Page 78 - TBE_Book_V2_2019

Chapter 3: Transmission / Natural cycle

stage, whether it is unfed or engorged. This is, Tick species involved in TBEV
however, different in each species and can
also differ within a tick species in different transmission
geographic areas. As an example, oviposition
Of the 54 species of ixodid ticks known from
can be delayed in D. marginatus. Engorged 47
females that feed in late summer, early fall or the Western Palearctic, 8 species from 3
7
in winter oviposit only in the following spring. genera are known to be able to transmit TBEV,
and the virus has been isolated from at least
14 other species (Table 1). Ixodes ricinus, the
Seasonal activity
most commonly encountered European tick
Ixodes persulcatus inhabits mainly coniferous species, is considered to be the principal
48
49
forests of Asia and Eastern Europe, while I. vector of TBEV there. Lichard and Kozuch
ricinus inhabits deciduous and mixed forests in were able to show TBEV persistence and
the British Isles, in Continental Europe, and transmission to white mice by Ixodes
western Asia. 7,28,40–42 Ixodes persulcatus adult arboricola, which is considered 50 to a
females and eggs are unable to survive the secondary amplifying vector of TBEV. Ixodes
winter, due to the severe climate in the persulcatus is also known to transmit
51,52
inhabited regions. In contrast, eggs as well as TBEV. Adult females of I. persulcatus are
unfed and satiated females of I. ricinus are the primary vectors for any zoonotic
capable of overwintering, a principal differ- pathogens to humans, and not the nymphs or
7
ence between the life-cycles of the 2 tick larvae. Both D. marginatus and D. reticulatus
53–55
species. Thus, the overall period of activity of are also vectors of TBEV.
I. persulcatus adults is usually longer than that
of I. ricinus. 40–42 Vector tick activity is well Haemaphysalis concinna is a known vector of
56,57
TBEV as well. Experimental proof for the
correlated with the seasonal pattern of TBE
occurrence. In eastern TBE foci, people vectorial capacity of Haemaphysalis inermis
58
become infected in spring and the first half of for TBEV is available from Nosek et al. Ixodes
gibbosus is a marginal vector in the
summer, during the highest activity of I. 59
Mediterranean.
persulcatus adults. In such a focus, it is
common for 2–3% of the ticks to be virus- In addition, TBEV has been found in numerous
43
infected. In Northern and Central Europe, other tick species, but transmission has not
the seasonal activity of I. ricinus often has 2 been demonstrated, as for example in I.
peaks, one in spring (May–June) and the other 52,60,61
frontalis.
one at the end of summer (September-
October). The virus has been isolated in the Czech
Republic from female and nymphal I.
Unfed Dermacentor reticulatus adults are hexagonus infesting a hedgehog. TBEV also
62
mostly active in spring and autumn, occasion- has been detected in Haemaphysalis
ally in winter but usually not in summer (June punctata. 63,64
to early August). 44–46
The role of Dermacentor ticks (Table 1) in the

circulation of TBEV in the environment is
unclear and poorly studied. 65,66 D. reticulatus

appears to be spreading and population
density increasing during recent decades. 65-69
In eastern Poland, the mean prevalence of

infection with TBEV found in questing adult D.

73