Isolation of tick-borne encephalitis virus from wild rodents and a seroepizootiologic survey in Hokkaido, Japan

Seroepidemiological surveys of tick-borne encephalitis virus and novel tick-borne viruses in wild boar in Nagasaki, Japan

In Japan, tick-borne viruses such as tick-borne encephalitis virus (TBEV) and severe fever with thrombocytopenia syndrome virus have been identified in humans, animals, and ticks. In addition, novel tick-borne viruses have been isolated from ticks in Japan. This study aimed to determine the seroprevalence of TBEV and novel viruses, particularly Tofla virus (TFLV), Kabuto Mountain virus (KAMV), and Muko virus (MUV) in wild boar in Nagasaki, Japan. Enzyme-linked immunosorbent assays and neutralization tests were performed to detect antibodies against each virus. Wild boar serum tested positive for antibodies against KAMV, TFLV, and TBEV, but not MUV. This study revealed the seroprevalence of newly identified tick-borne viruses and TBEV in animals residing in the Nagasaki area. The seroprevalence of these viruses in sentinel animals may inform policies aimed at preventing tick-borne virus disease outbreaks.

A Retrospective Epidemiological Study of Tick-Borne Encephalitis Virus in Patients with Neurological Disorders in Hokkaido, Japan

Tick-borne encephalitis (TBE) is a zoonotic disease that usually presents as a moderate febrile illness followed by severe encephalitis, and various neurological symptoms are observed depending on the distinct central nervous system (CNS) regions affected by the TBE virus (TBEV) infection. In Japan, TBE incidence is increasing and TBEV distributions are reported in wide areas, specifically in Hokkaido. However, an extensive epidemiological survey regarding TBEV has not been conducted yet. In this study, we conducted a retrospective study of the prevalence of antibodies against TBEV in patients with neurological disorders and healthy populations in a TBEV-endemic area in Hokkaido. Among 2000 patients, three patients with inflammatory diseases in the CNS had TBEV-specific IgM antibodies and neutralizing antibodies. The other four patients diagnosed clinically with other neurological diseases were positive for TBEV-specific IgG and neutralizing antibodies, indicating previous TBEV infection. In a total of 246 healthy residents in a TBEV-endemic region, one resident had TBEV-specific antibodies. These results demonstrated undiagnosed TBEV infections in Japan. Further surveys are required to reveal the actual epidemiological risk of TBE and to consider preventive measures, such as a vaccine program, for the control of TBE in Japan.

Characterization of tick-borne encephalitis virus isolated from a tick in central Hokkaido in 2017

Tick-borne encephalitis virus (TBEV) is a zoonotic virus in the genus Flavivirus, family Flaviviridae. TBEV is widely distributed in northern regions of the Eurasian continent, including Japan, and causes severe encephalitis in humans. Tick-borne encephalitis (TBE) was recently reported in central Hokkaido, and wild animals with anti-TBEV antibodies were detected over a wide area of Hokkaido, although TBEV was only isolated in southern Hokkaido. In this study, we conducted a survey of ticks to isolate TBEV in central Hokkaido. One strain, designated Sapporo-17-Io1, was isolated from ticks (Ixodes ovatus) collected in Sapporo city. Sequence analysis revealed that the isolated strain belonged to the Far Eastern subtype of TBEV and was classified in a different subcluster from Oshima 5-10, which had previously been isolated in southern Hokkaido. Sapporo-17-Io1 showed similar growth properties to those of Oshima 5-10 in cultured cells and mouse brains. The mortality rate of mice infected intracerebrally with each virus was similar, but the survival time of mice inoculated with Sapporo-17-Io1 was significantly longer than that of mice inoculated with Oshima 5-10. These results indicate that the neurovirulence of Sapporo-17-Io1 was lower than that of Oshima 5-10. Using an infectious cDNA clone, the replacement of genes encoding non-structural genes from Oshima 5-10 with those from Sapporo-17-Io1 attenuated the neuropathogenicity of the cloned viruses. This result indicated that the non-structural proteins determine the neurovirulence of these two strains. Our results provide important insights for evaluating epidemiological risk in TBE-endemic areas of Hokkaido.

