Some tick species are specialists – they are host-specific and only feed on defined hosts; like the cattle tick Boophilus microplus, which is a one host tick, with all development stages feeding and developing on the same host
However, many ticks are generalists and feed opportunistic. For example, Amblyomma americanum feeds on mammals, birds and reptiles (Sonenshine, 1991) and similarly, Ixodes ricinus is known to have a wide host-spectrum. Also, the most relevant ticks for companion animals are three-host-ticks, with each development stage finding a new host. This wide host spectrum forms a reservoir for tick-borne diseases, including the risk to transmit new pathogens, that has not been reported before to a host.
A contact between ticks and hosts is regulated by at least 5 conditions. They consist of seasonal and daily effects (abiotic parameters) like day length, temperature, humidity. The rest includes biotic parameters as biological processes within the tick, the biological processes within the host and interactions between the two.
Host-seeking ticks recognize a variety of stimuli from prospective hosts which in turn excites their host-finding behavior.
Odors are undoubtedly the most important and best studied stimuli. Host-originated odors provide specific and, when carried on wind currents, also directional information. Among the most important host-originated odorants are carbon dioxide, a component of animal breath and ammonia, common in urine and other animal wastes. They bring hungry ticks into close proximity to potential hosts, whereupon other, shorter range stimuli become effective.
Close range stimuli include radiant heat, such as body heat from the host, odorants characteristic of sweat and other body odors (e.g., lactic acid or butyric acid) and contact. Radiant heat is synergistic when combined with odors, as Lees (1948) showed in his experiments with adult Ixodes ricinus which responded more strongly to a cloth-wrapped tube containing circulating water at 37°C and emitting sheep body odors than to either the cloth or the warm tube alone.
Some species respond to sounds within a particular range of frequencies. Boophilus microplus larvae are highly responsive to sounds in the 80-800 Hz range, frequencies commonly emitted by feeding cattle, while Rhipicephalus sanguineus are attracted to the sounds made by barking dogs (Waladde and Rice, 1982).
Other stimuli in host-finding activities, like visual cues and vibrations have been little studied. Visual images are probably more important in hunter ticks, which are believed to discriminate dark shapes against the bright background of the sky. However also questing ticks of many species will respond to distinct shadows. Vibrations are also excitatory; rustling the grassy or weedy stems on which ticks are perched in ambush will provoke the characteristic "questing" behavior, with the forelegs outstretched to cling to a passing host.
Finally, tactile stimuli come into play only upon host contact, contributing, along with short-range odorants and body heat, to the selection of the feeding site and the commencement of blood-sucking activity.
In some instances, tick-originated rather than host-originated stimuli are of critical importance in tick host-seeking behavior. Thus, Amblyomma variegatum and Amblyomma hebraeum are excited by CO2 from cattle but select tick-infested animals when they detect the aggregation-attachment pheromone emitted by previously attached, feeding ticks (Norval et al., 1989).
Host finding strategies
There are two different host finding strategies in ticks, an endophilic (nidicolous) – from nidis, Latin for nest - strategy, where ticks live in burrows, nests, hollows or cracks near resting or breeding places of hosts, and an exophilic (non-nidicolous) strategy in which ticks must search for their hosts. Due to this diverging host seeking behavior, endophilic and exophilic ticks differ in their exposure to environmental conditions. While endophilic ticks remain better protected from environmental changes and hosts are easily accessible, exophilic ticks are highly exposed to environmental conditions and the accessibility to the host is also dependent on host ecology and population dynamics (Ruiz-Fons and Gilbert, 2010).
Appetence initiates the series of behavioral responses that leads to host contact and successful parasitism. Appetence is the "locomotory hunting for a host or seeking one from a vantage point" (Waladde and Rice, 1982). Appetence is preceded by hunger, which in turn is influenced by the tick's physiological condition; appetence does not occur in diapausing ticks.
Non-nidicolous (exophilic) ticks
Non-nidicolous (or exophilic) ticks are species that occupy open, exposed habitats. Most occur in forest, savannah, scrub, brush, or meadow vegetation; others remain buried in sand or sandy soils, under stones, crevices, and elsewhere in the open environment.
Most non-nidicolous ticks use a passive host-finding strategy, i.e., by direct contact with a passing host (= ambush strategy). Such questing ticks living in grass, herb, or brush covered habitats climb the vegetation, clinging to the tips of stems or branches waiting in a typical pose - with the front legs extended, especially in response to a host passing by - for direct contact with hosts that brush against these vegetative supports. Vibrations caused by animal movements as well as odors, body heat, and shadows from such hosts excite tick responses, causing extension and rapid waving of the forelegs. If contact is made, questing ticks cling to the bodies of animals as they brush past.
The height at which ticks quest also plays an important role in the types of hosts they acquire. Generally, tick questing height is strongly correlated with the specific life stage and size of the most common hosts of each species or life stage (Loye and Lane, 1988; Fourie et al., 1991; Goddard, 1992); immature ticks tend to occur near the base of vegetation or leaf layer, where small mammals and birds are active, while adults generally quest near the tips of vegetation where they encounter larger animals.
Questing hard tick (Ixodes ricinus)
Some ticks exhibit an active host-finding strategy, these so-called hunting ticks; crawl or run towards their hosts. They emerge to attack hosts when these animals appear nearby and may follow them across distances of many meters to attack and feed.
