it borders Sverdlovsk and Kirov oblasts and Perm territory where TBE is also widespread. The climate is moderately continental with high humidity. The central and southern part of the republic is covered with boreal coniferous vegetation (taiga). The fauna is rich and diverse including 57 species of mammals, 239 species of birds, 5 species of amphibians and 22 species of rodents. Blood-sucking insects include 19 species of ticks, 13 species of fleas, 29 species of blackflies and 14 species of mosquitoes [1, 2].
Extensive deforestation is typical of forested regions in the southern and central parts of the republic. The population actively gathers wild harvest, and hunting and fishing are also very popular. The areas influenced by anthropogenic factors are characterized by a drastic change in prevailing species of rodents and insect-eaters.
The goal of this paper is to study ecological factors enhancing the spread of TBE in the region, to define epidemiological characteristics of the natural focus of the infection and to describe prevention measures in the Republic of Komi.
Materials and Methods
Climate data for the region were provided by the Centre for Hydrometeorology and Environmental Monitoring of the Republic of Komi and epidemiological data have been received from the Centre of Hygiene and Epidemiology in the Republic of Komi and subjected to analysis. TBE incidence and the number of patients visiting out-patient clinics for tick bites were also analyzed. Data were collected from endemic areas considered to be the natural reservoirs (foci) of TBE and bordering areas in the south of the republic were analyzed. The data were primarily from the reporting period (2001 to 2008). Information collected since 1970 when the first TBE case was registered was also taken into account.
Ticks were investigated for TBE pathogen by the enzyme immunoassay (EIA) method using the “Vektor Best“ test systems manufactured in Novosibirsk, Russia. A total of 1603 ticks were studied.
Results and Discussion
Environmental conditions contribute to the emergence and spread of the natural focus of vector-borne infections including TBE. Disease prevalence is known to depend on the distribution and spread of infection carriers and vectors (Ixodes persulcatus and Ixodes ricinus ticks) in addition to forest industry activities and efficiency of preventive measures. The main factor that determined the emergence and survival of the TBE pathogen in the natural focus was the wide distribution of the hosts: the taiga tick – Ixodes persulcatus and the European tick – Ixodes ricinus.
Both of the species are polyphages: they can feed all species of mammals, birds and reptiles. The larvae and nymphs of ticks live on murine rodents, chipmunks, squirrels, hares, other small mammals and insectivorous birds. Humans are mostly bitten by imago and nymphs of ticks. The natural biotopes of Ixodes ticks are mixed forests, spruce forests, cutover patches, and forest paths. Depending on conditions, the life cycle of ticks can last 2 or 3 years. Ticks in all development stages spend winters in cracks, surface layers of the soil or in the forest floor. The duration of each development stage depends on the air temperature and relative humidity .
The winter is longest in the north-east of the republic. In the south the cold period lasts 170-180 days. At the coldest time of the year the average monthly air temperature is -15оС. In spring the air temperature rises to over 10оС by late May and the weather is moderately warm during the summer. In autumn the air temperature falls below 10оС by the end of September. The average monthly air temperature during the warmest part of the year in the south is about 17оС. The long-term meteorological monitoring data show that the climate in the region has become a little warmer. From 1935 to 1990, the air temperature in the central part of the region increased by 0.2 – 0.4оС during winter and remained stable in summer . While the administrative territories in the south of the republic are most comfortable for living, the population of these areas faces the highest risk of TBE infection. Table 1 shows the major meteorological data from the southern territories (Koygorodok district and Priluzsk district) during the periods when the highest activity of tick-borne natural reservoir was observed.
Table 1. Meteorological characteristics of administrative territories with the most cases of TBE in the Republic of Komi (April-September 2001-2008)
Research addressing TBE indicates that the borders of natural foci may change due to the transformation of biogeocenoses under anthropogenic pressure and climate changes. These factors influence the circulation of the pathogen .
According to the results of long-term monitoring, Ixodes persulcatus and Ixodes ricinus ticks live in subzones of southern and middle taiga. The ticks are primarily found in mixed secondary forests and cutover patches. The undergrowth in these areas consists of honeysuckle, brier and juniper and has a thick grass cover. Such conditions are especially favorable for ticks. In the last decades, the southern and middle taiga was subject to extensive deforestation that resulted in the growth of secondary forests, which creates favorable conditions for the spread of ticks, and high human activity in the forest where people could be bitten by ticks.
The results of an entomological survey indicate that since 2006 the number of Ixodes persulcatus and Ixodes ricinus ticks has increased as compared to the median long-term level. The peak value was reached in 2008 (see Figure 1).