Exploring the Reservoir Hosts of Tick-Borne Encephalitis Virus

Tick-borne encephalitis virus (TBEV) is an important arbovirus, which is found across large parts of Eurasia and is considered to be a major health risk for humans. Like any other arbovirus, TBEV relies on complex interactions between vectors, reservoir hosts, and the environment for successful virus circulation. Hard ticks are the vectors for TBEV, transmitting the virus to a variety of animals. The importance of these animals in the lifecycle of TBEV is still up for debate. Large woodland animals seem to have a positive influence on virus circulation by providing a food source for adult ticks; birds are suspected to play a role in virus distribution. Bank voles and yellow-necked mice are often referred to as classical virus reservoirs, but this statement lacks strong evidence supporting their highlighted role. Other small mammals (e.g., insectivores) may also play a crucial role in virus transmission, not to mention the absence of any suspected reservoir host for non-European endemic regions. Theories highlighting the importance of the co-feeding transmission route go as far as naming ticks themselves as the true reservoir for TBEV, and mammalian hosts as a mere bridge for transmission. A deeper insight into the virus reservoir could lead to a better understanding of the development of endemic regions. The spatial distribution of TBEV is constricted to certain areas, forming natural foci that can be restricted to sizes of merely 500 square meters. The limiting factors for their occurrence are largely unknown, but a possible influence of reservoir hosts on the distribution pattern of TBE is discussed. This review aims to give an overview of the multiple factors influencing the TBEV transmission cycle, focusing on the role of virus reservoirs, and highlights the questions that are waiting to be further explored.

Detection of a novel tick-borne flavivirus and its serological surveillance

Tick-borne encephalitis virus (TBEV), a flavivirus that causes severe neurological symptoms in humans, has been found in Hokkaido, Japan. In the present study, we detected sequences from a novel tick-borne flavivirus, designated Yamaguchi virus (YGV), in liver and serum samples obtained from a wild boar in the Yamaguchi prefecture, Japan. Phylogenetic analysis revealed that YGV belongs to the TBEV complex and is closely related to Langat virus (LGTV). YGV was also detected by specific RT-PCR from 20 in 378 pools of ticks (2923 ticks) collected in Yamaguchi and Wakayama prefectures and from seven in 46 wild boar captured in Wakayama. The major ticks infected with YGV belong to the genus Haemaphysalis. Unfortunately, YGV could not be isolated from any samples from the RT-PCR positive wild boar or ticks. Therefore, ELISA for detection of antibodies against YGV was established using LGTV, and surveillance was performed among wild boar in 10 different prefectures on Honshu Island, the main island of Japan. The results showed that the seroprevalence of tick-borne flavivirus infection in the Wakayama and Hyogo prefectures of western Japan was significantly higher than that in the other prefectures, while antibodies against tick-borne flavivirus were not detected in any wild boar in the Tochigi prefecture in the eastern part of Japan. In addition, wild raccoons or masked palm civets in the Hyogo prefecture did not possess detectable antibodies against tick-borne flaviviruses. In conclusion, YGV appears to be maintained primarily among wild boar and ticks in the western part of Japan. YGV is the second flavivirus (after Japanese encephalitis virus) shown to be circulating on Honshu Island in Japan.

Distribution of tick-borne diseases in Japan: Past patterns and implications for the future

The rapid geographical spread of tick-borne diseases (TBDs) worldwide has recently provoked significant concerns amongst public health authorities. Tick-borne pathogens are maintained in enzootic cycles involving ticks and wild animal hosts, with epizootic spread to other mammals, including livestock and humans. Despite the increasing public health concern, current TBD diagnostic tests and treatments are inadequate, and predictive models of future risks posed by TBDs are limited by the heterogeneity of environmental, vector, and host factors, even in neighboring regions. In recent years, infections resulting in severe fever with thrombocytopenia syndrome (SFTS), Japanese spotted fever, and the scrub typhus pathogens have been reported frequently in addition to traditional TBDs in Japan. The Japanese archipelago is extremely elongated from north to south and its climate varies considerably, creating remarkable regional differences in tick species. The importance of continuous surveillance of TBDs has been growing in terms of geopathology - studies dealing with the relationships between geographic factors and the causes of specific diseases - in Japan and neighboring areas among eastern Asian countries, including China and Korea. In this review, we summarize detailed information regarding the history and epidemic status of human TBDs in Japan.

The Role of Mammalian Reservoir Hosts in Tick-Borne Flavivirus Biology

Small-to-medium sized mammals and large animals are lucrative sources of blood meals for ixodid ticks that transmit life-threatening tick-borne flaviviruses (TBFVs). TBFVs have been isolated from various organs obtained from wild-caught Myodes and Apodemus species in Europe and Asia. Thus, these rodents are well-established reservoirs of TBFVs. Wild-caught Peromyscus species have demonstrated seropositivity against Powassan virus, the only TBFV known to circulate in North America, suggesting that they may play an important role in the biology of the virus in this geographic region. However, virus isolation from Peromyscus species is yet to be demonstrated. Wild-caught medium-sized mammals, such as woodchucks (Marmota monax) and skunks (Mephitis mephitis) have also demonstrated seropositivity against POWV, and virus was isolated from apparently healthy animals. Despite the well-established knowledge that small-to-medium sized animals are TBFV reservoirs, specific molecular biology addressing host-pathogen interactions remains poorly understood. Elucidating these interactions will be critical for gaining insight into the mechanism(s) of viral pathogenesis and/or resistance.