Host-finding strategies may also differ in different life stages, e.g., larvae of A. variegatum and A. hebraeum find hosts by questing while nymphs and adults are hunter ticks. The Lone star tick, Amblyomma americanum, exhibit both the ambush and hunter type of host-finding, most encounter their hosts by direct contact (ambush); others crawl considerable distances when attracted by host odors and body temperature.
Nidicolous (endophilic) ticks
In general, nidicolous ticks respond to the same spectrum of host-originated stimuli as non-nidicolous ticks, e.g., CO2 body heat, and various odors. However, the range at which these stimuli are perceived is considerably shorter than in non-nidicolous ticks. Typically, these stimuli are most effective when presented simultaneously, i.e., ticks respond more strongly to synergized stimuli than to individual stimuli. In addition, gradients are important and, in some cases, provide essential directional information without which the ticks either fail to respond, or are misdirected and fail to find hosts.
Gravity is important in host-seeking behavior of several species. As is the case with many other argasid species, intense radiation, especially in the range corresponding to daylight conditions (450-580 nm), depresses tick activity, inducing the ticks to remain in tree roosts and various crevices of their natural habitat.
The relative importance of different stimuli may vary greatly among nidicolous species. In some, host body heat and odors are likely to be of paramount importance, since the distance from parasite to host is extremely short. For others, such as harborage-infesting parasites that must migrate over considerable distances (e.g., meters) to reach their hosts, gravity, CO2, and even sound (A. cooleyi) serve as general excitants, bringing the searching ticks to a point where shorter range stimuli, such as host body odors and radiant heat, can lead the parasites to the host body.
Most of the hard ticks (Ixodidae) are non-nidicolous (exophilic) ticks, at least in some stage of their life cycle, but some also follow endophilic strategies (e.g. Ixodidae of the prostriate genus Ixodes) Sometimes all instars follow the same strategy, and sometimes strategies differ between instars, e.g., some species in the genera Hyalomma, Rhipicephalus, and Dermacentor quest as larvae and adults in the vegetation looking for hosts while nymphs remain on the host (two-host tick).
Soft ticks (Argasidae) are mostly nidicolous parasites with an endophilic strategy. Some hosts by questing on low-lying vegetation, but the vast majority are nest parasites, residing in sheltered environments such as burrows, caves, or nests.
The mouthparts of hard ticks are readily visible from above; while the mouthparts of soft ticks are not. Both families show three visible components: the two outside jointed parts are the highly mobile palps; between these are paired chelicerae, which protect the center rod-shaped structure, the hypostome.
The palps move laterally while the tick is feeding and do not enter the skin of the host. The rough hypostome has many beak-like projections on it. This is the structure which plunges into the host's skin while feeding. The backward-directed projections prevent easy removal of the attached tick.
In addition, most hard ticks secrete a cement-like substance produced by the salivary glands which literally glues the feeding tick in place; the substance dissolves after feeding is complete. Depth of attachment and the amount of cement secreted vary between species.
Duration of Feeding
Hard ticks feed for extended periods of time – from several days to weeks – on their hosts, specifics depending on tick species, life stage and host type.
The outside surface, or cuticle, of hard ticks actually grows to accommodate the large volume of blood ingested, which, in adult ticks, may be anywhere from 200-600 times their unfed body weight.
Fully engorged hard tick
Soft ticks feed for short periods of time on their hosts, varying from several minutes to days, specifics also here depending on tick species, life stage and host type. The feeding behavior of many soft ticks can be compared to that of fleas or bedbugs, as once established, they reside in the nest of the host, feeding rapidly when the host returns. Soft ticks feed several times during each life stage, and females lay multiple small batches of eggs between blood meals during their lives.
During their blood meal soft ticks can increase their body weight 3-5 times because their highly folded integument allows extensive stretching without additional growth, to accommodate the volume of blood ingested, which may be anywhere from 5-10 times their unfed body weight.
Tick saliva is a complex mixture serving a variety of functions. Soon after attachment, ixodid ticks (with the exception of a few Ixodes species) secrete a milky white material that hardens into a latex-like cone surrounding the hypostome. This is the initial core of the cement cone.
Additional secretions over the next 48-72 hours add cortical layers to the cement; in some species this added cement secretion flows over the skin of the host to further strengthen the attachment of the parasite.
The chemical composition of the cement consists of a mixture of antigenic and non-antigenic proteins, with substantial lipid and carbohydrate in the innermost layers, the latter compounds mostly in the form of lipo- and glycoproteins. Following establishment of the cement cone, the salivary glands of the feeding tick expand and protein synthesis accelerates.
The feeding period is accompanied by copious secretion of salivary fluids. This pattern of salivary gland activity parallels the sequence of attachment, wound site formation, feeding, mating and repletion that characterizes the parasitic period.
In addition to the cement precursors described previously, histochemical tests have demonstrated a variety of enzymes in the secretory cells of the salivary glands.
Whether some or all of these enzymes are secreted and contribute to the formation of the feeding site or fluid uptake is unknown.
Tick salivary glands also secrete a veritable cornucopia of pharmacologically active substances, including anticoagulants, prostaglandin E2 (PGE2) and prostacyclin, vasodilators, apyrase, anti-inflammatory agents, anti-histamines (in some species) and others. In some species, enzymes that destroy bradykinins and anaphylatoxins, -host proteins that play crucial roles in modulating the inflammatory response - are secreted. In certain very successful host/parasite associations, e.g., the deer tick Ixodes scapularis, and the white footed mouse Peromyscus leucopus, unknown salivary agents suppress components of the host immune system, e.g., T.-cells, thereby minimizing rejection.
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