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Natural foci of tick-borne encephalitis and prerequisites for their existence
I
lnt,J.Med. Microbial. 291, Suppl. 33, 183-186 (2002) © Urban & Fischer Verlag http://WVNJ.urbanfischer.de/journals/ijmm
M
extended Abstract Natural foci of tick-borne encephalitis and prerequisites for their existence Vlasta Danielova * National Institute of Public Health, Prague, Srobarova 48, CZ-10042
Since 1948 when its causative agent was isolated in the Czech Republic for the first time in Europe, tick-borne encephalitis (TBE) has occured permanently in all Czech regions, and in comparison with other countries more frequently. This situation can be explained by the favourable conditions existing in the Czech Republic i. e. the existence of TBE natural foci where the virus life cycle is easily maintained. This means that the landscape is highly varied and tessellated. It consists of both forests, characterized by great diversity in species, structure and size, and farm land. This in turn leads to formation of frequent transitional areas called ecotones, providing an optimum environment for all developmental stages of ticks and their respective hosts and thus TBE circulation. The latter is facilitated by the abundance of game associated with traditional game keeping that dates back to the early middle ages. The presence of roe deer plays a key role in the TBE natural foci as reported by Zeman and januska (2000), based on their detailed analysis. The TBE natural foci differ markedly in their epidemiological activity: TBE morbidity in human population has been permanently notified in western and southern Bohemia while some eastern and northern Bohemian foci are characterized by long-term TBE latency. Similar data are available e. g. from Germany (Suss and Dorn, 1997). In the latent foci, the TBE virus can be maintained for a long time in animals of the elementary focus only, the human population remaining uninfected for different reasons. Latency may develop as a result of the natural biocenosis succession or human intervention but also may be due to the virus itself
that undergoes selection pressure, more precisely to the fact whether the peripherally invasive part of the viral population prevails or not. The epidemiological service does not consider such latent natural foci as such at risk, which in turn may lead to erroneous conclusions since some initially active natural foci may become latent to be reactivated later again. Natural foci of the districts OStl nad Labem and Decfn situated close to the north-western border to Germany can serve as examples. While in the 1950's and 1960's multiple TBE cases in humans were notified in these districts, the following decades were characterized only by isolated cases of TBE, mostly imported. The situation has been changing since 1993/94, as documented by the increasing TBE morbidity rates reported from the districts OStl nad Labem, Decfn and Litomerice (since 1995) (Table 1) (National Reference Laboratory for Epidemiological Analysis, National Institute of Public Health, Prague). There is one possible interpretation: during the latency period, the TBE virus must have shown low circulation among animals only. Our working hypothesis of the cause of new TBE outbreaks in northern Bohemian foci has been tested. The TBE virus could be considered as absent in northern Bohemia throughout the entire latency period. Conclusions on the geographical distribution of the TBE natural foci cannot be based on the human morbidity data only. Furthermore, the mode of infection is to be taken into account. In the Czech Republic, most people are infected while staying in recreational areas where many of them own holiday homes and others like walking in the forest and picking wild berries and mushrooms. That is why the best known
* Corresponding author: Vlasta Daniclova, National Institute of Public Health, Srobarova 48, CZ-10042 Prague 10, Czech Republic, Phone: 00420267082472, Fax: 00420267082466, E-mail: nernecekvts'auer.cz 1438-4221/01/291/8-000 $ 15.00/0
184
V. Danielova
recreational areas can be considered highly active foci. For instance, if comparing TBE morbidity in residents and holidaymakers in a recreational area south of Prague (valley of the Vita va and Sazava rivers), TBE appears clearly to be a recreational infection. Another source of distortion may be due to the fact that TBE infection may be transmitted via the alimentary intake of insufficiently pasteurized milk and dairy products, particularly of goats and sheep harboring infected ticks, as reported in 1999 in 22 patients from a north-eastern Moravian locality with usual zero TBE incidence rates (Orolinova, 2000). The active natural foci differ from the latent ones in the intensity and extent of virus circulation, as we have documented by the TBE virus prevalence rates in
Table 1• .TBE frequency in humans in three North·Bohemian districts and the Czech Republic. Years
UstUl.
