Page 415 - TBE_Book_V2_2019

Chapter 12b

TBE in Sweden

Åke Lundkvist

History and Current Situation

Tick-borne encephalitis virus (TBEV) was The sequence was characterized and
isolated for the first time in Sweden in 1958 compared with those of other tick-borne
(from ticks and from 1 tick-borne encephalitis flaviviruses, which led to classification of the
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[TBE] patient). In 2003, Haglund and virus as TBEV-EU. The same group reported in
colleagues reported the isolation and 2011 on the first complete genome of a
antigenic and genetic characterization of 14 Swedish TBEV strain by completing the earlier
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TBEV strains from Swedish patients (samples partial sequencing (see above). The total RNA
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collected 1991–1994). The first serum was sufficient for the sequencing of a
sample, from which TBEV was isolated, was complete TBEV genome (Torö-2003), without
obtained 2–10 days after onset of disease and conventional enrichment procedures such as
found to be negative for anti-TBEV cell culture or amplification in suckling mice.
immunoglobulin M (IgM) by enzyme-linked Sequence analyses also revealed that Torö-
immunosorbent assay (ELISA), whereas TBEV- 2003 belonged to the TBEV-EU subtype, being
specific IgM (and TBEV-specific immuno- most similar to TBE 263 with 97.4% and 98.8%
globulin G/cerebrospinal fluid [IgG/CSF] homologies at the nucleotide and amino acid
activity) was demonstrated in later serum levels, respectively.
samples taken during the second phase of the
disease. In 2014, Veje and coworkers reported 2 cases
of TBE in which TBEV RNA could be detected
Of 20 patient serum samples inoculated into in urine by real-time polymerase chain
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the brain of suckling mice, 14 induced obvious reaction (PCR) during the encephalitic phase.
signs of illness (death or clear physical signs in The TBEV RNA quantities from 1 patient
all cases, 5–7 days after inoculation), and TBEV allowed sequencing of 10,432 nucleotides (nt),
was isolated in all cases. Three earlier Swedish which confirmed the PCR finding in urine, and
1
TBEV patient isolates from 1958, 1959, and phylogenetic analysis showed that the virus
1966, respectively, were included in the same belonged to the TBEV-EU clade.
study. Phylogenetic analyses of the partial
sequence (domain III) of the E gene revealed In 2016, Henningsson and associates reported
that all Swedish TBEV strains grouped isolation and a complete TBEV sequence from
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together with the previously characterized a biting tick. By performing nt sequencing of
strains (Neudoerfl, Kumlinge-A52, Hypr, and the virus strain (Tick/SWE/ Habo/2011/1) via 2
TBE 263) of the Western or European subtype different strategies (deep sequencing of the
of TBEV (TBEV-EU). A549 isolate and direct sequencing of PCR
amplicons of RNA extracted from the tick,
In 2007, a partial TBEV sequence respectively), the authors showed that the 2
(approximately one-third of the viral genome) sequences were identical over 3,382 nt,
from a small pool of ticks collected in the thereby suggesting that the virus isolation
Stockholm archipelago on the island of Torö procedure did not introduce a selection bias
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was reported. with regard to the compared nt sequences.

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