Fig. 1. Number of ticks collected per flag-hour in the endemic area in the south of the Republic of Komi, 2006-2008
While the population of ticks is growing and becoming more and more active and the number of patients visiting healthcare institutions due to tick bites is increasing, the viral infectivity level of the ticks is getting lower. This may be viewed as a positive tendency in the epidemiological situation. The ticks found in Sysolsk, Koygorodok and Priluzsk districts in the south of the republic showed the highest infectivity level. Patients bitten by ticks are registered in most parts of the republic (17 out of 20 administrative territories) with the greatest number of cases in the south. The minimum number of cases was reported in Usinsk and Izhma districts and the maximum number in Priluzsk and Sysolsk districts.
Fig. 2. Number of cases seeking medical assistance after tick bites in relation to viral infectivity of ticks in the Republic of Komi, 2006-2008
From 1970 when the first case of TBE was registered to 2008, 83 people were infected in the Republic of Komi. Within this period the incidence rate varied from 0.09 to 1.03 per 100 000 population. The maximum incidence was registered in Priluzsk and Koygorodok districts in the south of the republic. The majority of patients (95%) were urban residents who temporarily left the city.
Fig. 3. TBE incidence in the Republic of Komi per 100,000 population, 1993 - 2008
Laboratory analyses confirmed 82.2% of the cases. The most affected groups were forest specialists, retired persons and unemployed people. This indicates a growing importance of social factors in the dynamics of disease incidence. The seasonal distribution shows that all reported TBE cases were registered during May to August, the 4 warmest months of the year, with peak values reached in June and July. The clinical presentation of the disease was characterized by considerable pathomorphism. The incubation time usually lasted up to 14 days. All patients had fever: 10% had temperature of up to 38оС, 84% – from 38 to 39оС; 5% – over 40оС. Two lethal outcomes (one adult and one child) were registered. Serologic surveys for antibodies to tick-borne encephalitis virus were conducted among healthy people living in the endemic area. Reactive results were obtained for 13% of the surveyed people. The majority were men living in the non-urban area.
The TBE incidence rate in the Republic of Komi is characterized by cyclicality. At present the disease incidence is decreasing despite the growing population of ticks and increasing frequency of reported bites. This may be explained by the current preventive measures .
From the 1990s, TBE immunization was introduced in the two endemic areas of the republic for certain groups of the population. Persons at risk were involved in activities including agriculture, hydrotechnical melioration, building and construction works, ground excavation and moving works, purveyance, development, geological investigations, exploratory works, expeditions, disinfestations and disinsectizations, timber harvesting, wood clearing and recreational development. In accordance with the regulation of the Chief State Sanitary Doctor of the Republic of Komi, the natural focus of the infection in 2007 was determined as lying within six administrative territories of the republic: Priluzsk, Koygorodok, Sysolsk, Syktydvinsk and Ust’-Kulom districts and the city of Syktyvkar. The regulation also specified the categories of population to be immunized and defined the acaricide treatment algorithm. In 1999 a special law was brought into effect to determine how additional immunization of the population was to be financed. This new law was advantageous for the planning of preventive measures. The population was immunized with a purified inactivated dry vaccine made in Russia by the Federal State Unitary Enterprise for Manufacturing Bacterial and Viral Medications of the Institute of Poliomyelitis and Viral Encephalites named after M.L. Chumakov at the Russian Academy of Medical Sciences.
These measures comply with the algorithm of preventive arrangements prescribed by the Regulation of the Chief State Sanitary Doctor of Russia concerning the same problem (the document came into force in 2005). Emergency preventive measures were taken and community awareness work was organized. The major scope of mass preventive measures is outlined in Figure 4.
Fig. 4. Scope of mass preventive measures against TBE in the Republic of Komi, 2005-2008
Acaricide treatment was mainly applied in the areas of summer camps, around child education institutions and in cemeteries. The acaricides were chosen with regard to safety for the population and efficiency.
Lyme disease, another infection associated with ixodic ticks, has been registered in the same areas since 1997. The epidemiological situation concerning Lyme disease is similar to TBE but the intensity of the epidemic process is much lower.
The natural conditions in the Republic of Komi are favorable for the permanent focus of TBE and other tick-borne infections such as monocytic ehrlichiosis, granulocytic anaplasmosis, babesiosis and West Nile fever. Currently, TBE poses the major epidemiological threat for this area. The infection causes considerable social and economic damages. The long-term monitoring of the epidemiological situation in the region demonstrates the increasing importance of the problem since the natural focus of the infection continues to expand because of various activities of the population. The interrelation between the focus expansion and climate changes is not yet fully understood and requires further investigations and monitoring.
Mass and individual prevention helps to stabilize the epidemiological situation. Effective measures include immunization of at least 95% of the population living in endemic areas, acaricide treatment in the areas of summer camps, around child education institutions, in cemeteries, garden plots and parks, and community awareness campaigns in the mass media.
In the future, it may be necessary to start epidemiological monitoring of all types of tick-borne infections in the area. The development of differential diagnostic tests and epidemiological monitoring for this purpose is warranted.
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