DiWn
lltomeike
Czech Republic
1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
0 1 0 0 2 0 0 0 0 0 1 0 1 1 6 9 0 2 10 15 17
0 0 0 0 1 0 0 0 0 0 0 0 1 1 2 5 9 5 7 6 14
0 0 1 0 1 0 0 0 0 0 1 0 0 1 0 4 4 2 7 7 11
246 139 348 172 320 350 333 178 191 166 193 356 337 629 613 745 570 415 422 490 709
Table 2. Prevalence of ticks I. ricinus withTBE virus in South-Bohemian natural focus and in the elementary natural foci. % Minimal infection rate
Total
Nymphs Females Males
South-Bohemian natural focus
0.59
0.37
2.59
3.15
Borovany locality total
1.30
0.82
3.92
5.00
Borovany habitat 1
1.37
0.51
11.11
6.90
Borovany habitat 2
1.65
1.00
Borovany habitat 3
1.05
1.25
9.10
ticks from highly active southern Bohemian natural foci compared to those of other regions of the Czech Republic. First of all, based on a predictive map of the Czech regions (52000 km 2 ) , created by the use of satellite imagery LANDSAT TM-5 and allowing delineation of nine categories of forest growths acording to TBE risk (Daniel et al., 1998 and 2000), we selected suitable habitats within the natural foci. This was feasible thanks to high space resolution of the satellite data (30 X 30 01) and their accuracy. As many as 2968 ticks Ixodes ricinus, i. e. 2725 nymphs (N), 116 females (F) and 127 males (M), originating from 16 habitats of 10 localities were screened by the indirect immunofluorescence assay and plaque assay (Danielova et al., 2002). The presence of the TBE virus was detected in 17 of the pooled samples tested. The prevalence rate is expressed as a percentage of the minimum infection rate (MIR), based on the assumption that at least one tick had been positive out of a pool that tested positive. The MIRs were 0.57 % for all ticks screened, 0.37 % for N, 2.59 % for F, and 3.15 % for M (Table 2). Nevertheless, the MIRs varied widely between different habitats, reaching up to 0.231.25 % for N, 11.11 % for F and 10.00 % for M. The frequency of infection of imagos was 10 times higher than in nymphs. The highest TBE virus prevalence in ticks was found in the Borovany locality (Fig. 1) with a MIR of 1.30%, which was more than 2 times higher than the total prevalence rate. In the year 2000, the highest TBE incidence in humans was reported from the same locality; 8 out of 10 individuals who had acquired the infection there were residents. For the local population of 3928, this TBE incidence is more than 20 times higher than the mean incidence rate for the Czech Republic. The rate of TBE infected ticks is high enough for the local population to acquire TBE while coming into contact with wildlife. The TBE infected nymphs are the major danger to humans. In this locality their mean MIR was 0.82 %, ranging from 0.51 % to 1.25 % in different habitats. The latter percentage presents the highest prevalence rate determined in this study. This locality is the elementary focus and the particular habitats are its nuclei (Pavlovskij, 1948; Radvan et aI., 1960). There arc several such elementary foci in the Ceske Budejovicc depression and among them there are areas with low TBE virus prevalence in ticks or temporarily even without virus presence. The results obtained in this study are compared with the mean values recorded between 1970 and 1999 (Januska, 2000) by the National Reference Laboratory for Arboviruses for 86956 ticks, regardless whether harboring the TBE virus or not, from different localities and habitats all over the Czech Republic, except
Natural foci of TBE
for the region analysed in this study Crable 3). We can see that the southern Bohemian region shows a higher MIR. Nevertheless, if the prevalence rates of different localities where the TBE virus was isolated from ticks are compared, the rates found for southern Bohemia do not fall beyond those recorded in some active natural foci of other localities in other years. For illustration, some older data are given, those recorded in 1977 and 1978 for a region south of Prague with known TBE morbidity in humans (Danielova and Malkova, 1980) and in 1996 for southern Moravia where TBE was not reported in humans (Danielova, in press). Both localities are characterized by lower rates of TBE infected ticks. The southern Moravian latent natural focus showed 100 times lower rates of TBE infected imagoes than the highly active natural focus of southern Bohemia (Table 3). The high proportion of infected imagoes in the southern Bohemian natural focus can be explained by the virus amplification via the non-viraemic transmission (Labuda et aI., 1993a) associated with co-feeding of immature stages of ticks. The most important factors involved in the TEE virus circulation seem to be the co-feeding of larvae (L) leading to amplification of the transovarially transmitted virus and enhanced replication of under-threshold amounts of the virus during blood sucking by ticks of different developmental stages, particularly larvae (Labuda et aI., 1993b). It is generally agreed that L host infestation shows higher rates and is regularly aggregated and synchronized. Non-viraernic transmission between Land N, which can take place bidirectionally, and between N while co-feeding subsequently amplify the virus resulting in a tenfold increase in imago infection rate. This circumstance is relevant with respect to the relatively high percentage of TEE maternal transmission (Benda, 1958; Danielova and Holubova, 1990) and is supportive of the importance of transovarial transmission of the virus. The non-viraernic transmission between ticks while co-feeding seems not to take place in the latent natural foci, or seems to be negligible, and we do not consider it to be a prerequisite for the circulation of the TEE virus in nature. All sucking tick stages possibly co-feed which in turn may lead to TEE non-viraemic transmission. Nevertheless, we do not believe that the nonviraernic transmission from N to L would be a prerequisite sine qua 11011 for the existence of a TEE natural focus, even if some studies are based on such an assumption (Randolph et aI., 1999; 2000; Green et al., 1999). The TEE virus prevalence rate in ticks is a good indicator of the natural focus activity, if specifically related to the biocenosis studied. Pooled data are not of this predictive value.
