Tamara S. Gritsun

Tick-borne encephalitis.

Tick-borne encephalitis (TBE) is one of the most dangerous human infections occurring in Europe and many parts of Asia. The etiological agent Tick-borne encephalitis virus (TBEV), is a member of the virus genus Flavivirus, of the family Flaviviridae. TBEV is believed to cause at least 11,000 human cases of encephalitis in Russia and about 3000 cases in the rest of Europe annually. Related viruses within the same group, Louping ill virus (LIV), Langat virus (LGTV) and Powassan virus (POWV), also cause human encephalitis but rarely on an epidemic scale. Three other viruses within the same group, Omsk hemorrhagic fever virus (OHFV), Kyasanur Forest disease virus (KFDV) and Alkhurma virus (ALKV), are closely related to the TBEV complex viruses and tend to cause fatal hemorrhagic fevers rather than encephalitis. This review describes the clinical manifestations associated with TBEV infections, the main molecular-biological properties of these viruses, and the different factors that define the incidence and severity of disease. The role of ticks and their local hosts in the emergence of new virus variants with different pathogenic characteristics is also discussed. This review also contains a brief history of vaccination against TBE including trials with live attenuated vaccine and modern tendencies in developing of vaccine virus strains.

Tick-borne flaviviruses.

Tick-borne encephalitis (TBE), one of the most dangerous neuroinfections in Europe and Asia, is caused by tick-borne encephalitis virus (TBEV) and currently involves approximately 11,000 human cases annually, mostly in Russia. This chapter describes the main problems associated with the epidemiology, ecology, pathogenesis, and control of this disease. We have attempted to review the factors that influence the incidence and distribution of TBE, and to discuss possible reasons for the different clinical manifestations including most commonly observed asymptomatic infections, fever forms, acute encephalitis, and the less frequently registered biphasic milk fever and chronic encephalitis. Epidemiologic data concerning the other tick-borne flaviviruses, namely Louping ill virus, Langat virus, and Powassan virus that also produce encephalitis on a smaller scale, are also presented. Here we describe the history and current epidemiological role of Omsk hemorrhagic fever virus and Kyasanur forest disease virus, two viruses that are genetically closely related to TBEV, but produce hemorrhagic fever instead of encephalitis, and provide possible explanations for these differences. The other viruses in the tick-borne flavivirus group are also included despite the fact that they do not play an essential epidemiologic role in humans. This chapter contains a brief history of vaccination against TBE including the trials with live attenuated vaccine and reviews the modern trends in development of vaccine virus strains.

Characterization of a Siberian Virus Isolated from a Patient with Progressive Chronic Tick-Borne Encephalitis

ABSTRACT A strain of Tick-borne encephalitis virus designated Zausaev (Za) was isolated in Siberia from a patient who died of a progressive (2-year) form of tick-borne encephalitis 10 years after being bitten by a tick. The complete genomic sequence of this virus was determined, and an attempt was made to correlate the sequence with the biological characteristics of the virus. Phylogenetic analysis demonstrated that this virus belongs to the Siberian subtype of Tick-borne encephalitis virus. Comparison of Za virus with two related viruses, a Far Eastern isolate, Sofjin, and a Siberian isolate, Vasilchenko, revealed differences among the three viruses in pathogenicity for Syrian hamsters, cytopathogenicity for PS cells, plaque morphology, and the electrophoretic profiles of virus-specific nonstructural proteins. Comparative amino acid alignments revealed 10 individual amino acid substitutions in the Za virus polyprotein sequence that were different from those of other tick-borne flaviviruses. Notably, the dimeric form of the Za virus NS1 protein migrated in polyacrylamide gels as a heterogeneous group of molecules with a significantly higher electrophoretic mobility than those of the Sofjin and Vasilchenko viruses. Two amino acid substitutions, T277→V and E279→G, within the NS1 dimerization domain are probably responsible for the altered oligomerization of Za virus NS1. These studies suggest that the patient from whom Za virus was isolated died due to increased pathogenicity of the latent virus following spontaneous mutagenesis.

CD8 + T-cells mediate immunopathology in tick-borne encephalitis

Epidemics of tick-borne encephalitis involving thousands of humans occur annually in the forested regions of Europe and Asia. Despite the importance of this disease, the underlying basis for the development of encephalitis remains undefined. Here, we prove the key role of CD8(+) T-cells in the immunopathology of tick-borne encephalitis, as demonstrated by prolonged survival of SCID or CD8(-/-) mice, following infection, when compared with immunocompetent mice or mice with adoptively transferred CD8(+) T-cells. The results imply that tick-borne encephalitis is an immunopathological disease and that the inflammatory reaction significantly contributes to the fatal outcome of the infection.

Understanding the alphaviruses: Recent research on important emerging pathogens and progress towards their control

