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Chapter 8: TBE in animals

longevity of a natural focus. As outlined Other mammals and birds
earlier, in the chapter on transmission and
natural cycle, infection of a tick can occur via a As most animals do not develop overt disease
viremic host, but another phenomenon has upon infection with TBEV, many mammal
been described which also applies to the species have never been investigated as to
infection of ticks while feeding on small whether or not they are susceptible to an
mammals. The so-called co-feeding allows the infection or capable of developing an immune
infection of Ixodes larvae when an infected response in terms of measurable antibody
Ixodes nymph feeds in close proximity. In this titers. According to the broad geographic
case, the rodent does not have to be infected, distribution of TBE covering most of Europe
because the virus finds its way from the and northern Asia, we consider that there may
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nymph directly to the larva. So, it is safe to be many mammal species not yet investigated
say that, in many ways, rodents are as that react to an infection in a similar manner
necessary as Ixodes ticks for maintaining the as described above for wild boars or roe deer,
TBEV life-cycle. i.e., seroconversion without clinical disease.
One exception is the Barbary macaque
Recent publications have reviewed the (Macaca sylvanus), a monkey species not
prevalence of either viral ribonucleic acid native to Eurasia, despite a small population in
(RNA) or specific antibodies against TBEV in Gibraltar, the southern-most tip of Spain. An
rodents in various countries. 47,73,74 The anti-
individual of a small group of these animals
body prevalence in endemic areas was found kept in southwest Germany in an outdoor area
to range between 0% and 5.9%. However, fell severely ill with central nervous symptoms
seroprevalence rates up to 12.5% were found and was euthanized for ethical reasons. A pan-
in some rodent species (e.g., the bank vole, encephalitis was diagnosed and TBEV was
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Myodes glareolus), suggesting a differing
demonstrated by immunohistochemistry, real-
role of particular rodent species in a TBE time RT-PCR, and virus isolation. 81,82 Other
focus. Viral RNA can also be found in wild individuals of this monkey group sero-
rodents, with an even higher prevalence of up converted without showing clinical signs.
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to 15%. Studies from Hungary identified Thus far, we are not aware of further case
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TBEV-RNA in 4.2% and TBEV-specific anti- reports of non-native species kept in semi-free
bodies in 5.19% and 4.93% of the tested small holdings or zoos.
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rodents. Experimentally infected common
voles (Microtus arvalis) harbored infectious Birds are known to be readily infested with
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TBEV for at least 3 months. Viral RNA could ixodid ticks and are prime suspects for long-
be found in the brain tissue of experimentally distance transportation of ticks. The first
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infected bank voles for up to 168 days. This studies investigating the prevalence of TBEV-
has important implications, as the brain (and harboring ticks on birds came from the
to a lesser extent other organs such as kidney Ottenby Bird Observatory at the southern tip
and spleen) seems to be the prime site of virus of the island Öland in Sweden. During the
persistence in rodents. Indeed, TBE viral RNA annual ringing, more than 1000 Ixodes spp.
was found in the brain tissue of naturally ticks were collected from birds, with 0.52%
infected field voles (Microtus agrestis) and showing TBEV RNA. Subsequent studies from
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bank voles in Finland, after the winter but Estonia (0.4% positive nymphs ), Switzerland
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before the tick season started. Thus rodents (0.27% TBE viral RNA positive ), Latvia (14%
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seem, along with transstadially-infected ticks, 88 ), Germany (no TBE virus found in almost
to play a role in the ‘overwintering’ of the 2500 Ixodes ricinus ticks collected from
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TBEV. birds ) and Slovakia (a brain sample in a
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