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Chapter 3: Transmission / Natural cycle

Argasidae with genera (soft ticks), and the well beyond the basis capituli. In Europe, TBEV
Nuttalliellidae, represented by only one has been recovered from Haemaphysalis
species, Nuttalliella namaqua. 6,7,8 All the tick punctata (‘the red sheep tick’), Haemaphysalis
species involved in the eco-epidemiological concinna in Europe and Asia, and
cycle of TBEV belong to the Ixodidae. Details Haemaphysalis longicornis in Asia, Australia,
of tick biology generally can be found in a New Zealand, and recently also in North
variety of publications, for example in America. 7,14,15
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Nicholson et al., Petney et al., and
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Sonenshine and Roe, and a list of valid The genus Hyalomma is relatively small with
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species names in Guglielmone and Nava. The 27 species of small- to large-sized ticks. They
following genera of ticks contain species are characterized by their elongated palps,
known to transmit TBEV. which are at least twice as long as wide. The
distinct eyes are located in sockets adjacent to
Ixodes is the largest tick genus, with 244 the postero-lateral edges of the scutum that is
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described species worldwide . Ixodes species unornamented. The distribution of Hyalomma
are characterized by a distinct groove that species is limited to the Old World, primarily
encircles the anus anteriorly and a lack of to arid or semiarid habitats. Hyalomma
eyes. Males have 7 sclerotized ventral plates marginatum (‘the Mediterranean Hyalomma’)
that are absent in the males of other genera. is the only member of this genus from which
The genus Ixodes has been subdivided in TBEV has been recovered.
roughly 15 subgenera (e.g. Ixodes,
Pholeoixodes) on the basis of morphology. 12,13
The genus has a worldwide distribution,
including parts of Antarctica. 7,14 Some species The biology of hard ticks
are particularly important as vectors of TBEV:
Ixodes ricinus the ‘castor bean tick’ or ‘sheep All the species known to transmit TBEV have a
tick’ in Europe and middle Asia, Ixodes 3-host life-cycle (Figure 1). Each active life
persulcatus ‘the taiga tick’ in northeastern stage requires a blood meal from a suitable
Europe and northern Asia, and Ixodes ovatus host, after which the tick detaches and molts
in middle Asia and Japan. on the ground. The arrows with broken lines
in the figure show the potential transmission
The genus Dermacentor has 35 species paths to humans. The line from larvae to
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worldwide. The basis capituli appears humans indicates that transovarial trans-
rectangular when viewed dorsally. A pair of mission from an infected female can happen
medially directed spurs occurs on the first pair which results in infective larvae. Infection of
of coxae. The palps are short and thick. The the tick can occur when larvae, nymphs, or
scutum is almost always ornamented. females feed on an infective host (see below).
Dermacentor species are found mostly in
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Europe, Asia, and North America. In Europe, The larva, nymph, and adult (female or male –
TBEV has been recovered from 2 species, Figures 2a, 2b, 2c, and 2d) are active stages
Dermacentor reticulatus (‘the ornate dog that require a host (this is not the case for
tick’), Dermacentor marginatus (‘the ornate males of the genus Ixodes, which can mate
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sheep tick’), and in Asia, Dermacentor nuttalli. off-host without feeding). Larvae are easily
Haemaphysalis is the second largest tick recognizable by the presence of only 3 pairs of
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genus. This eyeless genus can, in most cases, legs, and absent spiracular and genital
be identified by a pronounced lateral apertures (Figures 3a and 3b). Nymphs have 4
projection of palpal segment 2, which extends pairs of legs and spiracles (Figures 4a and 4b).

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