----- - - - - -
185
.' '--
::==:=::--:~t=====--.! .
....- :.....
,.
Fig.1. locality Borovany. High TBE risk habitats categories 5-7 indentified bysatellite data.
Table 3. Prevalence rates of tick I. ricinus with TBE virus in highly active, moderately active, and latent natural foci incomparison withihemean prevalence in the Czech Republic. % Minimal infedion rate
Total
Nymphs Females Ma!es
South·Bohemian natural focus
0.59
0.37
2.59
3.15
Czech Republic 1970-1999
0.15
0.09
0.42
0.26
Central-Bohemian natural focus 1977 0.03
0.02
0.29
<0.26
Central-Bohemian natural focus 1978 0.12
0.09
South·Moravian natural focus 1966
0.15
0.15
0.16
References Benda, R.: The common tick Ixodes ricinus L as a reservoir and vector of tick-borne encephalitis. I. Survival of virus (strain B 3) during the development of the tick under laboratory conditions. J. Hyg, EpidemioI. ImmunoI. (Prahal: 2, 314-330 (1958). Daniel, M., Kolar, J., Zeman, P., Pavelka, K., Sadlo, J.: Prediction map of Ixodes ricinus high-incidence habitats and tick-borne encephalitis risk assessment using satellite data. Exp. Appl, AcaroI. 22,417-433 (1998). Daniel, M., Ko13r, J., Benes, C.: Predictive maps of Ixodes ricinus high incidence habitats and tick-borne encephalitis
186
V. Danielova
risk assessment based on remote sensing data. RemediaClinical microbiology: 4, 252-257 (2000). Danielova, V., Holubova, ].: Transovarial transmission rate of tick-borne encephalitis virus in Ixodes ricinus ticks. In: E Dusbabek, V.Bukva (Eds.): Modern Acarology. Academia, Prague and SPBAcademic Publishing by, The Hague: Vol.2, 7-10 (1991). Danielova, V., Holubova.j., Daniel, M.: Tick-Borne Encephalitis Virus Prevalence in Ixodes ricinus Ticks in Selected Risk Habitats of a Natural Focus in the South-Bohemian Region. Exp. Appl. Acarol., submitted. Daniclova, V., Malkova, D.: Tick-borne arboviruses in recreation areas. In: M. Labuda, C. H. Calisher (Eds.): New Aspects in Ecology of Arboviruses. Proc. internat. symposium. Slovak Academy Sci., Bratislava: 445-449 (1980). Green, R. M., Randolph, S. E.: The use of satellite imagery to predict foci of tick-borne encephalitis. Proc, 3'd internat. conference: Ticks and tick-borne pathogens: Into the 21 st century: 209-213 (1999). januska.j.: Results of virological investigations of ticks from the Czech Republic tested in the National Reference Laboratory for Arboviruses in 1970-1999: in litt (2000). Labuda, M.,]ones, L. D., Williams, T., Danielova, V.,Nuttall, P.A.: Efficient transmission of tick-borne virus between cofeeding ticks.]. Med. Entomol.: 30, 295-299 (1993 a). Labuda, M., Danielova, V., Jones, L. D., Nuttall, P.A.: Amplification of tick-borne encephalitis virus infection during co-feeding of ticks. Med. Vet. Entomol. 7, 339-342 (1993 b). Orolinova, M.: "Sheep cheese" epidemic- mass incidence of tick-borne encephalitis in the Vsetin district. Proceedings
of the National Conference on Microbiology and Epidemiology, Bfeclav, 9.-11. 10.2000: 37 (2000). Pavlovski], E. N.: Manual of human parasitology. AS USSR, Moskva, Leningrad: 2, 522-1022 (1948). Radvan R., Hanzak, ]., Hejny, S., Rehn, E, Rosicky, B.: Demonstration of elementary foci of tick-borne infections on the basis of microbiological, parasitological and biocenological investigations. ]. Hyg. Epidemiol. Immunol. (Praha) 4,81-93 (1960). Randolph, S. E., Green, R. M., Peacey, M. E, Rogers, D.].: Seasonal synchrony: the key to tick-borne encephalitis foci identified by satellite data. Parasitology 121, 15-23 (2000). Randolph, S. E., Miklisova, D., LysY,]', Rogers, D.]., Labuda, M.: Incidence from coincidence: patterns of tick infestations on rodents facilitate transmission of tick-borne encephalitis virus. Parasitology 118, 177-186 (1999). SUss,]., Dorn, W.: Tick-borne encephalitis in Thuringia (Germany): prevalence of the virus in an old focus-35 years of surveillance. In: ]. SUss, O. Kahl (Eds.): Tick-borne encephalitis and Lyme borreliosis, Proc. 4 th Potsdam Symposium on Tick-borne Diseases; Pabst Science Publishers, Lengerich: 197-203 (1997). Zeman, P.,januska, ].: Epizootiologic background of dissimilar distribution of human cases of Lyme borreliosis and tick-borne encephalitis in a joint endemic area. Compo Immun. Microbiol. Infect. Dis. 22, 247-260 (1999). Notification of Infectious Diseases, Czech Republic (EPIDAT), Hygienic Service, National Reference Centre for Analysis of Epidemiological Data, National Institute of Public Health, Prague (2001).