Survey of tick-borne zoonotic viruses in wild deer in Hokkaido, Japan

Tick-borne encephalitis (TBE) and severe fever with thrombocytopenia syndrome (SFTS) are both tick-borne zoonotic diseases caused by TBE virus (TBEV) and SFTS phlebovirus (SFTSV). In 2016, a second domestic TBE case was reported in Hokkaido, Japan, after an absence of 23 years. We conducted IgG ELISA for TBEV and SFTSV on 314 deer (Cervus nippon yesoensis) serum samples collected from 3 places in Hokkaido. There were 7 seropositive samples for TBEV but none for SFTSV by ELISA. The specificity of the 7 positive samples was confirmed by neutralization tests against TBEV, and 5 sera showed 320 to 640 of 50% focus reduction endpoint titers. Our results provide information about the infectious status of TBEV in wild deer in Hokkaido, Japan.

Tick bite-Erythema migrans

This manuscript describes the typical presentation of erythema migrans caused by tick bite. The purposes of this article are (i) to encourage physicians to observe patients with tick bite carefully since it may cause fatal encephalitis, and (ii) to facilitate physicians, especially in endemic areas, to consider tick-borne encephalitis virus as one of the causes of encephalitis.

A Serosurvey of Flavivirus Infection in Horses and Birds in Slovakia

In central Europe, at least three flaviviruses circulate among vectors and vertebrate hosts. West Nile virus (WNV) and Usutu virus (USUV) are mosquito-borne viruses maintained in the nature by enzootic cycle between mosquitoes and birds. Tick-borne encephalitis virus (TBEV) is a flavivirus causing annual human cases in Slovakia. The aim of this study is the prevalence assessment of flavivirus infections in horses (n = 145) and birds (n = 109) by enzyme-linked immunosorbent assay (ELISA) and confirmation by neutralization test (VNT). WNV antibodies have been detected in 11.7% of tested horses and 11.9% of tested birds and confirmed in 6.9% of horse and 9.2% of bird samples. None of the WNV seropositive or dubious horses had WNV IgM (ELISA), and none of the tested horses had USUV neutralizing antibodies. Autochthonous WNV infections have been confirmed in 16.7% of horses without international travelling history. Most of them were from western Slovakia with known endemic WNV transmission. An autochthonous WNV infection in a horse from highland area of Kremnické vrchy (central Slovakia) with unknown data of WNV circulation and without travelling history was detected. TBEV antibody was detected in 6.2% of horses and in 3.4% has been confirmed. In two horses, WNV and TBEV infection could not be distinguished. Confirmed WNV seropositive were eight raptors showing nonspecific signs or suffering from trauma, one white stork, and one house sparrow. The sparrow was caught in a locality in eastern Slovakia, where WNV RNA had been previously detected in sparrows. USUV neutralizing antibodies were present in pooled sample from four Eurasian great tits. Because of insufficient volume, TBEV VNT was not carried out in birds. Results further prove the endemicity of WNV and other vector-borne flaviviruses in natural and accidental hosts in Slovakia, giving better insight in flavivirus epidemiology in European countries in general.

[Pathogenic mechanisms of Tick-borne Flaviviruses]

Many tick-borne flaviviruses causes fatal encephalitis in humans and animals with severe sequelae. However, it remains unclear how viral replication and pathogenicity contribute to the neurologic manifestations. In this paper, I summarized the specific replication mechanism of tick-borne flaviviruses in neurons and their effect on the pathogenicity of neurological disease. Our findings of the unique virus-host interaction in central nerve system will improve further understanding of the molecular mechanisms of viral replication and the pathogenicity of neurotropic viruses.

[Tick-borne encephalitis]

Tick-borne encephalitis virus (TBEV) belongs to the Flaviviridae family and Flavivirus genus. TBEV is maintained in transmission cycles between Ixodid ticks and wild mammalian hosts, particularly rodents. A wide range of animal species are also infected with TBEV by the bite of infected ticks, and TBEV infection causes fatal encephalitis in humans. TBEV is endemic widely in the Eurasian continent, and more than 10,000 cases of the disease are reported annually. In Japan, the 1st confirmed case of TBE was reported in the southern area of Hokkaido in 1993, and after 20 years, the 2nd to 4th cases were reported in Hokkaido in 2016 and 2017. Our sero-epizootiological survey indicated endemic foci of TBEV are widely distributed in Hokkaido and that those of TBEV or tick-borne flavivirus outside Hokkaido. In this review, I introduced recent topics of TBEV including newly developed diagnostic methods, epidemiology and pathogenesis of TBEV.

