TBE in Åland Islands 1959–2005: Kumlinge disease

Tick-Borne Encephalitis Vaccine: Recommendations of the Advisory Committee on Immunization Practices, United States, 2023

Tick-borne encephalitis (TBE) virus is focally endemic in parts of Europe and Asia. The virus is primarily transmitted to humans by the bites of infected

Ixodes species ticks but can also be acquired less frequently by alimentary transmission. Other rare modes of transmission include through breastfeeding, blood transfusion, solid organ transplantation, and slaughtering of viremic animals. TBE virus can cause acute neurologic disease, which usually results in hospitalization, often permanent neurologic or cognitive sequelae, and sometimes death. TBE virus infection is a risk for certain travelers and for laboratory workers who work with the virus. In August 2021, the Food and Drug Administration approved Ticovac TBE vaccine for use among persons aged ≥1 year. This report summarizes the epidemiology of and risks for infection with TBE virus, provides information on the immunogenicity and safety of TBE vaccine, and summarizes the recommendations of the Advisory Committee on Immunization Practices (ACIP) for use of TBE vaccine among U.S. travelers and laboratory workers.

The risk for TBE for most U.S. travelers to areas where the disease is endemic is very low. The risk for exposure to infected ticks is highest for persons who are in areas where TBE is endemic during the main TBE virus transmission season of April–November and who are planning to engage in recreational activities in woodland habitats or who might be occupationally exposed. All persons who travel to areas where TBE is endemic should be advised to take precautions to avoid tick bites and to avoid the consumption of unpasteurized dairy products because alimentary transmission of TBE virus can occur. TBE vaccine can further reduce infection risk and might be indicated for certain persons who are at higher risk for TBE. The key factors in the risk-benefit assessment for vaccination are likelihood of exposure to ticks based on activities and itinerary (e.g., location, rurality, season, and duration of travel or residence). Other risk-benefit considerations should include 1) the rare occurrence of TBE but its potentially high morbidity and mortality, 2) the higher risk for severe disease among certain persons (e.g., older persons aged ≥60 years), 3) the availability of an effective vaccine, 4) the possibility but low probability of serious adverse events after vaccination, 5) the likelihood of future travel to areas where TBE is endemic, and 6) personal perception and tolerance of risk

ACIP recommends TBE vaccine for U.S. persons who are moving or traveling to an area where the disease is endemic and will have extensive exposure to ticks based on their planned outdoor activities and itinerary. Extensive exposure can be considered based on the duration of travel and frequency of exposure and might include shorter-term (e.g., <1 month) travelers with daily or frequent exposure or longer-term travelers with regular (e.g., a few times a month) exposure to environments that might harbor infected ticks. In addition, TBE vaccine may be considered for persons who might engage in outdoor activities in areas where ticks are likely to be found, with a decision to vaccinate made on the basis of an assessment of their planned activities and itinerary, risk factors for a poor medical outcome, and personal perception and tolerance of risk. In the laboratory setting, ACIP recommends TBE vaccine for laboratory workers with a potential for exposure to TBE virus

The RNA Secondary Structure Analysis Reveals Potential for Emergence of Pathogenic Flaviviruses

The Flavivirus genus is divided into four groups: Mosquito-borne flaviviruses, Tick-borne flaviviruses, no-known vector flaviviruses, and Insect specific flaviviruses. Millions of people are affected worldwide every year due to the flaviviral infections. The 5' UTR of the RNA genome plays a critical role in the biology of flaviviruses. To explore any correlation between the topology of the 5' UTR and pathogenesis, a global scale study of the RNA secondary structure of different groups of flaviviruses has been conducted. We found that most of the pathogenic flaviviruses, irrespective of their mode of transmission, tend to form a Y shaped topology in the Stem loop A of the 5' UTR. Some of the current non-pathogenic flaviviruses were also observed to form Y shaped structure. Based on this study, it has been proposed that the flaviviruses having the Y shaped topology in their 5' UTR regions may have the potential to become pathogenic.