lnt,J.Med. Microbial. 291, Suppl. 33, 183-186 (2002) © Urban & Fischer Verlag http://WVNJ.urbanfischer.de/journals/ijmm
M
extended Abstract Natural foci of tick-borne encephalitis and prerequisites for their existence Vlasta Danielova * National Institute of Public Health, Prague, Srobarova 48, CZ-10042
Since 1948 when its causative agent was isolated in the Czech Republic for the first time in Europe, tick-borne encephalitis (TBE) has occured permanently in all Czech regions, and in comparison with other countries more frequently. This situation can be explained by the favourable conditions existing in the Czech Republic i. e. the existence of TBE natural foci where the virus life cycle is easily maintained. This means that the landscape is highly varied and tessellated. It consists of both forests, characterized by great diversity in species, structure and size, and farm land. This in turn leads to formation of frequent transitional areas called ecotones, providing an optimum environment for all developmental stages of ticks and their respective hosts and thus TBE circulation. The latter is facilitated by the abundance of game associated with traditional game keeping that dates back to the early middle ages. The presence of roe deer plays a key role in the TBE natural foci as reported by Zeman and januska (2000), based on their detailed analysis. The TBE natural foci differ markedly in their epidemiological activity: TBE morbidity in human population has been permanently notified in western and southern Bohemia while some eastern and northern Bohemian foci are characterized by long-term TBE latency. Similar data are available e. g. from Germany (Suss and Dorn, 1997). In the latent foci, the TBE virus can be maintained for a long time in animals of the elementary focus only, the human population remaining uninfected for different reasons. Latency may develop as a result of the natural biocenosis succession or human intervention but also may be due to the virus itself
that undergoes selection pressure, more precisely to the fact whether the peripherally invasive part of the viral population prevails or not. The epidemiological service does not consider such latent natural foci as such at risk, which in turn may lead to erroneous conclusions since some initially active natural foci may become latent to be reactivated later again. Natural foci of the districts OStl nad Labem and Decfn situated close to the north-western border to Germany can serve as examples. While in the 1950's and 1960's multiple TBE cases in humans were notified in these districts, the following decades were characterized only by isolated cases of TBE, mostly imported. The situation has been changing since 1993/94, as documented by the increasing TBE morbidity rates reported from the districts OStl nad Labem, Decfn and Litomerice (since 1995) (Table 1) (National Reference Laboratory for Epidemiological Analysis, National Institute of Public Health, Prague). There is one possible interpretation: during the latency period, the TBE virus must have shown low circulation among animals only. Our working hypothesis of the cause of new TBE outbreaks in northern Bohemian foci has been tested. The TBE virus could be considered as absent in northern Bohemia throughout the entire latency period. Conclusions on the geographical distribution of the TBE natural foci cannot be based on the human morbidity data only. Furthermore, the mode of infection is to be taken into account. In the Czech Republic, most people are infected while staying in recreational areas where many of them own holiday homes and others like walking in the forest and picking wild berries and mushrooms. That is why the best known
