Are ticks venomous animals?
© Cabezas-Cruz and Valdés; licensee BioMed Central Ltd. 2014
Received: 7 May 2014
Accepted: 20 June 2014
Published: 1 July 2014
As an ecological adaptation venoms have evolved independently in several species of Metazoa. As haematophagous arthropods ticks are mainly considered as ectoparasites due to directly feeding on the skin of animal hosts. Ticks are of major importance since they serve as vectors for several diseases affecting humans and livestock animals. Ticks are rarely considered as venomous animals despite that tick saliva contains several protein families present in venomous taxa and that many Ixodida genera can induce paralysis and other types of toxicoses. Tick saliva was previously proposed as a special kind of venom since tick venom is used for blood feeding that counteracts host defense mechanisms. As a result, the present study provides evidence to reconsider the venomous properties of tick saliva.
Based on our extensive literature mining and in silico research, we demonstrate that ticks share several similarities with other venomous taxa. Many tick salivary protein families and their previously described functions are homologous to proteins found in scorpion, spider, snake, platypus and bee venoms. This infers that there is a structural and functional convergence between several molecular components in tick saliva and the venoms from other recognized venomous taxa. We also highlight the fact that the immune response against tick saliva and venoms (from recognized venomous taxa) are both dominated by an allergic immunity background. Furthermore, by comparing the major molecular components of human saliva, as an example of a non-venomous animal, with that of ticks we find evidence that ticks resemble more venomous than non-venomous animals. Finally, we introduce our considerations regarding the evolution of venoms in Arachnida.
Taking into account the composition of tick saliva, the venomous functions that ticks have while interacting with their hosts, and the distinguishable differences between human (non-venomous) and tick salivary proteins, we consider that ticks should be referred to as venomous ectoparasites.
KeywordsTicks Venom Secreted proteins Toxicoses Pathogens Convergence
As haematophagous (blood sucking) arthropods, ticks are mainly considered as ectoparasites that use their salivary constituents to successfully obtain a blood meal by targeting major physiological pathways involved in host defense mechanisms . Ticks constitute an important pest affecting agricultural development, as well as domestic animal and human health since they transmit a variety of infectious agents. Tick saliva has been described as a complex mixture of pharmacologically active compounds with implications for pathogen transmission . From a functional and evolutionary point of view, Fry and colleagues , considered the feeding secretions of some haematophagous invertebrates (such as ticks) as a specialized subtype of venom. Certainly, Ixodida, that includes hard and soft tick species, is proven to be a venomous taxonomic Order in Chelicerata . In fact, the bite from a single tick can produce several types of toxicoses ; paralysis being the most common and recognized form of tick-induced toxicoses [3, 5].
Tick paralysis is an ascending motor paralysis produced by an impairment of neurotransmission, possibly due to the blockade of ion channels involved in the depolarization of nervous tissue . This form of polyneuropathy is mainly associated with the acquisition of a blood meal by female ticks and will spread to the upper limbs of the host, causing incoordination and, in some cases, ending with respiratory failure and death . Nevertheless, evident signs of toxicoses (e.g., paralysis) are not a sine qua non effect from the tick bite as in the case of other venomous taxa, such as snakes, spiders, scorpions or pseudoscorpions. This observational scarcity is perhaps the reason ticks are not considered venomous animals. Thus, tick saliva as venom has rarely been mentioned in parasitological literature, with the exception of a few examples (e.g., as in ).
Traditionally, venom was defined as a toxic fluid that inflicts an abrupt death or paralysis in the host and/or prey. This archaic concept, however, partially highlights the deleterious effects of venom on the host/prey and lacks ecological relevance. After investigating many venomous animals, Fry and colleagues [2, 8] extended this limited definition of venom “as secretions produced in specialized glands and delivered through a wound (regardless of the wound size), that interferes with normal physiological processes to facilitate feeding or defense by the animal that produces the venom”. By interfering with normal host physiological processes infers that all toxins are venomous, but not all venomous proteins are toxic. This new paradigm allows us to consider a wider spectrum of envenomation produced by a myriad of macromolecules. In our study we hypothesize that due to their salivary composition ticks are venomous animals within the phylum Chelicerata. We base our hypothesis on the following points: (i) the various toxic effects induced by ticks (ii) the convergent protein families present in spiders, scorpions and ticks; (iii) the immunomodulatory properties found in ticks saliva is also found in other venomous taxa (iv) the pattern of immune response against toxins by the host/prey is similar in both ticks and other venomous taxa; (v) the structural similarities in members of major protein families between known venomous taxa and ticks; (vi) the bimodal structural dichotomy between human (non-venomous) and tick saliva; and, finally (vii), the phylogenetic position of parasitiformes (Ixodida, Holothyrida and Mesostigmata) as a sister clade of pseudoscorpiones based on .
