Page 79 - TBE_Book_V2_2019

Chapter 3: Transmission / Natural cycle

reticulatus was 10.8% (range 7.3–14.3% in Khabarovsk Region from 3.4% to 9.4% over 4
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infected areas): This is considerably higher years. In the Novosibirsk Region, the preva-
than the prevalence found in questing adult I. lence of TBEV in unfed adult I. persulcatus was
ricinus (1.6%, range 0.7–4.3% in infected 3.6%, with 0.8% being pathogenic to
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areas) in the same area. laboratory mice. In the same study, 3.3% of
questing adult I. pavlovskyi were infected with
Prevalence in questing adult D. reticulatus the virus with 1.8% of the isolates being
ticks from Białowieża Primeval Forest was pathogenic. Information on less commonly
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similar (1.58%) to that in questing I. ricinus encountered vectors is rarely available and
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(1.30%), as was the case in Moldova (adult I. sample sizes are usually low, making such data
ricinus 3.8%, adult D. reticulatus 3.9%, but unreliable (e.g., Kim et al.) 86 Daniel et al.
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adult Haemaphysalis punctata 8.8%). The state that the prevalence of TBEV in ticks is
natural occurrence of TBEV in a D. reticulatus correlated with the time of the year when
tick population has also been proven for ticks are collected. Long-term studies and
Germany during 2016 to 2018 by the isolation statistical analyses showed that higher
of several TBEV strains from this tick species in average temperatures during the summer-
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a natural focus. autumn period may lead to higher levels of
TBEV found in ticks and consequently increase
The differences in TBEV prevalence in the the risk for humans to develop symptomatic
various vector species remain puzzling. TBE following an infected tick bite.
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Questing I. ricinus usually have a very low
prevalence of the virus, ranging from no virus
in many areas to less than 1% in most others,
and rarely reaching 2–5%, especially in unfed Vertebrate hosts
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adults. 75–79 Knap and Avsic-Zupanc showed
that over a 4-year period, the prevalence was The prevalence of antibodies to TBEV in hosts
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at the expected low level in the 8 areas is quite variable. TBEV has been found in
numerous mammal species from different
studied, but that no area was consistently
positive for the virus. This may be related to families, as well as in a large number of
the frequently low sample sizes used (14/30 passerine and non-passerine bird species
samples had fewer than 300 specimens). (Table 2). Virus infection, antibodies to the
virus, or viral ribonucleic acid (RNA) have also
Prevalence of virus in feeding ticks, although been found in ticks collected from a wide
81,82,88,89
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very variable, can be substantially higher. variety of bird species, with virus
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Waldenström et al. showed a low prevalence isolation from Turdus pilaris (fieldfare) and
(0.5%) in infected nymphs and larvae feeding Acrocephalus dumetorum (Blyth’s reed
on migratory birds in Sweden, while Kazarina warbler) opening the possibility of virus
82 transfer to new foci during bird dispersal or
et al. detected 14% nymphs and 7% larvae of 88
I. ricinus on migratory birds infected in Latvia. migration. Viremia has been induced
Data for I. persulcatus are more variable. experimentally in birds, reaching levels
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Korenberg and Kovalevskii reported a high theoretically sufficient to infect feeding ticks.
prevalence in unfed adults, ranging from Generally speaking, findings of TBEV in
10.9% to 38.7% over 6 years (mean 26.2%) in animals, whether indirect or direct, do not
unfed adults in the Pre-Ural Region, whereas mean that much eco-epidemiologically. Only
the prevalence in the Primorskii Region of the the demonstration of reservoir competence
Russian Far-East ranged from a little over 1% indicates an active role in the perpetuation of
to over 9% from 1970 to 1990, and in the TBEV.

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