* Corresponding author: Vlasta Daniclova, National Institute of Public Health, Srobarova 48, CZ-10042 Prague 10, Czech Republic, Phone: 00420267082472, Fax: 00420267082466, E-mail: nernecekvts'auer.cz 1438-4221/01/291/8-000 $ 15.00/0
184
V. Danielova
recreational areas can be considered highly active foci. For instance, if comparing TBE morbidity in residents and holidaymakers in a recreational area south of Prague (valley of the Vita va and Sazava rivers), TBE appears clearly to be a recreational infection. Another source of distortion may be due to the fact that TBE infection may be transmitted via the alimentary intake of insufficiently pasteurized milk and dairy products, particularly of goats and sheep harboring infected ticks, as reported in 1999 in 22 patients from a north-eastern Moravian locality with usual zero TBE incidence rates (Orolinova, 2000). The active natural foci differ from the latent ones in the intensity and extent of virus circulation, as we have documented by the TBE virus prevalence rates in
Table 1• .TBE frequency in humans in three North·Bohemian districts and the Czech Republic. Years
UstUl.
DiWn
lltomeike
Czech Republic
1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000
0 1 0 0 2 0 0 0 0 0 1 0 1 1 6 9 0 2 10 15 17
0 0 0 0 1 0 0 0 0 0 0 0 1 1 2 5 9 5 7 6 14
0 0 1 0 1 0 0 0 0 0 1 0 0 1 0 4 4 2 7 7 11
246 139 348 172 320 350 333 178 191 166 193 356 337 629 613 745 570 415 422 490 709
Table 2. Prevalence of ticks I. ricinus withTBE virus in South-Bohemian natural focus and in the elementary natural foci. % Minimal infection rate
Total
Nymphs Females Males
South-Bohemian natural focus
0.59
0.37
2.59
3.15
Borovany locality total
1.30
0.82
3.92
5.00
Borovany habitat 1
1.37
0.51
11.11
6.90
Borovany habitat 2
1.65
1.00
Borovany habitat 3
1.05
1.25
9.10
ticks from highly active southern Bohemian natural foci compared to those of other regions of the Czech Republic. First of all, based on a predictive map of the Czech regions (52000 km 2 ) , created by the use of satellite imagery LANDSAT TM-5 and allowing delineation of nine categories of forest growths acording to TBE risk (Daniel et al., 1998 and 2000), we selected suitable habitats within the natural foci. This was feasible thanks to high space resolution of the satellite data (30 X 30 01) and their accuracy. As many as 2968 ticks Ixodes ricinus, i. e. 2725 nymphs (N), 116 females (F) and 127 males (M), originating from 16 habitats of 10 localities were screened by the indirect immunofluorescence assay and plaque assay (Danielova et al., 2002). The presence of the TBE virus was detected in 17 of the pooled samples tested. The prevalence rate is expressed as a percentage of the minimum infection rate (MIR), based on the assumption that at least one tick had been positive out of a pool that tested positive. The MIRs were 0.57 % for all ticks screened, 0.37 % for N, 2.59 % for F, and 3.15 % for M (Table 2). Nevertheless, the MIRs varied widely between different habitats, reaching up to 0.231.25 % for N, 11.11 % for F and 10.00 % for M. The frequency of infection of imagos was 10 times higher than in nymphs. The highest TBE virus prevalence in ticks was found in the Borovany locality (Fig. 1) with a MIR of 1.30%, which was more than 2 times higher than the total prevalence rate. In the year 2000, the highest TBE incidence in humans was reported from the same locality; 8 out of 10 individuals who had acquired the infection there were residents. For the local population of 3928, this TBE incidence is more than 20 times higher than the mean incidence rate for the Czech Republic. The