Peromyscus leucopus mouse brain transcriptome response to Powassan virus infection

Powassan virus (POWV) is a tick-borne Flavivirus responsible for life-threatening encephalitis in North America and some regions of Russia. The ticks that have been reported to transmit the virus belong to the Ixodes species, and they feed on small-to-medium-sized mammals, such as Peromyscus leucopus mice, skunks, and woodchucks. We previously developed a P. leucopus mouse model of POWV infection, and the model is characterized by a lack of clinical signs of disease following intraperitoneal or intracranial inoculation. However, intracranial inoculation results in mild subclinical encephalitis from 5 days post infection (dpi), but the encephalitis resolves by 28 dpi. We used RNA sequencing to profile the P. leucopus mouse brain transcriptome at different time points after intracranial challenge with POWV. At 24 h post infection, 42 genes were significantly differentially expressed and the number peaked to 232 at 7 dpi before declining to 31 at 28 dpi. Using Ingenuity Pathway Analysis, we determined that the genes that were significantly expressed from 1 to 15 dpi were mainly associated with interferon signaling. As a result, many interferon-stimulated genes (ISGs) were upregulated. Some of the ISGs include an array of TRIMs (genes encoding tripartite motif proteins). These results will be useful for the identification of POWV restriction factors.

Tick-borne encephalitis in Japan, Republic of Korea and China

Tick-borne encephalitis virus (TBEV) causes mild or moderate febrile illness in humans that may progress to encephalitis, leading to severe long-term complications and sometimes death. TBEV is prevalent in the Eurasian continent and has been isolated in China, Japan and Republic of Korea (ROK). The TBEV isolates from Japan are of the Far-Eastern subtype; in ROK, the isolates are of the Western subtype; and all TBEV isolates in China are of the Far-Eastern subtype, except one strain that was identified most recently as the Siberian subtype. TBE is endemic to the northeast, northwest and southeast of China; only two confirmed TBE cases have been reported in Japan to date; and no TBE case has been confirmed in ROK. For TBE patients in China, the onset of disease is acute with no biphasic course for disease presentation. The clinical spectrum of disease phenotypes may be wider than currently understood, since serological evidence suggests the presence of TBEV infections in healthy people, indicating that asymptomatic or unspecific manifestations of TBEV infection may exist. The current treatment for TBE is supportive care. In China, vaccines against TBEV have been developed and are available with demonstrated immunogenicity and safety, although efficacy data are lacking. No vaccines are available in ROK or Japan.

Characteristics of far eastern strains of tick-borne encephalitis virus

A comparative study of biological, molecular and genetic characteristics of a collection of ten strains of tick-borne encephalitis virus (TBEV) isolated in Primorsky Krai before 1960 and stored in a lyophilized state for a prolonged period (over 65 years) is presented. The collection includes the Sofjin strain isolated from the brain of a fatal case in Primorsky Krai in 1937 and transferred to the Scientific Research Institute of Epidemiology and Microbiology (Vladivostok) in 1953. All lyophilized viral strains demonstrated great preservation and high infectious activity in the model of 2-day-old non-inbred mice. Whole-genome sequencing showed that all strains belong to the Far East TBEV subtype, comprising three clusters of Sofjin-, Oshima- and Senzhang-like strains. We show that SofjinPYB, Sofjin (Vector) and Sofjin-HO strains form a separate branch of the phylogenetic tree and are closely related to Khabarovsk-Obor-4, but not to the original Sofjin strain. The Sofjin-1953, Sofijin-Chumakov, SofjinKSY and SofjinCDC strains are genetically close to each other and can be used as reference strains for comparative analysis of the tick-borne encephalitis virus population.

Modeling Powassan virus infection in Peromyscus leucopus, a natural host

The tick-borne flavivirus, Powassan virus (POWV) causes life-threatening encephalitis in humans in North America and Europe. POWV is transmitted by ixodid tick vectors that feed on small to medium-sized mammals, such as Peromyscus leucopus mice, which may serve as either reservoir, bridge or amplification hosts. Intraperitoneal and intracranial inoculation of 4-week old Peromyscus leucopus mice with 103 PFU of POWV did not result in overt clinical signs of disease. However, following intracranial inoculation, infected mice seroconverted to POWV and histopathological examinations revealed that the mice uniformly developed mild lymphocytic perivascular cuffing and microgliosis in the brain and spinal cord from 5 to 15 days post infection (dpi), suggesting an early inflammatory response. In contrast, intracranial inoculation of 4-week old C57BL/6 and BALB/c mice was lethal by 5 dpi. Intraperitoneal inoculation was lethal in BALB/c mice, but 40% (2/5) of C57BL/6 mice survived. We concluded that Peromyscus leucopus mice infected i.c. with a lethal dose of POWV support a limited infection, restricted to the central nervous system and mount an antibody response to the virus. However, they fail to develop clinical signs of disease and are able to control the infection. These results suggest the involvement of restriction factors, and the mechanism by which Peromyscus leucopus mice restrict POWV infection remains under study.