Clinical Characteristics of Patients with Tick-Borne Encephalitis (TBE): A European Multicentre Study from 2010 to 2017

Tick-borne encephalitis (TBE) virus is a major cause of central nervous system infections in endemic countries. Here, we present clinical and laboratory characteristics of a large international cohort of patients with confirmed TBE using a uniform clinical protocol. Patients were recruited in eight centers from six European countries between 2010 and 2017. A detailed description of clinical signs and symptoms was recorded. The obtained information enabled a reliable classification in 553 of 555 patients: 207 (37.3%) had meningitis, 273 (49.2%) meningoencephalitis, 15 (2.7%) meningomyelitis, and 58 (10.5%) meningoencephalomyelitis; 41 (7.4%) patients had a peripheral paresis of extremities, 13 (2.3%) a central paresis of extremities, and 25 (4.5%) had single or multiple cranial nerve palsies. Five (0.9%) patients died during acute illness. Outcome at discharge was recorded in 298 patients. Of 176 (59.1%) patients with incomplete recovery, 80 (27%) displayed persisting symptoms or signs without recovery expectation. This study provides further evidence that TBE is a severe disease with a large proportion of patients with incomplete recovery. We suggest monitoring TBE in endemic European countries using a uniform protocol to record the full clinical spectrum of the disease.

A Serosurvey of Tick-Borne Encephalitis Virus in Sweden: Different Populations and Geographical Locations

Background: New risk areas for tick-borne encephalitis (TBE) are emerging and the spread of disease and vaccine coverage is unclear in Sweden. We wanted to study the prevalence and levels of TBE-virus (TBEV) antibodies in southern Sweden, and to investigate whether there were individuals with undiagnosed TBE. Materials and Methods: Two cohorts of sera were collected: One group of anonymous individuals in rural areas (AIRA) in Skåne and one group of volunteers who often got tick-bites (tick-bitten individuals [TBI]). An enzyme-linked immunosorbent assay for TBEV IgM and IgG was performed, as well as a TBEV neutralization test (NT) in selected individuals. Results: In the AIRA group, there was an IgG seropositivity of 5.3%. There were individuals with high antibody levels both in areas previously considered as risk areas (Bromölla and Knislinge), as well as in another area (Tyringe). In the TBI group, 45% of the individuals were vaccinated according to the questionnaires and IgG seropositivity was 28%. A lower seroprevalence and levels of antibodies were seen in the middle-aged group (50-69 years) compared with younger or elderly study participants. A positive NT revealed several individuals with suspected undiagnosed episodes of TBE. Conclusion: Subclinical or misdiagnosed cases have probably occurred in Skåne. Middle-aged individuals had lower levels of IgG, which could indicate either less tick exposure or a lower vaccine response. Less than half of the TBI were vaccinated, an indication that more information about the disease and vaccine might be needed. We conclude that the study motivates an increased awareness of TBEV in the region.

Surveillance of endemic foci of tick-borne encephalitis in Finland 1995-2013: evidence of emergence of new foci

The geographical risk areas for tick-borne encephalitis (TBE) in Finland remained the same until the beginning of the 21st century, but a considerable geographical expansion has been observed in the past 10 years. In order to support public health measures, the present study describes the number of laboratory-confirmed TBE cases and laboratory tests conducted and the associated trends by hospital district, with a particular emphasis on the suspected geographical risk areas. An additional investigation was conducted on 1,957 clinical serum samples throughout the country taken from patients with neurological symptoms to screen for undiagnosed TBE cases. This study identified new TBE foci in Finland, reflecting the spread of the disease into new areas. Even in the most endemic municipalities, transmission of TBE to humans occurred in very specific and often small foci. The number of antibody tests for TBE virus more than doubled (an increase by 105%) between 2007 and 2013. Analysis of the number of tests also revealed areas in which the awareness of clinicians may be suboptimal at present. However, it appears that underdiagnosis of neuroinvasive TBE is not common.

Siberian subtype tick-borne encephalitis virus in Ixodes ricinus in a newly emerged focus, Finland

The first tick-borne encephalitis (TBE) cases in Kotka, Finland appeared in 2010. Altogether ten human cases have been diagnosed by 2014. Four had long-lasting sequelae. We collected 195 Ixodes ricinus ticks, nine rodents, and eleven shrews from the archipelago of Kotka in 2011. Three Siberian subtype TBE virus (TBEV) strains were isolated from the ticks and three mammals were positive for TBEV antibodies. The archipelago of Kotka is a newly emerged TBE focus of Siberian subtype TBEV circulating notably in I. ricinus. The patients had on average longer hospitalization than reported for the European subtype infection.