rate of TBE infected ticks is high enough for the local population to acquire TBE while coming into contact with wildlife. The TBE infected nymphs are the major danger to humans. In this locality their mean MIR was 0.82 %, ranging from 0.51 % to 1.25 % in different habitats. The latter percentage presents the highest prevalence rate determined in this study. This locality is the elementary focus and the particular habitats are its nuclei (Pavlovskij, 1948; Radvan et aI., 1960). There arc several such elementary foci in the Ceske Budejovicc depression and among them there are areas with low TBE virus prevalence in ticks or temporarily even without virus presence. The results obtained in this study are compared with the mean values recorded between 1970 and 1999 (Januska, 2000) by the National Reference Laboratory for Arboviruses for 86956 ticks, regardless whether harboring the TBE virus or not, from different localities and habitats all over the Czech Republic, except
Natural foci of TBE
for the region analysed in this study Crable 3). We can see that the southern Bohemian region shows a higher MIR. Nevertheless, if the prevalence rates of different localities where the TBE virus was isolated from ticks are compared, the rates found for southern Bohemia do not fall beyond those recorded in some active natural foci of other localities in other years. For illustration, some older data are given, those recorded in 1977 and 1978 for a region south of Prague with known TBE morbidity in humans (Danielova and Malkova, 1980) and in 1996 for southern Moravia where TBE was not reported in humans (Danielova, in press). Both localities are characterized by lower rates of TBE infected ticks. The southern Moravian latent natural focus showed 100 times lower rates of TBE infected imagoes than the highly active natural focus of southern Bohemia (Table 3). The high proportion of infected imagoes in the southern Bohemian natural focus can be explained by the virus amplification via the non-viraemic transmission (Labuda et aI., 1993a) associated with co-feeding of immature stages of ticks. The most important factors involved in the TEE virus circulation seem to be the co-feeding of larvae (L) leading to amplification of the transovarially transmitted virus and enhanced replication of under-threshold amounts of the virus during blood sucking by ticks of different developmental stages, particularly larvae (Labuda et aI., 1993b). It is generally agreed that L host infestation shows higher rates and is regularly aggregated and synchronized. Non-viraernic transmission between Land N, which can take place bidirectionally, and between N while co-feeding subsequently amplify the virus resulting in a tenfold increase in imago infection rate. This circumstance is relevant with respect to the relatively high percentage of TEE maternal transmission (Benda, 1958; Danielova and Holubova, 1990) and is supportive of the importance of transovarial transmission of the virus. The non-viraernic transmission between ticks while co-feeding seems not to take place in the latent natural foci, or seems to be negligible, and we do not consider it to be a prerequisite for the circulation of the TEE virus in nature. All sucking tick stages possibly co-feed which in turn may lead to TEE non-viraemic transmission. Nevertheless, we do not believe that the nonviraernic transmission from N to L would be a prerequisite sine qua 11011 for the existence of a TEE natural focus, even if some studies are based on such an assumption (Randolph et aI., 1999; 2000; Green et al., 1999). The TEE virus prevalence rate in ticks is a good indicator of the natural focus activity, if specifically related to the biocenosis studied. Pooled data are not of this predictive value.
----- - - - - -
185
.' '--
::==:=::--:~t=====--.! .
....- :.....
,.
Fig.1. locality Borovany. High TBE risk habitats categories 5-7 indentified bysatellite data.