Review: Sentinels of tick-borne encephalitis risk

Tick-borne encephalitis (TBE) is a viral zoonotic disease endemic in many regions of Eurasia. The definition of TBE risk areas is complicated by the focal nature of the TBE virus transmission. Furthermore, vaccination may reduce case numbers and thus mask infection risk to unvaccinated persons. Therefore, additional risk indicators are sought to complement the current risk assessment solely based on human incidence. We reviewed studies published over the past ten years investigating potential new sentinels of TBE risk to understand the advantages and disadvantages of the various sentinel animal surveys and surrogate indicator methods. Virus prevalence in questing ticks is an unsuitable indicator of TBE infection risk as viral RNA is rarely detected even in large sample sizes collected at known TBE endemic areas. Seroprevalence in domestic animals, on the other hand, showed good spatial correlation with TBE incidence in humans and might also uncover presently unknown TBEV foci.

Louping ill virus (LIV) in the Far East

This study focused on finding, culturing, and identifying the biological and genetic characteristics of three louping ill virus (LIV) strains in the south of the Russian Far East. The Primorye-155-77 and Primorye-20-79 virus strains were isolated from Ixodes persulcatus ticks, and the Primorye-185-91 strain was isolated from the blood of a person after a tick bite. According to the hemagglutination and neutralization tests, Primorye-155-77, Primorye-20-79 and Primorye-185-91 had weak reactivity with antibodies in an antiserum against tick-borne encephalitis virus. In Primorye-155-77 and Primorye-20-79, the sequences of the 5' ends of the 2456-nucleotide-long viral RNA including the 5' untranslated region (UTR) and genes of the capsid protein, prM protein and envelope E protein were determined. The complete genome sequence of Primorye-185-91 was determined. The E protein gene of the Negishi strain differed from those of three analyzed strains, as there were mutations resulting in the replacement of three amino acids: Ala163Thr, Asp193Asn and Ala313Thr. The homology of Primorye-185-91 to LIV 369/T2 was 97.57 %, and to the Penrith strain, it was 98.36 %. Phylogenetic analysis showed that Primorye-155-77, Primorye-20-79 and Primorye-185-91 are related to LI/A and LI/K strains isolated in England and Scotland and to the Negishi strain; these strains have a common progenitor. Negishi-like strains were represented by one subtype of louping ill virus, i.e. the British subtype (LIV-Brit). The possibility is discussed of a single introduction of the virus to the Far Eastern region (Japan and Primorsky Krai) from a single natural locus more than 50 years ago.

The three subtypes of tick-borne encephalitis virus induce encephalitis in a natural host, the bank vole (Myodes glareolus)

Tick-borne encephalitis virus (TBEV) infects bank voles (Myodes glareolus) in nature, but the relevance of rodents for TBEV transmission and maintenance is unclear. We infected colonized bank voles subcutaneously to study and compare the infection kinetics, acute infection, and potential viral persistence of the three known TBEV subtypes: European (TBEV-Eur), Siberian (TBEV-Sib) and Far Eastern (TBEV-FE). All strains representing the three subtypes were infective and highly neurotropic. They induced (meningo)encephalitis in some of the animals, however most of the cases did not present with apparent clinical symptoms. TBEV-RNA was cleared significantly slower from the brain as compared to other organs studied. Supporting our earlier findings in natural rodent populations, TBEV-RNA could be detected in the brain for up to 168 days post infection, but we could not demonstrate infectivity by cell culture isolation. Throughout all time points post infection, RNA of the TBEV-FE was detected significantly more often than RNA of the other two strains in all organs studied. TBEV-FE also induced prolonged viremia, indicating distinctive kinetics in rodents in comparison to the other two subtypes. This study shows that bank voles can develop a neuroinvasive TBEV infection with persistence of viral RNA in brain, and mount an anti-TBEV IgG response. The findings also provide further evidence that bank voles can serve as sentinels for TBEV endemicity.

