Ticks animals worldwide, able to transmit the largest number

Ticks

            Ticks are temporary obligate haematophagous ectoparasites
that feed on 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 have originated 146–65 million years ago (mya) in the
mid-Cretaceous period, with reptiles as possible primeval hosts (Klompen and
Grimaldi, 2001; Grimaldi et al., 2002; Poinar and Brown, 2003; Nava et al.,
2009). However, other reports proposed 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 blood-feeding behaviour (Mans and
Neitz, 2004). The earliest written information on ticks is dated back to the
year 850 BCE, whereas ´tick fever´ was mentioned for the first time in an
Egyptian papyrus scroll approximately 1550 BCE (de la Fuente, 2003).
Apparently, ticks have been known as a serious 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). Currently, ticks are among the most important vectors of pathogens
affecting humans and animals worldwide, able to transmit the largest number of
infectious organisms than any other blood-feeding arthropod (Jongejan and
Uilenberg, 2004, Pfäffle et al., 2013).

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2.1.1. Classification,
biology and ecology

            Ticks are
closely related to mites and belong to the class Arachnida, subclass Acari, order
Parasitoformes and suborder Ixodida (Metastigmata). In total, 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 comprises 14 genera (including
two fossil representatives) which can further be grouped into Prostriata (genus
Ixodes) and Metastriata (other 13 genera)
lineages, whilst generic placement for most species of the Argasidae 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. Contrary, the Argasidae or soft ticks have leathery and
folded cuticle, the scutum is absent 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, and it shares some
features with the ticks from the other two families (Latif et al., 2012).
Ixodid ticks are considered far more important as vectors of diseases compared
to argasids, being involved in the transmission cycles of many pathogens of
veterinary and public health relevance (Jongejan and Uilenberg, 2004). This
could be attributed to their prolonged feeding habits that enable both pathogen
acquisition and its transmission to a suitable host (Mans and Neitz, 2004).
Therefore, the current thesis and the following paragraphs will only be focused
on biology and ecology of ixodid ticks.

            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 blood of
their vertebrate host. Each active stage of ixodid ticks feed only once and
ingests a large volume of blood over a prolonged period of time (up to 14
days), which provides them with energy required for subsequent moulting and
reproduction (Estrada-Peña and de la Fuente, 2014). In general, immature
instars mainly feed on small- and medium-sized animals such as rodents, birds
and lagomorphs, while adults commonly prefer large animals like carnivores and
ungulates. However, this general rule cannot be applied to all species of
ticks, as some of them are opportunistic feeding on different groups of
animals, and others are highly host specific and restricted to a certain animal
species (Jongejan and Uilenberg, 2004). Based on the number of host individuals
that ixodid ticks require to complete their development, they can be classified
as one- (monophasic), two- (diphasic), and three-host (triphasic) ticks. Most
tick species undergo three-host life cycle pattern in which each of the three
parasitic instars feed 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.
Two-host ticks moult from larva to nymph on the same animal host, once engorged
nymph detaches from the host, and then moults into the adult stage that use a
second host for the final feeding. In one-host cycle, all active stages feed
and moult on the same animal. After the ticks reach maturity, adults of
metastriate ticks mate on-host during female feeding, while in prostriate ticks
copulation can take place either on-host or off-host in vegetation, even before
the female is attached to the vertebrate (Ioffe-Uspensky and Uspensky, 2017). Life
cycle of ixodid ticks is characterized by having only one gonotrophic cycle,
which means that engorged and mated female detaches the host, drops to the
ground and dies after laying thousands of eggs. 

            Over the evolutionary history, ticks have adapted their
biology to a wide variety of ecological conditions and different biotopes
resulting in their cosmopolitan distribution (Black and Kondratieff, 2004).
Moreover, ixodid ticks have evolved two different strategies to come into
contact with a suitable animal host and to enhance 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 (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 temperature is considered as 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 conditions and consequently more susceptible
to desiccation (McCoy et al., 2013). Ticks survive in the field for such long
periods mainly on account of their energy reserves from a previous blood meal,
and an extraordinary ability to minimize water loss and to replenish it from
the atmosphere by descending to the litter 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). Second group of ticks have
developed nidicolous behaviour and includes the ticks that inhabit host-dwelling
enclosures (endophilic ticks), such as burrows, nests or caves where they are
exposed to higher relative humidity, and thus are less prone to desiccation and
better protected from the extreme environmental changes. Apart from the
climatic changes, ecology of ticks may also be influenced by host population
and certain individual host-related factors (Sobrino et al., 2012).

2.1.1. Ticks as disease
vectors

Very complex biology and ecology
of ticks, along with their genetic and physiological determinants developed
during the course of evolution, make these macroparasites one of the most
efficient vectors of multiple pathogens (Hoogstraal, 1985; Mans and Neitz,
2004). The exploitation of different vertebrate hosts by ticks in each active
stage over the extended periods of feeding enhances their vector capacity or
potential to transmit pathogens, influenced by the tick behavioural and abiotic
environmental factors (Randolph, 2004; de la Fuente et al., 2017). During the
feeding, ticks are exposed to different microorganisms present in the blood of
vertebrate hosts, becoming potentially incorporated in the epidemiological
cycle of the pathogen transmission (Estrada-Peña and de la Fuente, 2014).
Therefore, the tick microbial community can comprise viruses, bacteria,
parasites and fungi which display little or no impact on the tick itself (Beerntsen
et al., 2000; de la Fuente et al., 2017). Some of these agents serve as
symbionts or commensals, while others are disease-causing pathogens (Baneth,
2014; Vayssier-Taussat et al., 2015). Furthermore, tick microbiome can
influence the pathogen acquisition, transmission, survival and even virulence.
For instance, the spirochetes of the Borrelia
burgdorferi sensu lato complex initiate the synthesis of an outer surface
protein C (OspC) in the midguts of an infected tick, which in combination with
the tick salivary protein 15 (Salp15) facilitates the bacterial survival,
transmission and host infection (Ramamoorthi et al., 2005; Dai et al., 2009),
protecting them from the host’s immune response (Garg et al., 2006). On the
other hand, some pathogens have the potential to enhance tick fitness ensuring
its vector capacity (de la Fuente et al., 2016), such as gram-negative
bacterium Anaplasma phagocytophilum
that by inducing the expression of an antifreeze glycoprotein (AFGP) in Ixodes scapularis, increases the tick
cold tolerance and survival (Neelakanta et al., 2016).

During the
prolonged feeding activity of ixodid ticks, they inject a plethora of
pharmacologically active molecules which suppress the immune response of the
exploited vertebrate host which ensures the tick success in the pathogen
transmission (McCoy et al., 2013). However, a pathogen must survive
transstadial (stage-to-stage) and transovarial (female-to-egg) transmission
within a tick to use it as a vector or even as a reservoir (Pfäffle et al.,
2013; Estrada-Peña and de la Fuente, 2014). In addition, ticks saliva also
contains toxic substances that may cause severe skin irritation, deadly
paralysis and toxicosis in infested vertebrate hosts (Jongejan and Uilenberg,
2004). More recent studies have shown that certain tick species may be involved
in a severe allergic reaction to galactose-alpha-1,3-galactose (alpha-gal),
commonly known as red meat allergy in human patients (Commins and Platts-Mills,
2013; Hamsten et al., 2013). 

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