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Chapter 3: Transmission / Natural cycle

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level depends on the rodent species and age, Danielova et al. detected TBEV in 2 out of
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and exhibits individual variability. 647 flagged larvae of I. ricinus, which
indicates transovarial transmission.
Co-feeding transmission
Transstadial persistence
TBEV transmission is also possible from
infected to non-infected ticks during feeding TBEV was not detectable in I. ricinus nymphs
close to each other on a non-viremic host. 98,102 14 days after molting from larvae that had
Cellular infiltration of tick feeding sites, and engorged on viremic A. flavicollis, but TBEV
the migration of cells from such sites, can was present in these ticks 2 months post
provide a vehicle for transmission between co- ecdysis. Many nymphs contained the virus,
feeding ticks that is independent of host indicating that the latter undergoes a
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viremia. The non-viremic route of trans- quiescent phase during metamorphosis. The
mission between co-feeding ticks can even threshold of viremia in the vertebrate host for
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occur in rodents that are immune to TBEV. infection of feeding tick larvae was estimated
The degree of co-feeding virus transmission to correspond to a virus concentration slightly
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may be influenced by local climatic factors below 100 LD 50/0.02 mL.
that affect the seasonal timing of tick host-
seeking activity and, as such, can be used to Sexual transmission in ticks
predict the focal distribution of TBEV. 107,109
Transmission of TBEV from males to
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Transovarial transmission females is successful in only 10% of
copulations in infected I. persulcatus, but it
Another possible way for ticks to transmit may provide notable support for the transfer
TBEV involves transovarial transmission and of the virus to the following generation of ticks
transstadial persistence (see below), which if transovarial transmission follows. A
were described for the first time as early as mathematical model of sexual transmission of
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1940. However, only some eggs in the egg the virus was developed long before
batch of a TBEV-infected vector tick female determining that such a sort of transmission
become infected. 111 In addition, virus can occurs. Virus exchange between a non-
partly be lost during transition from stage to engorged female and an infected male of I.
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stage, and not all tick individuals reach the persulcatus that ‘feeds’ on (i.e., attaches to)
next life history stage irrespective of the the female before or after copulation is quite
presence or absence of the pathogen. probable, and it has been proven that the
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Danielova and Holubova found that only saliva of starved males contains a fairly large
0.23% of larvae coming from infected females amount of virus, sufficient for infecting not
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were TBEV-positive. Other studies showed only animals but also humans. The feeding
that 0.58% to 0.75% of the larvae were transo- of I. persulcatus males on females with which
varially infected. Thus, the rate of transovarial they later copulate can be observed in 2–10%
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transmission remains below 1%. Nuttall et of cases.
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al. suggest that transovarial transmission is
important for the maintenance of a natural
focus even if it occurs at a very low rate.

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