Detection and characterization of Babesia species in Ixodes ticks in Estonia.

The presence of Babesia spp. was studied in 2603 Ixodes ricinus and Ixodes persulcatus ticks collected at seven sites in Estonia. By reverse line blot screening, Babesia spp. was detected in 36 (1.4%) ticks, among them 18 (0.7%) were further recognized by a Babesia microti probe, 3 (0.1%) by a Babesia divergens probe, and the other 15 (0.6%) were recognized only by the universal Babesia spp. "catch all" probe. Sequence analyses of 6 of these 15 samples revealed that all of them belonged to Babesia sp. EU1. B. microti was detected in both tick species I. ricinus and I. persulcatus at the seven sites, whereas B. divergens-like and Babesia sp. EU1 were found only in I. persulcatus and I. ricinus, respectively. Genetic characterization based on partial 18S rRNA showed that the Estonian sequences of B. microti, B. divergens-like, and Babesia sp. EU1 share a high rate of similarity and are closely related to sequences from other European countries, Siberia, and United States. The present study demonstrated for the first time the existence and distribution of Babesia spp. in I. persulcatus and I. ricinus ticks in Estonia.

Variable spikes in tick-borne encephalitis incidence in 2006 independent of variable tick abundance but related to weather.

BACKGROUND: The incidence of tick-borne encephalitis showed a dramatic spike in several countries in Europe in 2006, a year that was unusually cold in winter but unusually warm and dry in summer and autumn. In this study we examine the possible causes of the sudden increase in disease: more abundant infected ticks and/or increased exposure due to human behaviour, both in response to the weather. METHODS: For eight countries across Europe, field data on tick abundance for 2005-2007, collected monthly from a total of 41 sites, were analysed in relation to total annual and seasonal TBE incidence and temperature and rainfall conditions. RESULTS: The weather in 2006-2007 was exceptional compared with the previous two decades, but neither the very cold start to 2006, nor the very hot period from summer 2006 to late spring 2007 had any consistent impact on tick abundance. Nor was the TBE spike in 2006 related to changes in tick abundance. Countries varied in the degree of TBE spike despite similar weather patterns, and also in the degree to which seasonal variation in TBE incidence matched seasonal tick activity. CONCLUSION: The data suggest that the TBE spike was not due to weather-induced variation in tick population dynamics. An alternative explanation, supported by qualitative reports and some data, involves human behavioural responses to weather favourable for outdoor recreational activities, including wild mushroom and berry harvest, differentially influenced by national cultural practices and economic constraints.

Behavioural responses to perceived risk of tick-borne encephalitis: vaccination and avoidance in the Baltics and Slovenia.

Tick-borne encephalitis (TBE) incidence increased markedly in the Baltics and Slovenia in the early 1990s, but then declined again in some places. Our analyses of temporal and spatial data on TBE incidence and vaccination revealed that over 1970-2005 up-take of vaccination varied in both time and space according to incidence, i.e. was apparently responsive to perceived risk. Since 1999, however, decreases in incidence in many counties within each country have far exceeded vaccination rates or immunity through natural exposure, and in Latvia and Lithuania these changes are correlated with previous incidence. Survey data on human activities in Latvia revealed that people in socio-economic groups whose behaviour put them at highest risk of exposure to ticks in forests, including people with lower education and lowest incomes, are least likely to be vaccinated. We conclude that risk avoidance through changing human behaviour has driven incidence-dependent decreases in TBE infection, but targeted vaccination campaigns could provide more secure protection.

Socio-economic factors in the differential upsurge of tick-borne encephalitis in Central and Eastern Europe.

