Ticks are obligate non-permanent haematophagous ectoparasites that
feed on the blood of various terrestrial vertebrates, including mammals, birds,
reptiles and occasionally amphibians. However, their main importance resides in
their ability to maintain and transmit a multitude of disease-causing agents of
medical and veterinary importance (Jongejan and Uilenberg, 2004). According to fossil
records, ticks originated 146–65 million years ago (mya) in the mid-Cretaceous
period, with reptiles as possible primeval hosts (Klompen and Grimaldi, 2001;
Poinar and Brown, 2003; Nava et al., 2009). However, other reports have suggested
that ticks occurred on amphibians much earlier (ca. 390 mya) in the Devonian
period (Oliver, 1989; Dobson and Barker, 1999), being the first organisms to
evolve the blood-feeding behaviour (Mans and Neitz, 2004). The earliest written
information on ticks is dated back to the year 850 BCE, while ´tick fever´ was
mentioned for the first time in an Egyptian papyrus scroll approximately 1550
BCE (de la Fuente, 2003). Apparently, ticks are known as a severe pest since
ancient times, but their role as disease vectors was discovered only at the end
of the 19th century after Rhipicephalus
(Boophilus) annulatus was recognized to be involved in the transmission of Babesia bigemina (Smith and Kilbourne,
1893). Being capable to transmit a larger number of infectious organisms than
any other blood-feeding arthropod, ticks are currently regarded as the most
important vectors of pathogens affecting humans and animals (Jongejan and
Uilenberg, 2004, Pfäffle et al., 2013).

Ticks are
closely related to mites and they belong to the class Arachnida, subclass
Acari, order Parasitoformes and suborder Ixodida (Metastigmata). A total of 896
valid tick species that have been described to date are subdivided into three
families: Ixodidae (702 species), Argasidae (193 species) and Nuttalliellidae
(1 species) (Guglielmone et al., 2010). The family Ixodidae is represented by
14 genera (including two fossil representatives) which can further be grouped
into Prostriata (genus Ixodes) and
Metastriata (other 13 genera) lineages, while generic placement for most Argasidae
species is currently uncertain and disputable (Guglielmone et al., 2010;
Estrada-Peña, 2015). Members of the family Ixodidae, commonly known as hard
ticks, are morphologically characterized by a sclerotized dorsal shield
(scutum) and anteriorly located mouthparts (gnathosoma). The scutum covers the
entire dorsal body surface in males, and only about one-third of the dorsum in
unfed females, nymphs and larvae. Conversely, the Argasidae or soft ticks do
not possess the scutum, and the mouthparts are situated anteriorly on the
ventral side of the body (Estrada-Peña, 2015). The third family is monotypic,
comprising only one tick species found in Africa, i.e., Nuttalliella namaqua which shares several features with the
representatives from the other two families (Latif et al., 2012). Ixodid ticks
are involved in the transmission cycles of a substantial number of pathogens of
veterinary and public health relevance, and thus considered far more important
as vectors of diseases compared to argasids (Jongejan and Uilenberg, 2004).
This could be attributed to their prolonged feeding practice that enables pathogen
acquisition and its transmission to a suitable host (Mans and Neitz, 2004). Therefore,
the present thesis and the following paragraphs will only be focused on the biology
and ecology of ixodid ticks.

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The developmental
cycle of ticks is very complex and includes four life stages: egg, larva, nymph,
and adult, of which latter three are parasitic and strictly depend on the blood
of a vertebrate host. Each active stage of ixodid ticks feeds only once and
ingests a large volume of blood over an extended period of time (up to 14
days), which provides them with the energy indispensable for the following
moulting and reproduction processes (Estrada-Peña and de la Fuente, 2014). In
general, immature instars mainly feed on small- and medium-sized animals such
as rodents, birds and insectivores, while adults commonly prefer carnivores and
ungulates. However, the feeding pattern cannot be applied to all species of
ticks, as some of them are opportunistic and feed on miscellaneous groups of
animals (e.g. Ixodes ricinus), and
others are highly host specific and restricted to a particular animal host
species (e.g. Rhipicephalus microplus)
(Jongejan and Uilenberg, 2004). Considering the number of host individuals that
ixodid ticks require to complete the development, they can be classified as
one- (monophasic), two- (diphasic), and three-host (triphasic) ticks. Most tick
species undergo three-host life cycle in which each of the three parasitic
instars feeds on a different host; after feeding the tick drops to the ground,
moults into the following stage and seeks for another host to feed on it.
Larvae and nymphs of two-host ticks feed on the same individual, whereas adults
use a second host for the final feeding. In one-host cycle, all active stages
feed and moult on the same animal. After reaching maturity, adults of
Metastriata mate on-host during female feeding, while copulation in Prostriata
ticks can take place either on-host or off-host in vegetation, before the
female is attached to the animal (Ioffe-Uspensky and Uspensky, 2017). The life
cycle of ixodid ticks is characterized by only one gonotrophic cycle, which
means that once engorged and mated female detaches the host, drops to the
ground and dies after laying thousands of eggs (Estrada-Peña and de la Fuente,
2014). 

Over the evolutionary history, ticks have adapted their biology to
different groups of vertebrates and ecological conditions, resulting in the
cosmopolitan distribution (Black and Kondratieff, 2004). Furthermore, they have
evolved several strategies to come into contact with a suitable vertebrate host
which is a critical point for their survival and perpetuation. The majority of
ticks are exophilic, adapted to open environments (e.g. forests, meadows,
public gardens, semi-deserts) where they passively wait for a host by questing
on vegetation. This is also known as ambush strategy. However, some exophilic
ticks, such as Hyalomma and Ambylomma are true hunters, highly
mobile and able to crawl or even run over short distances to attack and feed on
an available host. The questing activity is regulated by the environmental
conditions, and the temperature seems to be a key factor influencing the
questing behaviour (Estrada-Peña and Venzal, 2007; Tomkins et al., 2014). Most
of their lifetime ixodid ticks spend off-host in open areas (?90%), being
exposed to different environmental factors and consequently more susceptible to
desiccation (McCoy et al., 2013). They survive in the field for such long
periods on account of the energy reserves obtained from a previous blood meal,
and also because of their extraordinary ability to minimize water loss and to
replenish it from the atmosphere by descending to the litter humid zone (Perret
et al., 2004; Estrada-Peña, 2015). Diapause or period of reduced questing and
development activity is another survival strategy regularly used by ticks as a
response to unfavourable temperature conditions (Gray et al., 2016). Another
group of ticks have developed nidicolous behaviour and includes species that
inhabit host-dwelling enclosures (endophilic ticks) such as dens, nests or
caves where they are better protected from the extreme environmental changes
and thus less prone to desiccation. Apart from the climatic changes, the
ecology of ticks is also influenced by host population and individual
host-related factors (Sobrino et al., 2012).

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