Page 157 - TBE_Book_V2_2019

Chapter 8: TBE in animals

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autochthonous TBE, 2,59,60 this study demon- RNA. It would be interesting and necessary
strates the power of using animal surveillance to perform a seroprevalence study in a known
data for pinpointing TBE-endemic areas. Like endemic area to shed light on the role of the
the roe deer described above, wild boars are fox in the natural transmission cycle of TBEV
rather territorial, allowing the geographical and to prove the putative positive correlation
allocation of such data. Only the renegade between fox abundance and TBE inci-
wild sows are known to travel across large dence. 66,68
areas when they are searching for a new herd.
A study from the Czech Republic, traditionally Studies trying to detect a correlation between
a country with a high TBE incidence, found a human TBE incidence and abundance of
positive association between the number of certain animals are contradictory. A Swedish
hunted wild boar and human cases. Conse- study revealed that, with one year of time-lag,
quently, the authors concluded that wild boar the abundance of roe deer, red deer,
must play a role in TBEV transmission either mountain hare (Lepus timidus) and European
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directly or indirectly. hare (Lepus europaeus) showed positive
covariance with the incidence of human TBE.
In Finland, moose (Alces alces) and white- In contrast, moose and fallow deer (Dama
tailed deer (Odocoileus virginianus) were dama) showed negative covariance and wild
found to harbor TBEV-specific antibodies boar, lynx (Lynx lynx) and red fox showed no
(0.74%) and the use of such seroprevalence significant covariance with human TBE
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data as an indicator for local risk of human incidence. In Slovenia, red deer abundance
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TBE infection is recommended. One single was correlated with human TBE incidence
case report describes the pathological and when including a three-year time-lag, whereas
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immunohistological findings in a mouflon (Ovis roe deer showed no significant correlation.
ammon musimon) with marked encephalitis An Italian study found roe deer density to
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due to TBEV. A polish study analyzed D. have a better predictive value for a model
reticulatus collected from the lowland explaining the increasing human TBE incidence
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European bison (Bison bonasus bonasus) in a than red deer density.
known endemic focus and found 18.42% of
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these ticks to be positive for TBEV RNA.
Interestingly, not much is known about the Small mammals
role of foxes (Vulpes vulpes) in natural TBE
foci. Although it is a highly prevalent predator
Small mammals have an essential role in the
of small mammals (see below), and is regularly maintenance of TBE foci in 2 ways. Firstly,
infested with Ixodes ticks, there are no recent rodents and, to a lesser extent, shrews are the
studies investigating virus or antibodies
main hosts for Ixodes larvae. Without this first
against TBEV in foxes. Older studies from
blood meal, a tick population would die out
Germany were mostly performed in non- over time. They are also host for nymphs
endemic areas on the German-Dutch border when they take their blood meal, which is
and Brandenburg, and consequently revealed needed before they can molt into adult ticks.
no seroprevalence or a single sero-reactive
65,66 Secondly, they are reservoir hosts for TBEV
serum sample only. However, the latter
and thus responsible for re-infections of ticks
report found every third fox in South-Western via transovarial transmission, i.e., the transfer
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Germany to have antibodies against TBEV. In of TBEV from a female tick to her eggs,
Croatia, a study found at least 1.6% of ticks on although this is negligible for the epidemi-
red foxes and 1.1% of spleen samples of red
ology of the virus. The reservoir function,
deer (Cervus elaphus) to be positive for TBEV-
however, has large implications for the

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