Factors determining immunological response to vaccination against tick-borne encephalitis virus in older individuals

We performed a cross-sectional study including 533 individuals (median age 61) from the highly TBE endemic Åland Islands in the archipelago between Sweden and Finland. Blood samples, questionnaires and vaccination records were obtained from all study participants. The aim was to investigate if there was any association between TBEV antibody titer and 12 health-related factors. Measurement of TBEV IgG antibodies was performed using two commercial ELISA assays (Enzygnost and Immunozym), and a third in-house rapid fluorescent focus inhibition test was used to measure TBEV neutralizing antibodies. The age of the individual and the number of vaccine doses were the two most important factors determining the immunological response to vaccination. The response to each vaccine dose declined linearly with increased age. A 35 year age difference corresponds to a vaccine dose increment from 3 to 4 to achieve the same immunological response. Participants previously vaccinated against other flaviviruses had lower odds of being seropositive for neutralizing TBEV antibodies on average, while participants with self-reported asthma had higher odds of being seropositive. By comparing the 3 serological assays we show that the Enzygnost and Immunozym assay differ due to choice of cutoffs, but not in overall accuracy.

Prevalence of tick-borne encephalitis virus in Ixodes ricinus and Dermacentor reticulatus ticks collected from the Lublin region (eastern Poland)

A total of 875 nymphal and adult Ixodes ricinus ticks and 148 adult Dermacentor reticulatus ticks were collected by flagging lower vegetation in the Lublin region (eastern Poland) and examined for the presence of RNA of tick-borne encephalitis virus (TBEV) by nested RT-PCR. The minimum infection rate of I. ricinus ticks with TBEV amounted to 1.6% while the infection rate of D. reticulatus ticks was 10.8%. The results suggest that D. reticulatus may be a potential vector of TBEV in Central Europe.

A newly detected tick-borne encephalitis (TBE) focus in south-east Sweden: A follow-up study of TBE virus (TBEV) seroprevalence

In 2002, two cases of tick-borne encephalitis (TBE) were diagnosed in inhabitants of a tick endemic area on the island of Aspö in south-east Sweden. During the previous 25 y, only 2 other cases of TBE had been diagnosed in that region of Sweden. To investigate the presence and evolution of seroprevalence of antibodies to TBE virus (TBEV), we compared inhabitants' anti-TBEV immunoglobulin G (IgG) levels in blood samples drawn in 1991 and 2002. A significant increase in IgG antibody levels was observed in 24/200 (12.0%) 2002 blood samples compared to 7/200 (3.5%) 1991 samples. However, neutralizing antibodies were detected in only 4 of the 2-step enzyme-linked immunosorbent assay (ELISA) IgG positive sera against TBEV, corresponding to a TBEV neutralizing test (NT) prevalence of 2%. Significantly more men than women were seropositive for TBEV antibodies. Compared to other age groups, the greatest increase in TBEV antibody levels was observed in the 20-29-y age group. However, the majority of seropositive samples were from participants >50 y of age. Recommending preventative measures, including vaccination against TBE, to individuals who reside in or regularly visit TBEV endemic areas is suggested.

Tick-borne encephalitis

We review the epidemiological and clinical characteristics of tick-borne encephalitis, and summarise biological and virological aspects that are important for understanding the life-cycle and transmission of the virus. Tick-borne encephalitis virus is a flavivirus that is transmitted by Ixodes spp ticks in a vast area from western Europe to the eastern coast of Japan. Tick-borne encephalitis causes acute meningoencephalitis with or without myelitis. Morbidity is age dependent, and is highest in adults of whom half develop encephalitis. A third of patients have longlasting sequelae, frequently with cognitive dysfunction and substantial impairment in quality of life. The disease arises in patchy endemic foci in Europe, with climatic and ecological conditions suitable for circulation of the virus. Climate change and leisure habits expose more people to tick-bites and have contributed to the increase in number of cases despite availability of effective vaccines. The serological diagnosis is usually straightforward. No specific treatment for the disease exists, and immunisation is the main preventive measure.