Abstract The alphaviruses were amongst the first arboviruses to be isolated, characterized and assigned a taxonomic status. They are globally very widespread, infecting a large variety of terrestrial animals, insects and even fish, and circulate both in the sylvatic and urban/peri-urban environment, causing considerable human morbidity and mortality. Nevertheless, despite their obvious importance as pathogens, there are currently no effective antiviral drugs with which to treat humans or animals infected by any of these viruses. The EU-supported project—VIZIER (Comparative Structural Genomics of Viral Enzymes Involved in Replication, FP6 Project: 2004-511960) was instigated with an ultimate view of contributing to the development of antiviral therapies for RNA viruses, including the alphaviruses [Coutard, B., Gorbalenya, A.E., Snijder, E.J., Leontovich, A.M., Poupon, A., De Lamballerie, X., Charrel, R., Gould, E.A., Gunther, S., Norder, H., Klempa, B., Bourhy, H., Rohayemj, J., L’hermite, E., Nordlund, P., Stuart, D.I., Owens, R.J., Grimes, J.M., Tuckerm, P.A., Bolognesi, M., Mattevi, A., Coll, M., Jones, T.A., Åqvist, J., Unger, T., Hilgenfeld, R., Bricogne, G., Neyts, J., La Colla, P., Puerstinger, G., Gonzalez, J.P., Leroy, E., Cambillau, C., Romette, J.L., Canard, B., 2008. The VIZIER project: preparedness against pathogenic RNA viruses. Antiviral Res. 78, 37–46]. This review highlights some of the major features of alphaviruses that have been investigated during recent years. After describing their classification, epidemiology and evolutionary history and the expanding geographic distribution of Chikungunya virus, we review progress in understanding the structure and function of alphavirus replicative enzymes achieved under the VIZIER programme and the development of new disease control strategies.

Analysis of flavivirus envelope proteins reveals variable domains that reflect their antigenicity and may determine their pathogenesis

Studies on the molecular basis of flavivirus neutralisation, attenuation and tropism indicate that amino acid substitutions, in different parts of the envelope gene, may be responsible for the altered phenotypes. However, the association of particular substitutions with individual characteristics has proven difficult. Comparative analysis of all known tick-borne flavivirus envelope proteins through sequence alignment and a sliding window, reveals clusters of amino acid variation distributed throughout the envelope protein coding region. Further comparison with mosquito-borne flaviviruses reveals essentially the same profile of variability throughout the envelope protein sequence although there is a major difference within the postulated B domain of these viruses which may reflect their different evolutionary development. Most phenotypically variant properties, such as serotypic differences, variants characteristic of vaccine strains, altered tropisms and neutralisation escape mutants, map within the variable clusters. Thus, we propose that natural mutagenesis and selection may occur at specific sites that do not destroy the secondary and tertiary E protein structure and that the variable clusters represent the exposed surface amino acids of the envelope protein defining antigenicity, tropicity and pathogenesis.

Potential Arbovirus Emergence and Implications for the United Kingdom

Climate change can cause arthropodborne diseases to emerge.

Sequencing and antigenic studies of a Norwegian virus isolated from encephalomyelitic sheep confirm the existence of louping ill virus outside Great Britain and Ireland

We have carried out an antigenic analysis and nucleotide sequence comparison of the envelope glycoprotein of recognized louping ill virus strains isolated from Scotland with that of a Norwegian virus known to cause encephalomyelitis in sheep. Monoclonal antibodies with defined specificity for the louping ill virus envelope glycoprotein failed to distinguish between the Norwegian virus and prototype louping ill virus in indirect immunofluorescence, haemagglutination inhibition and neutralization tests. Nucleotide sequencing of the envelope glycoprotein and alignment of the deduced amino acid sequence with other known sequences revealed that the Norwegian virus closely resembles (> 95% identity for nucleotide and > 98% identity for amino acid sequences) louping ill virus. Maximum variation in identities among four strains of louping ill virus were 4.4% and 1.8% respectively for nucleotide and amino acid alignments. We conclude that sheep encephalomyelitis in Norway is caused by louping ill virus. These results imply that other viruses present in Europe and known to cause encephalitis/encephalomyelitis of sheep could be caused by louping ill virus.

Mutations in the NS2B and NS3 genes affect mouse neuroinvasiveness of a Western European field strain of tick-borne encephalitis virus

An attenuated strain (263) of the tick-borne encephalitis virus, isolated from field ticks, was either serially subcultured, 5 times in mice, or at 40 degrees C in PS cells, producing 2 independent strains, 263-m5 and 263-TR with identical genomes; both strains exhibited increased plaque size, neuroinvasiveness and temperature-resistance. Sequencing revealed two unique amino acid substitutions, one mapping close to the catalytic site of the viral protease. These observations imply that virus adaptation from ticks to mammals occurs by selection of pre-existing virulent variants from the quasispecies population rather than by the emergence of new random mutations. The significance of these observations is discussed.

Nucleotide and deduced amino acid sequence of the envelope gene of the Vasilchenko strain of TBE virus; comparison with other flaviviruses☆

A strain of tick-borne encephalitis virus known as Vasilchenko (Vs) exhibits relatively low virulence characteristics in monkeys, Syrian hamsters and humans. The gene encoding the envelope glycoprotein of this virus was cloned and sequenced. Alignment of the sequence with those of other known tick-borne flaviviruses and identification of the recognised amino acid genetic marker EHLPTA confirmed its identity as a member of the TBE complex. However, Vs virus was distinguishable from eastern and western tick-borne serotypes by the presence of the sequence AQQ at amino acid positions 232-234 and also by the presence of other specific amino acid substitutions which may be genetic markers for these viruses and could determine their pathogenetic characteristics. When compared with other tick-borne flaviviruses, Vs virus had 12 unique amino acid substitutions including an additional potential glycosylation site at position (315-317). The Vs virus strain shared closest nucleotide and amino acid homology (84.5% and 95.5% respectively) with western and far eastern strains of tick-borne encephalitis virus. Comparison with the far eastern serotype of tick-borne encephalitis virus, by cross-immunoelectrophoresis of Vs virions and PAGE analysis of the extracted virion proteins, revealed differences in surface charge and virus stability that may account for the different virulence characteristics of Vs virus. These results support and enlarge upon previous data obtained from molecular and serological analysis.