Results and discussion
Toxicoses phenomena within ixodida
Additionally, several lethal and paralysis inducing toxins have been identified in ticks. For example, the 15.4 kDa acidic salivary toxin secreted by Ornithodoros savignyi is highly abundant and its purified form kills a mouse within 90 minutes at a concentration of 400 μg/10 g of mouse weight . Another purified basic toxin from the same tick species was shown to kill a 20 g mouse within 30 minutes after administration of 34 μg of the toxin . Verified via Western blot, a 20 kDa trimeric neurotoxin was identified in the salivary glands of Rhipicephalus evertsi evertsi that paralyzed muscle contractions in an in vitro assay [22, 23]. Maritz and colleagues (2001) identified a 60 kDa toxin in Argas walkerae that reduces [3H]glycine release from crude rat brain synaptosomes, indicating a paralytic effect. Other toxins have also been identified in tick egg extract from Amblyomma hebraeum, R. e. evertsi, R. microplus, R. decoloratus and Hyalomma truncatum, (revised in ). The presence of these toxins in tick eggs may be related to the protection of the egg mass against predation in natural environments – adding a new function for venoms in ticks, i.e., defense.
Besides tick paralysis, other types of toxicoses can be induced by a particular tick species, including sand tampan toxicoses by O. savignyi, sweating sickness, Mhlosinga, Magudu, and necrotic stomatitis nephrosis syndrome by H. truncatum, spring lamp paralysis in South Africa by R. e. evertsi, and, finally, specific toxicoses induced by R. microplus, D. marginatus, R. appendiculatus, I. rekicorzevi and O. gurneyi (revised in ). Toxicoses by R. microplus, H. truncatum and R. appendiculatus induce an anorexigenic effect , as induced by the secreted toxin Bv8 from the skin of the fire-bellied toad, Bombina variegata. Symptoms of general toxicoses were also reported after soft tick bites that include pain, blisters, local irritation, oedema, fever, pruritus, inflammation and systemic disturbances . Recently, human and canine toxicoses induced by the argasid tick O. brasiliensis, known as “mouro” tick, were reported and the most frequent symptoms of toxicoses induced by this tick species were local pruritus, slow healing lesions, local edema and erythema, and local skin rash . Different types of immune reactions can also be included in the general scope of tick toxicoses [3, 27]. Immediate and delayed skin hypersensitivity was reported in cattle exposed to R. microplus and R. decoloratus antigens [28, 29], and in dogs exposed to A. cajennense antigens .
There are important factors in considering the severity of tick-induced toxicoses. (i) As stated by Paracelsus, the dose makes the poison. For example, I. rubicundus induces Karoo paralysis in South African livestock only when critical infestation densities are reached during repletion . (ii) The anatomical location where the tick saliva is inoculated also seems to play a role in the toxic output. Although the tick species was not identified, a case report described a 3 year-old Indian boy with an acute onset of left-sided facial palsy secondary to tick infestation in the left ear . Therefore, the proximity to a nerve (in this case the facial nerve) was important for the clinical toxic output (left-sided facial palsy). A similar case was also reported in a 3 year-old Turkish girl . (iii) The duration of tick feeding is also an important factor of induced toxicoses . Venzal and colleagues  showed that, after 3 days, laboratory mice infested with Ornithodoros aff. puertoricensis had initial signs of hyperaemia, followed by respiratory symptoms on day 4, and finally after 4 days the mice displayed nervous incoordination. A final factor (iv) to consider is the presence of common antigens between tick saliva and hosts. Recent episodes of human anaphylaxis after allergic sensitizations induced by bites of A. americanum have been reported. Patients with a history of A. americanum bites produced increased levels of pro-allergenic immunoglobulin E (IgE). The increased anti-tick IgE levels in these patients were correlated to anaphylactic reactions to one anti-cancer monoclonal antibody (Cetuximab) and red meats . Anaphylaxis induced by A. americanum is provoked by the presence of specific IgE to the carbohydrate galactose-alpha-1,3-galactose (alpha-gal) that is also present in Cetuximab and red meat . Interestingly, alpha-gal was recently found in the gut of I. ricinus, a tick that also induces anaphylaxis .