Genetic and biological characterization of tick-borne encephalitis virus isolated from wild rodents in southern Hokkaido, Japan in 2008

Tick-borne encephalitis virus (TBEV) is a zoonotic agent causing severe encephalitis in humans. A recent epizootiological survey indicated that endemic foci of TBEV have been maintained in the southern part of Hokkaido until recently. In this study, we sought to isolate TBEV from wild rodents in the area. One virus, designated Oshima 08-As, was isolated from an Apodemus speciosus captured in Hokuto in 2008. Oshima 08-As was classified as the Far Eastern subtype of TBEV and formed a cluster with the other strains isolated in Hokkaido from 1995 to 1996. Thirty-six nucleotide differences resulted in 12 amino acid changes between Oshima 08-As and Oshima 5-10 isolated in 1995. Oshima 08-As caused high mortality and morbidity in a mouse model compared with Oshima 5-10. Although similar transient viral multiplication in the spleen was observed in the mice infected with Oshima 08-As and Oshima 5-10, greater viral multiplication with an inflammatory response was noted in the brains of mice infected with Oshima 08-As than those infected with Oshima 5-10. These data indicate that a few naturally occurring mutations affect the pathogenicity of the Oshima strains endemic in the southern part of Hokkaido.

Synanthropy of wild mammals as a determinant of emerging infectious diseases in the Asian-Australasian region

Humans create ecologically simplified landscapes that favour some wildlife species, but not others. Here, we explore the possibility that those species that tolerate or do well in human-modified environments, or 'synanthropic' species, are predominantly the hosts of zoonotic emerging and re-emerging infectious diseases (EIDs). We do this using global wildlife conservation data and wildlife host information extracted from systematically reviewed emerging infectious disease literature. The evidence for this relationship is examined with special emphasis on the Australasian, South East Asian and East Asian regions. We find that synanthropic wildlife hosts are approximately 15 times more likely than other wildlife in this region to be the source of emerging infectious diseases, and this association is essentially independent of the taxonomy of the species. A significant positive association with EIDs is also evident for those wildlife species of low conservation risk. Since the increase and spread of native and introduced species able to adapt to human-induced landscape change is at the expense of those species most vulnerable to habitat loss, our findings suggest a mechanism linking land conversion, global decline in biodiversity and a rise in EIDs of wildlife origin.

Prevalence of tick-borne encephalitis virus in ticks from southern Korea

The prevalence of tick-borne encephalitis virus (TBEV) in southern Korea was determined by collecting ticks using tick drags. A total of 4,077 of 6,788 ticks collected were pooled (649 pools) according to collection site, species, and developmental stage and assayed for TBEV. The TBEV protein E and NS5 gene fragments were detected using RT-nested PCR in six pools of nymphs collected from Jeju Island (2,491 ticks). The minimum field detection rates for TBEV were 0.17% and 0.14% for Haemaphysalis longicornis and Haemayphysalis flava nymphs, respectively. The 252 bp NS5 and 477 bp protein E gene amplicons were sequenced. Phylogenetic analysis showed that the NS5 and protein E genes of the Jeju strain were clustered with Western subtype (98.0% and 99.4% identity, respectively). The Western subtype of TBEV is endemic in Korea, including Jeju Island. The study of vector and zoonotic host susceptibility to TBEV is required to better understand its potential impact on public health.

Prevalence of tick-borne encephalitis virus antibodies in dogs from Denmark

Background

Large regions of central and eastern Europe are recognized as areas where tick-borne encephalitis virus (TBEV) is endemic, including countries neighbouring Denmark. It is therefore timely and relevant to determine if TBEV infections occur in Denmark. This study investigates the presence of antibodies against TBEV in a cross-section of the Danish canine population to assess the level of exposure to TBEV and possibly identify TBEV microfoci in Denmark.

Methods

Blood samples were collected from 125 dogs originating from five regions of Denmark between November 2005 and March 2006. Serum was tested by indirect ELISA. All positive and borderline samples were re-evaluated by neutralisation test (NT).

Results

The prevalence of TBEV serocomplex antibodies was 30% by ELISA and 4.8% by NT (with 100%-neutralising capacity). The island of Bornholm was the only area in Denmark with NT positive samples.

Conclusions

The island of Bornholm is an area with a high risk of encountering TBEV microfoci. The presence of TBEV serocomplex antibodies in many sentinel animals from other parts of Denmark points toward existence of other TBEV microfoci. Discrepancies found between ELISA and NT results stress the importance of careful evaluation of serological tests, when interpreting results.

Tick-borne disease systems: mapping geographic and phylogenetic space

Evidence is presented that the evolution of the tick-borne flaviviruses is driven by biotic factors, principally the exploitation of new hosts as transmission routes. Because vector-borne diseases are limited by climatic conditions, however, abiotic factors have the potential to direct and constrain the evolutionary pathways. This idea is explored by testing the hypothesis that closely related viruses occupy more similar eco-climatic spaces than do more distantly related viruses. A statistical comparison of the conventional phylogenetic tree derived from molecular distances and a novel phenetic tree derived from distances between the climatic spaces within which each virus circulates, indicates that these trees match each other more closely than would be expected at random. This suggests that these viruses are indeed limited in the degree to which they can evolve into new environmental conditions.