Tick-borne encephalitis (TBE), the most serious widespread vector-borne disease of humans in Europe, increased from 2- to 30-fold in many Central and Eastern European countries from 1992 to 1993, coinciding with independence from Soviet rule. Unemployment and low income have been shown in Latvia to be statistically associated with high-risk behaviour involving harvest of wild foods from tick-infested forests, and also with not being vaccinated against TBE. Archival data for 1970--2005 record major changes in the agricultural and industrial sectors, and consequent changes in the abiotic and biotic environment and socio-economic conditions, which could have increased the abundance of infected ticks and the contact of humans with those ticks. For example, abandoned agricultural fields became suitable for rodent transmission hosts; use of pesticides and emissions of atmospheric industrial pollutants plummeted; wildlife hosts for ticks increased; tick populations appear to have responded; unemployment and inequality increased in all countries. These factors, by acting synergistically but differentially between and within each country, can explain the marked spatio-temporal heterogeneities in TBE epidemiology better than can climate change alone, which is too uniform across wide areas. Different degrees of socio-economic upheaval caused by political transition in Estonia, Latvia, Lithuania, Slovenia and the Czech Republic can apparently explain the marked variation in TBE upsurge. Causal linkage between national socio-economic conditions and epidemiology is strongly indicated by striking correlations across eight countries between the degree of upsurge of TBE and both poverty and household expenditure on food (R2 = 0.533 and 0.716, respectively).

Climate change cannot explain the upsurge of tick-borne encephalitis in the Baltics.

BACKGROUND: Pathogens transmitted by ticks cause human disease on a greater scale than any other vector-borne infections in Europe, and have increased dramatically over the past 2-3 decades. Reliable records of tick-borne encephalitis (TBE) since 1970 show an especially sharp upsurge in cases in Eastern Europe coincident with the end of Soviet rule, including the three Baltic countries, Estonia, Latvia and Lithuania, where national incidence increased from 1992 to 1993 by 64, 175 and 1,065%, respectively. At the county level within each country, however, the timing and degree of increase showed marked heterogeneity. Climate has also changed over this period, prompting an almost universal assumption of causality. For the first time, we analyse climate and TBE epidemiology at sufficiently fine spatial and temporal resolution to question this assumption. METHODOLOGY/PRINCIPAL FINDING: Detailed analysis of instrumental records of climate has revealed a significant step increase in spring-time daily maximum temperatures in 1989. The seasonal timing and precise level of this warming were indeed such as could promote the transmission of TBE virus between larval and nymphal ticks co-feeding on rodents. These changes in climate, however, are virtually uniform across the Baltic region and cannot therefore explain the marked spatio-temporal heterogeneity in TBE epidemiology. CONCLUSIONS/SIGNIFICANCE: Instead, it is proposed that climate is just one of many different types of factors, many arising from the socio-economic transition associated with the end of Soviet rule, that have acted synergistically to increase both the abundance of infected ticks and the exposure of humans to these ticks. Understanding the precise differential contribution of each factor as a cause of the observed epidemiological heterogeneity will help direct control strategies.

Characterization of hemorrhagic fever with renal syndrome caused by hantaviruses, Estonia.

Thirty cases of hemorrhagic fever with renal syndrome (HFRS) due to Puumala virus (PUUV), Saaremaa virus (SAAV), and Dobrava virus infection were confirmed in Estonia. Except for the levels of serum creatinine, no remarkable differences were found in the clinical course of HFRS caused by PUUV and SAAV.

Tick-borne encephalitis in the Baltic States: identifying risk factors in space and time.

This paper presents preliminary results in our investigations of the biological (abiotic and biotic) and non-biological causes of the spatial heterogeneity and temporal change of tick-borne encephalitis (TBE), both within and between Estonia, Latvia, and Lithuania. Spatial analysis revealed that the land cover and precise seasonal patterns of climatic indices (temperature and normalized difference vegetation index) can explain 55% of the observed spatial variation in TBE incidence over the period 1993-98 across all the Baltic States. Temporal analysis of climatic variables indicates a very specific change in spring temperature conditions from 1993 onwards that could enhance the transmission of TBE virus. Further time series analysis of climate, together with analysis of biotic factors, socio-economic conditions, and human behaviour is being undertaken to explain the epidemiological patterns more fully.

