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Tick-borne encephalitis virus in ticks detached from humans and follow-up of serological and clinical response.
Linköping University, Department of Clinical and Experimental Medicine, Division of Microbiology and Molecular Medicine. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Division of Microbiology and Molecular Medicine. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Division of Inflammation Medicine. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Division of Microbiology and Molecular Medicine. Linköping University, Faculty of Health Sciences. Östergötlands Läns Landsting, Heart and Medicine Center, Department of Infectious Diseases.
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2014 (English)In: Ticks and Tick Borne Diseases, ISSN 1877-959X, Vol. 5, no 1, 21-28 p.Article in journal (Refereed) Published
Abstract [en]

The risk of tick-borne encephalitis virus (TBEV) infection after a tick bite remains largely unknown. To address this, we investigated the presence of TBEV in ticks detached from humans in an attempt to relate viral copy number, TBEV subtype, and tick feeding time with the serological and clinical response of the tick-bitten participants. Ticks, blood samples, and questionnaires were collected from tick-bitten humans at 34 primary health care centers in Sweden and in the Aland Islands (Finland). A total of 2167 ticks was received from 1886 persons in 2008-2009. Using a multiplex quantitative real-time PCR, 5 TBEV-infected ticks were found (overall prevalence 0.23%, copy range <4 X 10(2)-7.7 X 10(6) per tick). One unvaccinated person bitten by a tick containing 7.7 x 10(6) TBEV copies experienced symptoms. Another unvaccinated person bitten by a tick containing 1.8 x 10(3) TBEV copies developed neither symptoms nor TBEV antibodies. The remaining 3 persons were protected by vaccination. In contrast, despite lack of TBEV in the detached ticks, 2 persons developed antibodies against TBEV, one of whom reported symptoms. Overall, a low risk of TBEV infection was observed, and too few persons got bitten by TBEV-infected ticks to draw certain conclusions regarding the clinical outcome in relation to the duration of the blood meal and virus copy number. However, this study indicates that an antibody response may develop without clinical symptoms, that a bite by an infected tick not always leads to an antibody response or clinical symptoms, and a possible correlation between virus load and tick feeding time. (C) 2013 Elsevier GmbH. All rights reserved.

Place, publisher, year, edition, pages
Jena, Germany: Elsevier, 2014. Vol. 5, no 1, 21-28 p.
National Category
Microbiology in the medical area
Identifiers
URN: urn:nbn:se:liu:diva-103491DOI: 10.1016/j.ttbdis.2013.07.009ISI: 000329007300004PubMedID: 24275477OAI: oai:DiVA.org:liu-103491DiVA: diva2:689309
Available from: 2014-01-20 Created: 2014-01-20 Last updated: 2015-08-10Bibliographically approved
In thesis
1. Epidemiological and Ecological Studies of Tick-borne Encephalitis Virus
Open this publication in new window or tab >>Epidemiological and Ecological Studies of Tick-borne Encephalitis Virus
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ticks are blood-sucking parasites that are an inconvenience for both humans and animals. The tick by itself is normally harmless unless they attack in excessive numbers. The harm from ticks stems from them being excellent vectors for other parasites, in the form of bacteria and virus that via the ticks are provided a bridge to move across the blood streams of different animals, including humans.

One of the most pathogenic tick-borne disease for humans is caused by a flavivirus, the tick-borne encephalitis virus (TBEV). Each year approximately 10 000 individuals on the Eurasian continent develop neurological disease, in the form of meningitis, encephalitis, myelitis and radiculitis, following a bite by a TBEV infected tick.

To evaluate the risk of TBEV infection after a tick-bite, we have developed a study to investigate ticks that have bitten humans and to follow up the tick-bitten humans to investigate if they get infected, and if they develop symptoms, and further trace the virus back to the tick that is infected with TBEV. Ticks, blood samples, and questionnaires were collected in collaboration with 34 primary health care centers in Sweden and on the Åland Islands during 2008 and 2009.

Several demographical and biological factors were investigated regarding the interaction between ticks and humans. The main finding was that men removed the ticks later than women, and that both older men and older women removed the ticks later than younger individuals. This could in part explain why older individuals in general, and men in particular, are at greater risk of acquiring tick-borne encephalitis (TBE).

Furthermore, the prevalence of TBEV in ticks that have bitten humans were investigated, in order to correlate the copy number of TBEV in the tick and the tick feeding-time to the risk of developing symptomatic and asymptomatic infection. This entailed the development of new methodology for tick analysis and TBEV real-time PCR. The result showed a very low risk of TBEV infection in the studied areas, only 5 of 2167 investigated ticks contained TBEV. Three of the individuals bitten by TBEV infected ticks were vaccinated and did not develop symptoms of TBEV infection. One unvaccinated individual got bitten by a tick containing 1800 virus copies, with a feeding-time of 12-24h, and interestingly showed no signs of infection. Another unvaccinated individual got bitten by a tick containing 7.7 million virus copies, with a feedingtime of >60h. This individual developed symptoms consistent with a 1st phase of TBE, including fever and headache, but did not develop the 2nd neurological phase of TBEV infection. Despite only  finding 5 ticks infected with TBEV, a correlation between the virus load in the tick and the tick feeding-time was observed. In 2 other individuals, TBEV antibody seroconversion was detected during the 3 month study period, one without symptoms, while the other experienced symptoms consistent with the 1st phase of TBE. These observations support the hypothesis that a higher virus amount in the tick and a longer feeding time increases the risk of TBEV infection.

To further examine TBEV in ticks that have bitten humans and find factors that may predict the risk of human infection and development of TBE, we characterized several TBEV strains genetically. Including TBEV strains isolated from ticks that have bitten human, from questing field-collected ticks, and TBEV strains isolated from patients with TBE. In one of the ticks detached from a human after >60h of feeding, there was a heterogeneous population of TBEV quasispecies with varying poly(A) length in the 3’ untranslated region of the genome was observed. These variations might have implications for differences in virulence between TBEV strains, and the heterogeneous quasispecies population observed could be the virus adapting from replication in tick cells to mammalian cells.

We also investigated the response to TBEV vaccination in relation to 14 health-related factors in a population of older individuals on the Åland Islands. Blood samples, questionnaires, and vaccination records were collected from 533 individuals. Three different serological assays to characterize antibody response to TBEV vaccination were used. The main finding was that the number of vaccine doses in relation to age was the most important factor determining successful vaccination. The response to each vaccination dose declined linearly with age, and as much as 47%  of individuals 50 years or older that had taken 3 vaccine doses were seronegative, compared to 23% that had taken 4 doses and 6% with 5 doses. Comparison between the serological assays revealed that the cutoffs determining the balance between sensitivity and specificity differed, but not the overall accuracy.

Taken together, these results contribute to a better understanding of the TBEV epidemiology and can provide tools in the prevention of TBE.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 72 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1399
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-105921 (URN)10.3384/diss.diva-105921 (DOI)978-91-7519-381-6 (ISBN)
Public defence
2014-05-21, Berzeliussalen, Hälsouniversitetet, Campus US, Linköpings universitet, Linköping, 09:00 (English)
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Supervisors
Available from: 2014-04-14 Created: 2014-04-14 Last updated: 2014-04-14Bibliographically approved

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Lindblom, PontusWilhelmsson, PeterFryland, LindaSjowall, JohannaErnerudh, JanForsberg, PiaLindgren, Per-Eric

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Division of Microbiology and Molecular MedicineFaculty of Health SciencesDivision of Inflammation MedicineDepartment of Infectious DiseasesClinical ImmunologyDepartment of Clinical Immunology and Transfusion Medicine
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