Page 76 - TBE_Book_V2_2019

Chapter 3: Transmission / Natural cycle

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Questing height is also important. Ticks to their humid microenvironments each day.
questing on or near the ground are exposed Dense ecotonal vegetation provides shade,
mostly to small animals, while those questing increased moisture, protection from intense
higher in the vegetation are more likely to solar radiation, and plants that support the
encounter larger animals. The extent to which tick’s hosts.
different hosts are utilized depends on host
behavior and opportunities for contact, such There have been various studies showing the
as foraging range, time of day and time spent relationship between I. ricinus and vegetation
33,34
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foraging, habitats visited, and other factors. type in central Europe and the capacity of
this species to adapt to a large variety of
Acceptance of a vertebrate animal is also biotopes with low temperature (e.g. Sweden)
dependent on physiological factors and the and high altitudes, up to 1500 m. 35–37
ability of the ticks to recognize it as a host.
Host utilization may be influenced by the Normally, temperature and relative humidity
ability of ticks to evade or suppress host in a burrow, cave, or similar type of shelter are
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homeostatic systems and avoid rejection. more uniform throughout the year than in the
external macro- environment. The higher
Hard tick ecology, environmental relative humidity in such microenvironments
factors is due in part to the presence of hosts, their
wastes, and the plant materials they use to
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Ticks occur in many terrestrial habitats ranging construct or line their nests. Nidicolous ticks
from cool, arboreal northern forests to hot, exhibit behavioral patterns that restrict their
arid deserts. Each species, however, has distribution to these sheltered locations. They
become adapted to the specific types of avoid bright sunlight and low humidity, the
habitat where it is generally found in highest type of conditions prevailing at the entrances
abundance. All I. ricinus stages are exophilic of burrows or caves. Confined within these
and depend entirely on a suitable combination hidden, restricted locations, nidicolous ticks
of climatic variables, making them vulnerable become active when hosts are present.
to climate changes and especially to desic- However, when the hosts are absent, they
cation. Thus, they are found in mainly cool, may wait for up to several years for hosts to
moist forests. 7,21,29,30 return, or until they die of starvation.

Water balance is a critical determinant of a Diapause
tick’s ability to wait for hosts. Ticks may quest
for weeks or even months while waiting for a An important physiological trait that enables
host. When they have a body water deficit, ticks to survive adverse environmental
they retreat to more sheltered, humid micro- conditions and conserve energy until condi-
environments, such as the rotting vegetation tions improve is diapause as a form of
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in a meadow or damp leaf litter on the forest dormancy. Diapause is induced by an
floor. They secrete a hygroscopic salivary external cue before adverse conditions occur.
secretion onto their external mouthparts that It is not terminated by favorable external
collects atmospheric water at relative conditions – as it is the case with quiescence –
humidities ≥80-85% (active water vapor but there is some diapause development
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sorption). Rehydrated ticks are able to before its termination. During diapause ticks
resume host-seeking. Some ticks are able to become inactive, reduce their metabolic rates,
remain in the questing position for many days and do not feed on hosts even when given the
7,21
without rehydration, while others must return opportunity. Diapause can occur in each life

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