Emerging zoonotic encephalitis viruses: lessons from Southeast Asia and Oceania

The last decade of the 20th Century saw the introduction of an unprecedented number of encephalitic viruses emerge or spread in the Southeast Asian and Western Pacific regions (Mackenzie et al, 2001; Solomon, 2003a). Most of these viruses are zoonotic, either being arthropod-borne viruses or bat-borne viruses. Thus Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, has spread through the Indonesian archipelago to Papua New Guinea (PNG) and to the islands of the Torres Strait of northern Australia, to Pakistan, and to new areas in the Indian subcontinent; a strain of tick-borne encephalitis virus (TBEV) was described for the first time in Hokkaido, Japan; and a novel mosquito-borne alphavirus, Me Tri virus, was described from Vietnam. Three novel bat-borne viruses emerged in Australia and Malaysia; two, Hendra and Nipah viruses, represent the first examples of a new genus in the family Paramyxoviridae, the genus Henipaviruses, and the third, Australian bat lyssavirus (ABLV) is new lyssavirus closely related to classical rabies virus. These viruses will form the body of this brief review.

[Phylogenetic analysis and pathogenicity of tick-borne encephalitis virus from Japan and far-east Russia]

Phylogenetic analysis of tick-borne encephalitis (TBE) virus revealed that Hokkaido strain of TBE virus evolved several hundreds years ago in far-east Russia. TBE virus strains in Irkutsk area were identified as Siberian subtype of TBE virus. BHK-cell adapted mutant of TBE virus showed lower neuro-invasive virulence in mice than parent virus. The mutant carried one amino acid substitution in envelope protein which resulted in increase of positive charge of the protein. The mutant-infected mice showed lower virus titers in bloods and spleens than the parent-infected mice. Infectious c-DNA clone of TBE virus Hokkaido strain was successfully generated and was applied to examine the neurovirulence in mice. One amino acid change in envelope protein and 2 amino acid changes in Ns5 protein showed a synergistic effect on reduced neurovirulence in mice.

A multigene analysis of the phylogenetic relationships among the flaviviruses (Family: Flaviviridae) and the evolution of vector transmission

The genus Flavivirus (family Flaviviridae) presently comprises around 70 single-strand positive-sense RNA viruses. These replicate in a range of vertebrate and invertebrate cells and may be mosquito-borne, tick-borne or have no-known-vector. Since transmission mode correlates strongly with phylogeny, the flaviviruses constitute a valuable model for the evolution of vector-borne disease. Attempts to resolve the higher-level taxonomic relationships of the flaviviruses through molecular phylogenetics have thus far proved inconclusive because of conflicting positions for the three main transmission groups. We conducted the most comprehensive phylogenetic study to date, involving maximum likelihood analyses of the NS3 and NS5 genes and the entire genome sequences available at present. For the first time, we use and test a variety of more robust methods of sequence alignment and appropriate models of amino acid replacement to study these highly divergent sequences, and explicitly test specific hypotheses of tree topology. We show that (i) the NS5 gene contains insufficient phylogenetic signal to choose between competing topological hypotheses, (ii) the NS3 gene and whole genome data indicate that the mosquito-borne flaviviruses represent an outgroup to the remaining flaviviruses, and (iii) that tick-borne transmission is probably a derived trait within the genus.

Amino acid changes responsible for attenuation of virus neurovirulence in an infectious cDNA clone of the Oshima strain of Tick-borne encephalitis virus

A stable full-length infectious cDNA clone of the Oshima strain of Tick-borne encephalitis virus (Far-Eastern subtype) was developed by a long high-fidelity RT-PCR and one-step cloning procedure. The infectious clone (O-IC) had four amino acid substitutions and produced smaller plaques when compared with the parent Oshima 5-10 strain. Using site-directed mutagenesis, the substitutions were reverted to restore the parent virus sequence (O-IC-pt). Although genetically identical, parent virus Oshima 5-10 and virus recovered from O-IC-pt demonstrated some biological differences that are possibly explained by the presence of quasispecies with differing virulence characteristics within the original virus population. These observations may have implications for vaccines based on modified infectious clones. It was also demonstrated that the amino acid substitution E-S40→P at position 40 in the envelope (E) glycoprotein was responsible for plaque size reduction, reduced infectious virus yields in cell culture and reduced mouse neurovirulence. Additionally, two amino acid substitutions in the non-structural (NS)5 protein (virus RNA-dependent RNA polymerase) NS5-V378→A and NS5-R674→K also contributed to attenuation of virulence in mice, but did not demonstrate a noticeable biological effect in baby hamster kidney cell culture. Comparative neurovirulence tests revealed how the accumulation of individual mutations (E-S40→P, NS5-V378→A and NS5-R674→K) can result in the attenuation of a virus.