Characterization of tick-borne encephalitis virus from Estonia.

Tick-borne encephalitis virus (TBEV) is a severe problem in Estonia. In the present article the first genetic analysis of Estonian TBEV strains is described. In total, seven TBEV strains were isolated from ticks (Ixodes ricinus and I. persulcaus), rodents (Apodemus agrarius and Cletrionomys glareolus), and serum from a tick-borne encephalitis (TBE) patient. The nucleic acid sequences of the viral genome encoding almost the complete E protein (nt 41-1250) and the 3'-NCR-termini of the Estonian TBEV strains were determined by direct sequencing of RT-PCR products. The results showed that all three known TBEV subtypes, Western TBEV (W-TBEV), Far-Eastern TBEV (FE-TBEV), and Siberian TBEV (S-TBEV), co-circulate in Estonia. The Estonian TBEV strains of the S-TBEV and W-TBEV subtypes clustered with the previously reported strains from Latvia and Lithuania. Within the FE-TBEV subtype, however, the Estonian strain Est2546 clustered together with the strain Sofjin, originating from the Far-East of Russia, but not with the strain RK1424, isolated in the neighboring Latvia. This suggests a different evolutionary history for the Estonian and the Latvian strains in the FE-TBEV subtype. The Estonian TBEV strain (Est3535), which belonged to the S-TBEV subtype, had an organization of the 3'-NCR similar to that of strains from the Far-East of Russia (Irkutsk). The 3'-NCRs of Estonian strains of the W-TBEV subtype (Est3051, Est3053, Est3476, and Est3509) were very similar to those of the strain Ljubljana I from the Balkans. In the 3'-NCR sequence of the Estonian strain Est2546, which belonged to the FE-TBEV subtype, a deletion from position 10461 to 10810 extending approximately 10 nucleotides into the core element, was detected.

Hantaviruses in Estonia.

Human serum samples collected from healthy individuals in 14 counties were screened by ELISA in order to investigate the presence of hantavirus infections in Estonia. Out of 1,234 serum samples, 124 were found positive for hantavirus-specific IgG and were subsequently serotyped by a focus reduction neutralization test. A total of 112 samples neutralized at least one of the examined hantaviruses-Puumala (PUUV), Saaremaa (SAAV), Dobrava (DOBV), Hantaan, and Seoul viruses-and thereby, the focus reduction neutralization test confirmed the overall hantavirus seroprevalence rate in Estonia to be 9.1%. Most of the sera showed a specific reaction (at least 4-fold higher endpoint titer) of neutralizing antibodies to PUUV (5.1%), while 3.4% showed a SAAV- or SAAV/DOBV-specific reaction. The fact that seven sera (0.6%) could not be serotyped may indicate the presence of an unknown hantavirus serotype. Hantavirus infections were confirmed in 13 of 14 investigated counties, with highly varying seroprevalence rates (1.0-28.4%). The sex ratio was 1.8:1.0 (M:F), and the antibody prevalence peaked in the age group 45-54 years. A total of 513 rodents of seven species trapped in seven counties were examined for the presence of hantavirus antigen, in order to study the distribution of hantavirus natural carriers. Two species, Clethrionomys glareolus and Apodemus agrarius, were found positive for hantaviral antigen in 13.7% and 4.5% of the investigated rodents, respectively. Analyses of viral sequences recovered from infected C. glareolus tissue samples showed that the infecting virus belonged to the PUUV genotype, confirming that PUUV circulates in mainland Estonia. The Estonian PUUV strains were placed in the closest proximity to Russian PUUV strains in phylogenetic trees, suggesting a common evolutionary history. Together with earlier data on SAAV in A. agrarius, the results revealed that two hantaviruses, PUUV and SAAV, are common in Estonia and that the incidence of human infection is high in both cases.