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Chapter 3: Transmission / Natural cycle

large vertebrates, such as wild ungulate families, including migratory species, which
species, wild boar, or foxes, can also be may be important for the distribution of the
important for the spread and maintenance of virus. A common strategy for migratory birds
TBEV. Additionally, changing climatic patterns, is to rest at certain stopover sites along their
as well as changes in ecosystems, may not routes. At these sites, the birds can be
only affect the spatial distribution of TBEV, but infested with ticks or engorged ticks can
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also the maintenance of small natural TBEV detach after engorgement. Sándor et al.
foci. 128,129 Small rodents like A. flavicollis are detected 4 different tick species on 11
important hosts for the larvae of I. ricinus, the different bird species in the Danube Delta,
probably most important TBEV amplifying host including larvae, nymphs, and females of I.
in Central Europe. Apodemus flavicollis ricinus.
develops high virus titers necessary to infect
ticks. Detailed studies by Radda et al., 90,115 Roe deer, red deer, wild boar, and other wild
who trapped small rodents and collected the animal species are also involved in the spread
engorged ticks in a natural TBE focus for 2
of TBEV, although they are regarded as
years, showed that given certain prerequisites
reservoir-incompetent hosts, i.e., they are tick
are fulfilled (high numbers of rodents, vector hosts, but they are not directly involved in the
tick larvae and nymphs feeding on these transmission of the virus to the ticks. Usually
rodents), such a natural TBEV focus is able to adult ticks feed on larger animals and
sustain itself without any significant input of
therefore the role of such animals has been
other hosts. This may explain why many of 128,131,134
intensively discussed. In large
these natural foci are stable, but restricted to domestic and wild animals (e.g., cows, sheep,
small areas, and why they harbor TBEV- goats, swine, deer, and badgers) viremia rarely
positive ticks over a long period of time. exceeds the threshold level, meaning that
Forest structure, especially deforestation and
they are not a source of the virus for feeding
reforestation, are known to have a huge 96,135
ticks.
impact on ticks and vertebrate reservoir hosts
for many tick-borne pathogens. 130,131
This may account for the variability found
Experimental transstadial maintenance of between areas and years with respect to viral
TBEV in D. marginatus and D. reticulatus ticks prevalence in both vertebrate hosts and
emphasizes the role of both species can be vector tick populations, while consistent
vectors and indicates the importance of these differences between vectors. For example the
ticks in certain areas. TBEV infection and generally higher TBEV prevalences in I.
transmission rates in Dermacentor species are persulcatus compared with those in I. ricinus
somewhat lower than in species of the genera may relate to the ecology/biology of the
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Ixodes and Haemaphysalis. Larvae and individual vectors. This complexity is well
nymphs of I. persulcatus may be infected with defined by the various mathematical models
TBEV if the virus titer in the host blood aimed at exploring the dynamics of TBEV
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reaches at least 3.0 log 10 LD 50/0.03 mL. Such ecology. 98,136,137 Hartemink et al. list 19
levels of viremia occur only in small rodents parameters based on field data to define the
and are a critical factor in the virus circulation basic reproduction number (R o) of tick-borne
between vertebrates and ticks in natural foci. infections, while Rosà et al. list 32
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In small rodents, the infection is asymp- parameters in a more comprehensive model.
tomatic. 91 Unfortunately, no single study has been able
In addition, TBEV has been isolated from a to comprehensively measure all the
wide range of birds from many different parameters needed to test these models,
although approximations are available.

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