Bohumír Kříž

An attempt to elucidate the increased incidence of tick-borne encephalitis and its spread to higher altitudes in the Czech Republic

In 1993 the incidence of tick-borne encephalitis (TBE) showed a sharp rise in Central Europe and has remained high since--with some slight fluctuation. This increase is clearly evident in the Czech Republic and it could be roughly characterised as twofold in 1993-2001 in comparison with 1984-1992 (5,240:2,441 human cases). As yet in the Czech Republic the TBE increase has been manifested by: a) higher number of cases in areas well known for TBE occurrence in humans; b) re-emergence in areas where TBE human cases were not observed, or only sporadically, for a long time; c) emergence of TBE in places unknown previously (including highly elevated areas). This phenomenon has not been fully elucidated yet and we would like to contribute to a better understanding of its cause by comparing the present situation with historical data. Besides TBE epidemiological data (1965-2001) we use the twenty-year all season dynamics data of Ixodes ricinus host-seeking activity (1953-1972) supplemented by the data in 1982, 1992 and by meteorological data characterising periods under study. The fluctuation in annual averages of I. ricinus occurrence was irregular. These irregularities when compared with meteorological data can be explained by different meteorological conditions in particular years. In corresponding long-time series, the peaks of I. ricinus occurrence and TBE incidence were reached at the same time. Analyses of relevant meteorological data displayed that joint I. ricinus and TBE maximal occurrence values had been preceded by mild (or warm) autumn seasons allowing a prolongation of I. ricinus activities (including development) up to November at least, thus resulting in a higher tick volume the following year. Based on these data we conclude that the increased TBE incidence rates reported in 1993 and afterwards are attributable to a more abundant occurrence of I. ricinus ticks and that their higher abundance is due to modified climatic conditions in the last decade. Such a situation has appeared in the past as well; however, it was rare and in isolated years only. At that time, although the tick population had been more abundant, in the following year it reached the usual level again. In the 1990s the prolonged mild autumn for consecutive several years led to permanently more abundant populations of I. ricinus. The same conclusion explains the shift of I. ricinus occurrence and of TBE distribution to higher altitudes in Sumava and Krkonose Mountains.

Socio-economic conditions and other anthropogenic factors influencing tick-borne encephalitis incidence in the Czech Republic

Laboratory confirmed cases of tick-borne encephalitis (TBE) have been reported in the Czech Republic since 1971. Peaks of incidence occurred at 2 to 5-year intervals. The incidence in the eighties was generally lower than in seventies. In the nineties there was a sharp increase in incidence that peaked in 1995 (7.2/100,000). TBE cases occur from April to November with maximum incidence in July. During the last decade the second peak of incidence occurred in most years in September and/or October. In the last decade, an extension of the TBE season towards the spring and autumn periods has been observed. In addition to a general increase in TBE incidence in the last decade, some other changes in the epidemiology of TBE were observed in the Czech Republic. During the whole period sex-specific incidence was higher in men than in women (men to women ratio 1.5:1). Age-specific incidence over recent years increased steadily in children and adolescent age groups. In the ten-year age groups from 25 to 65, it remains practically at the same level (6-8/100,000). In older persons it fell to 2-3/100,000. Tick bites infect patients mainly during their recreational activities. A very small proportion (less than 1%) acquires the infection through the alimentary route. We have tested the hypothesis whether the increase in TBE incidence in the nineties was due to economical or social changes after the velvet revolution of 1989. Between 1991 and 1995 unemployment largely remained on the same level (between 2-3%). Over the next years the percentage of unemployed persons increased rapidly to 9.3% in 1999 (7.8% in 2001). This trend differs significantly from the trend of TBE incidence that peaked in 1995. No correlation between the district incidence of TBE and the district percentage of unemployment in the years 1997-2001 was found (r=-0.20). The percentage of unemployed persons among the TBE cases was 1-3% in contrast to the Czech Republic figures which were 5-9% for the same period. The gross domestic product in USD per capita increased from dollars 2,600 in 1991 to dollars 5,000 in 1995. Since then it has varied between dollars 4,800 and dollars 5,600. This trend, therefore, differs from the trend of TBE incidence as well. Among the TBE cases the percentage of foresters and other persons working in the forests in the years 1997-2001 was 0.5-1%. The behavioral and socio-economic aspects of TBE cases remained stable despite the political changes which have take place in the Czech Republic since the beginning of the nineties. They are not, therefore, responsible for the increased TBE incidence. In the industrial areas most polluted by SO2 in the past in northern Bohemia the sharp increase in TBE incidence seems to be connected with measures aiming to eliminate SO2 emissions from brown coal power stations.