Enzyme-linked immunosorbent assay using recombinant antigens expressed in mammalian cells for serodiagnosis of tick-borne encephalitis

A recombinant plasmid that expresses the tick-borne encephalitis (TBE) virus premembrane (prM) and envelope (E) proteins in mammalian cells was constructed. Recombinant proteins retained antigenic and conformational structures similar to those of native virus proteins, and transfected cells released virus-like particles (VLPs), which were 1.13-1.14 g/ml in density and 20-30 nm in diameter, into the culture medium. Recombinant E proteins were used for the development of an enzyme-linked immunosorbent assay (ELISA) to detect TBE virus-specific IgM and IgG antibodies in serum. The results of this ELISA correlated well with the results of commercial ELISA, when tested with 95 serum samples from clinically TBE-suspected patients. In addition, ELISA using recombinant antigens showed no cross-reactivity against serum from Japanese encephalitis (JE) patients, despite the cross-reactivity shown by commercial ELISA systems. These observations indicated that this newly developed ELISA system could distinguish tick-borne encephalitis from Japanese encephalitis infection, and that it constitutes a useful and safe alternative to conventional ELISA systems.

Characterization of monoclonal antibodies against Hokkaido strain tick-borne encephalitis virus

A tick-borne encephalitis (TBE) patient was found in Hokkaido in 1993, and TBE viruses were isolated from animals and ticks in our previous studies. To develop a diagnostic reagent to identify TBE viruses, monoclonal antibodies (Mabs) were produced against the TBE virus strain Hokkaido (Oshima 5-10). Seven Mabs were obtained which reacted with the envelope protein of the Oshima 5-10 strain. These Mabs were flavivirus genus-specific, TBE virus complex-specific or TBE virus type-specific. The Mabs are applicable for identification of TBE virus strains.

Phylogeny of the genus flavivirus using complete coding sequences of arthropod-borne viruses and viruses with no known vector

Attempts to define the evolutionary relationships and origins of viruses in the genus Flavivirus are hampered by the lack of genetic information particularly amongst the non-vectored flaviviruses. Using a novel protocol for sequence determination, the first complete coding sequence of St Louis encephalitis virus and those of two representative non-vectored flaviviruses, Rio Bravo (isolated from bat) and Apoi (isolated from rodent), are reported. The encoded polyproteins of Rio Bravo and Apoi virus are the smallest described to date within the genus FLAVIVIRUS: The highest similarities with other flaviviruses were found in the NS3 and NS5 genes. The proteolytic cleavage sites for the viral serine protease were highly conserved among the flaviviruses completely sequenced to date. Comparative genetic amino acid alignments revealed that p-distance cut-off values of 0.330-0.470 distinguished the arthropod-borne viruses according to their recognized serogroups and Rio Bravo and Apoi virus were assigned to two distinct non-vectored virus groups. Within these serogroups, cladogenesis based on the complete ORF sequence was similar to trees based on envelope and NS5 sequences. In contrast, branching patterns at the deeper nodes of the tree were different from those reported in the previous study of NS5 sequences. The significance of these observations is discussed.

Trends in flavivirus infections in Japan

Although Japanese encephalitis has declined as an important cause of illness and death in Japan, infection with other flaviviruses has become a public health concern. Recently, reports of imported dengue cases, as well as isolations of tick-borne encephalitis virus, have increased.

Phylogenetic and virulence analysis of tick-borne encephalitis viruses from Japan and far-Eastern Russia

We have previously reported that tick-borne encephalitis (TBE) is endemic in a specific area of Hokkaido, Japan. In Oshima, the southern part of Hokkaido, TBE virus was isolated from sentinel dogs, ticks and rodents in 1995 and 1996. To identify when these TBE viruses emerged in Hokkaido, the times of divergence of TBE virus strains isolated in Oshima and far-eastern Russia were estimated. TBE virus was isolated in Khabarovsk in 1998 and the nucleotide sequences of viral envelope protein genes of isolates from Oshima and Khabarovsk were compared. From the synonymous substitution rate of these virus strains, the lineage divergence time of these TBE virus strains was predicted phylogenetically to be about 260-430 years ago. Furthermore, the virulence of TBE virus isolates from Oshima and Khabarovsk were compared in a mouse model. The results showed that the isolates possessed very similar virulence in mice. This report provides evidence that the Oshima strains of TBE virus in Hokkaido emerged from far-eastern Russia a few hundred years ago and this explains why these strains possess virulence similar to the TBE viruses isolated in Russia.