Table 3. Prevalence rates of tick I. ricinus with TBE virus in highly active, moderately active, and latent natural foci incomparison withihemean prevalence in the Czech Republic. % Minimal infedion rate
Total
Nymphs Females Ma!es
South·Bohemian natural focus
0.59
0.37
2.59
3.15
Czech Republic 1970-1999
0.15
0.09
0.42
0.26
Central-Bohemian natural focus 1977 0.03
0.02
0.29
<0.26
Central-Bohemian natural focus 1978 0.12
0.09
South·Moravian natural focus 1966
0.15
0.15
0.16
References Benda, R.: The common tick Ixodes ricinus L as a reservoir and vector of tick-borne encephalitis. I. Survival of virus (strain B 3) during the development of the tick under laboratory conditions. J. Hyg, EpidemioI. ImmunoI. (Prahal: 2, 314-330 (1958). Daniel, M., Kolar, J., Zeman, P., Pavelka, K., Sadlo, J.: Prediction map of Ixodes ricinus high-incidence habitats and tick-borne encephalitis risk assessment using satellite data. Exp. Appl, AcaroI. 22,417-433 (1998). Daniel, M., Ko13r, J., Benes, C.: Predictive maps of Ixodes ricinus high incidence habitats and tick-borne encephalitis
186
V. Danielova
risk assessment based on remote sensing data. RemediaClinical microbiology: 4, 252-257 (2000). Danielova, V., Holubova, ].: Transovarial transmission rate of tick-borne encephalitis virus in Ixodes ricinus ticks. In: E Dusbabek, V.Bukva (Eds.): Modern Acarology. Academia, Prague and SPBAcademic Publishing by, The Hague: Vol.2, 7-10 (1991). Danielova, V., Holubova.j., Daniel, M.: Tick-Borne Encephalitis Virus Prevalence in Ixodes ricinus Ticks in Selected Risk Habitats of a Natural Focus in the South-Bohemian Region. Exp. Appl. Acarol., submitted. Daniclova, V., Malkova, D.: Tick-borne arboviruses in recreation areas. In: M. Labuda, C. H. Calisher (Eds.): New Aspects in Ecology of Arboviruses. Proc. internat. symposium. Slovak Academy Sci., Bratislava: 445-449 (1980). Green, R. M., Randolph, S. E.: The use of satellite imagery to predict foci of tick-borne encephalitis. Proc, 3'd internat. conference: Ticks and tick-borne pathogens: Into the 21 st century: 209-213 (1999). januska.j.: Results of virological investigations of ticks from the Czech Republic tested in the National Reference Laboratory for Arboviruses in 1970-1999: in litt (2000). Labuda, M.,]ones, L. D., Williams, T., Danielova, V.,Nuttall, P.A.: Efficient transmission of tick-borne virus between cofeeding ticks.]. Med. Entomol.: 30, 295-299 (1993 a). Labuda, M., Danielova, V., Jones, L. D., Nuttall, P.A.: Amplification of tick-borne encephalitis virus infection during co-feeding of ticks. Med. Vet. Entomol. 7, 339-342 (1993 b). Orolinova, M.: "Sheep cheese" epidemic- mass incidence of tick-borne encephalitis in the Vsetin district. Proceedings
of the National Conference on Microbiology and Epidemiology, Bfeclav, 9.-11. 10.2000: 37 (2000). Pavlovski], E. N.: Manual of human parasitology. AS USSR, Moskva, Leningrad: 2, 522-1022 (1948). Radvan R., Hanzak, ]., Hejny, S., Rehn, E, Rosicky, B.: Demonstration of elementary foci of tick-borne infections on the basis of microbiological, parasitological and biocenological investigations. ]. Hyg. Epidemiol. Immunol. (Praha) 4,81-93 (1960). Randolph, S. E., Green, R. M., Peacey, M. E, Rogers, D.].: Seasonal synchrony: the key to tick-borne encephalitis foci identified by satellite data. Parasitology 121, 15-23 (2000). Randolph, S. E., Miklisova, D., LysY,]', Rogers, D.]., Labuda, M.: Incidence from coincidence: patterns of tick infestations on rodents facilitate transmission of tick-borne encephalitis virus. Parasitology 118, 177-186 (1999). SUss,]., Dorn, W.: Tick-borne encephalitis in Thuringia (Germany): prevalence of the virus in an old focus-35 years of surveillance. In: ]. SUss, O. Kahl (Eds.): Tick-borne encephalitis and Lyme borreliosis, Proc. 4 th Potsdam Symposium on Tick-borne Diseases; Pabst Science Publishers, Lengerich: 197-203 (1997). Zeman, P.,januska, ].: Epizootiologic background of dissimilar distribution of human cases of Lyme borreliosis and tick-borne encephalitis in a joint endemic area. Compo Immun. Microbiol. Infect. Dis. 22, 247-260 (1999). Notification of Infectious Diseases, Czech Republic (EPIDAT), Hygienic Service, National Reference Centre for Analysis of Epidemiological Data, National Institute of Public Health, Prague (2001).