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Chapter 3: Transmission / Natural cycle

Vertical TBEV transmission in Outbreaks due to alimentary virus
vertebrates transmission are known from Eastern, Central
and Southern Europe, 122,123 and have to be
considered particularly in cases of local
TBEV transmission from mother to her epidemics. 123–125
offspring in small rodents, e.g., red voles (M.
rutilus), was shown for naturally infected
reservoir hosts as well as after experimental In summary, all these ways of transmission
together ensure the long-term perpetuation
infection with different sublethal doses of the
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virus. TBEV RNA was detected in up to 90% of TBEV in many foci.
of the newborn rodents, 240–280 days after
experimental infection of their parents, by Changes in environmental conditions, e.g.
real-time polymerase chain reaction (RT-PCR), temperature, strengthen the importance of
enzyme-linked immunosorbent assay (ELISA), one possibility or of another. Thus, the
and bioassays. The small amounts of TBEV development of eggs in infected females
RNA detected in the embryos, placenta, and under high temperature conditions leads to an
blood serve as evidence of prenatal trans- increase in the proportion of transovarially
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mission. Postnatal transfer of the virus might infected larvae. An increase in summer
occur through the rodent’s milk. Vertical virus temperature increases the frequency of
transmission may occur before, during, and/or occurrence of engorged vector tick females
after birth of the baby rodents with a high with traces of male bites, with the occurrence
frequency. In natural foci, this could ensure of bites being higher among southern
long-term persistence of TBEV in mammal populations of I. persulcatus (up to 10% in the
hosts without involving any arthropod region of Zhiguli) than among northern ones
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vectors. (less than 2% in the Krasnoyarsk popular-
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tion). Therefore, under changing abiotic
conditions, different homeostatic mechanisms
Reservoir incompetent hosts do not directly
participate in virus transmission, but can play become apparent. Tick behavior can also be
an important factor affecting the effectiveness
an important role in the maintenance of
of TBEV transmission, with the TBEV infection
natural foci. The density of reservoir-
incompetent hosts may have either a positive in the tick itself possibly serving to stimulate
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effect on virus transmission, by amplifying the the questing activity of the tick.
tick population, or a negative (‘dilution’)
effect, as tick bites on a reservoir-incompetent
host cannot lead to virus transmission. 98,120
The natural cycle
Alimentary route of transmission The natural cycle of TBEV is highly complex,
and many details remain obscure. The three
prevailing TBEV subtypes overlap in some
Humans mostly become infected with TBEV areas, they all have multiple mammalian
via tick bites, but viral transmission is also reservoir hosts and various tick vectors, and in
possible via the consumption of unpasteurized some areas these subtypes occur sym-
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goat, cow and sheep milk. Approximately 1%
patrically. Humans are not included in these
of all TBEV infections in humans are probably natural cycles, but may enter those trans-
acquired by consuming infected unpasteurized mission cycles inadvertently.
milk and milk products from infected livestock,
particularly goats. 121 Small mammals and vector ticks play a central
role in the natural cycle of TBEV, but birds and

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