Increased Relative Risk of Tick-Borne Encephalitis in Warmer Weather

Tick-borne encephalitis (TBE) is a serious acute neuroinfection of humans caused by a tick-borne flavivirus. The disease is typically seasonal, linked to the host-seeking activity of Ixodes ricinus (predominantly nymphs), the principal European tick vector species. To address the need for accurate risk predictions of contracting TBE, data on 4,044 TBE cases reported in the Czech Republic during 2001–2006 were compared with questing activity of I. ricinus nymphs monitored weekly at a defined location for the same 6-year period. A time shift of 21 days between infected tick bite and recorded disease onset provided the optimal model for comparing the number of cases of TBE with numbers of questing nymphs. Mean annual distribution of TBE cases and tick counts showed a similar bimodal distribution. Significantly, the ratio of TBE cases to questing nymphs was highest in the summer-autumn period even though the number of questing nymphs peaked in the spring-summer period. However, this pattern changed during a period of extreme meteorological events of flooding and abnormally high temperatures, indicating that changes in climate affect the incidence of TBE. Previous studies failed to link human behavior with changes in incidence of TBE but showed extrinsic temperature impacts arbovirus replication. Hence, we hypothesize the apparent discrepancy between peak nymphal tick activity and greatest risk of contracting TBE is due to the effect of temperature on virus replication in the tick vector. Relative proportions of questing nymphs and the numbers of weeks in which they were found were greater in summer-autumn compared with spring-summer at near-ground temperatures >5°C and at standard day and weekly average temperatures of >15°C. Thus, during the summer-autumn period, the virus dose in infected tick bites is likely greater owing to increased virus replication at higher microclimatic temperatures, consequently increasing the relative risk of contracting TBE per summer-autumn tick bite. The data support the use of weather-based forecasts of tick attack risk (based on daytime ambient temperature) supplemented with weekly average temperature (as a proxy for virus replication) to provide much-needed real-time forecasts of TBE risk.

Analysis of meteorological effects on the incidence of tick-borne encephalitis in the Czech Republic over a thirty-year period

The goal is to analyse the effect of climatic factors on the incidence of tick-borne encephalitis (TBE) in three adjacent, high-incidence regions of the Czech Republic (CZ) that differed considerably in the baseline incidence and in the timing of high incidence period. The basis for the analysis were the meteorological data from a 30-year database (1982-2011) of daily air temperature and precipitation measurements and time-matched reports of TBE cases defined by the onset of symptoms. A semi-parametric Poisson regression model revealed a statistically significant relationship between the average daily ambient air temperature and incidence of TBE. The shapes of the dependence were similar in all three regions under study. The analysis of the available data did not show a statistically significant relationship between the precipitation amount and incidence of TBE. Other statistically significant predictors were the trend over calendar years, monthly seasonality and trendseasonality interaction (the shape of the monthly seasonality varies over years), and day of week. The climate changes had differentiated effects on TBE increase depending on the local environmental conditions as can be illustrated by the variations between the three regions not only in the TBE incidence level but also in the shape of the trend over years. Correspondence to: Marek Malý, National Institute of Public Health, Šrobárova 48, 100 42 Prague 10, Czech Republic, Tel: +42

A serological study of antibodies to Anaplasma phagocytophilum and Borrelia burgdorferi sensu lato in the sera of healthy individuals collected two decades apart

Sera collected from healthy individuals from the general population in the Czech Republic during repeated cross-sectional surveys were analyzed. Samples collected in the same six districts in two time periods, 1978–1989 and 2001, were compared. The study subjects were divided into six age categories between 10 and 59 years. Overall, 434 samples from 1978–1989 and 270 samples from 2001 were screened for Anaplasma phagocytophilum (AP) and Borrelia burgdorferi sensu lato (BB). The antiAP positivity rates were 13.1% and 11.5% in the first and second period, respectively, and did not differ significantly between the periods (P = 0.559). The anti-BB antibodies were detected in 33.9% and 14.8% of study subjects, respectively. The positivity rates were significantly lower in the second period (P>0.001). No considerable changes were observed in the sex distribution of positive findings between the two periods. The highest positivity rates of anti-AP antibodies were found in the 10–14 year age group: 16.0% in 1978–1989 and 16.7% in 2001. The age distribution of the anti-AP antibody positivity rates did not change substantially (P = 0.872). In 1978–1989, the lowest anti-BB antibody positivity rate (26.7%) was found in the 10–14 year age group, with a gradual increase with age to 41.1% in 50–59 year-olds. In 2001, the positivity rate in the 10–14 year age group was 26.2% and was not significantly different from that in the first period (P = 0.955). However, the positivity rates in the older age groups 15–59 years decreased significantly (P<0.001) and varied between 8.3% and 15.1%.