The unified view of venom immune modulation and anti-venom immune responses
The haematotoxic and neurotoxic effects associated with venom exposure are widely recognized (revised in ). Nevertheless, all venomous animals are also constantly challenged by the host/prey or predator immune response. Studies have shown that the immune response of laboratory animals successfully counteracted venomous toxins [37, 38]. In fact, natural resistance to snake venom was reported in both prey [39, 40] and predator . Thus, the immune system of the host/prey must constitute an important target of venoms in order to be effective. In fact, manipulating host defense mechanisms by venoms has been reported for some venomous animals like the parasitoid wasp, Nasonia vitripennis. N. vitripennis, like ticks, are considered to be an ectoparasite since the Nasonia larvae feed on their hosts (invertebrates) without entering the host body . The venom of N. vitripennis must suppress the immune response of the hosts in such a way that the host “allows” the parasitoid infection while simultaneously, the host will be able to control infections by other microorganism that otherwise would compete with the N. vitripennis larvae development . Two major host defense cascades were suppressed by N. vitripennis venom: the phenoloxidase cascade and the coagulation cascade . Several components of N. vitripennis venom have been suggested to modulate the host immune system, e.g., serine protease inhibitors, serine proteases, cystein-rich/Kunitz venom proteins and cysteine-rich/trypsin inhibitor-like venom proteins .
Manipulation of the host/prey immune system is not restricted to venomous Hymenoptera, e.g., N. vitripennis; for example, the haematophagous bat Desmodus rotundus, a venomous animal based on its salivary composition and feeding behavior , possesses two members of TNF-α-stimulated gene 6 (TSG-6) family that are highly expressed in its salivary glands. The TSG-6 family members have specific anti-inflammatory properties, such as the inhibition of neutrophil migration to interact with macrophage CD44 and modulation of NF-κB signaling . This suggests that TSG-6 may play a feeding-facilitating role by suppressing the immune system. One well-studied example is the immune modulation induced by ticks in their hosts. The immune system manipulation by ticks is a complex process that has been recently revised .
Ticks are unique among hematophagous arthropods since they attach to host skin and feed for several days, while other blood-feeding arthropods (e.g. Triatomes or mosquitoes) feed little and often. Therefore, ticks need to counteract both the immediate innate immunity and the slower-developing adaptive immune responses in their vertebrate hosts. One first line of defense will be to counteract pain and itching responses of the host by targeting, for example, histamine, an immune-related mediator of pain and itch (revised in ). A few histamine-binding lipocalins was reported in the hard tick, R. appendiculatus. In this regard, tick venom differs from canonical venoms since most venomous animals (e.g., wasps, bees, snakes, scorpions, spiders and jellyfish) will induce pain or an itch response. These venomous animals use their venom systems as a defensive or predatory function  with the desired effects of pain or itch to produce a deterrent effect. In contrast, similar to ticks, venomous haematophagous animals, like D. rotundus or triatomes bugs, should counteract prey/host awareness in order to feed until repletion.
After the skin is injured by a tick bite, the inflammatory response of the host will be activated. Ticks require a molecular arsenal to suppress both the cellular and molecular components of the host defenses. Tick salivary extract have been shown to reduce endothelial cell expression of the adhesion molecules ICAM-I and VCAM-I (Dermacentor andersoni) and P-selectin (I. scapularis). Reduction in adhesion molecules will reduce the extravasation of leukocytes at the site of tick attachment. The alternative pathway of complement activation is also one of the targets of the immunomodulation induced by tick saliva and thus complement inhibition activity has been reported in saliva of D. andersoni, I. scapularis, I. ricinus, I. hexagonus, I. uriae and O. moubata (revised in ). In addition, as a general trend, the saliva from haematophagous arthropods, including ticks, inhibit the proliferation of naïve T cell and the production of Th1 citokines . One interesting example of modulating the adaptative immune response by hard ticks is Japanin. Japanin is a lipocalin that specifically reprograms human dendritic cells by hijacking the normal maturation process, even in the presence of “danger” signals like bacterial lipopolysaccharide . Interestingly, Japanin promotes secretion of the anti-inflammatory cytokine IL-10 and increases expression of programmed death-ligand 1 (PD-L1), and both are involved in suppressing T cell immunity and induction of tolerogenic responses . Such degree of molecular specialization has neither been described in other haematophagous arthropods nor in other venomous taxa. However, despite the immune suppression induced by tick saliva, some tick-host interactions result in immune-mediated acquired resistance to ticks after subsequent tick challenge.
Given the dynamics between induction of immune suppression by venoms and host/prey resistance development, an arms race between the host immune system and venomous components has been proposed . The balance of this arms race will result in a susceptible or resistant host, prey or predators. In our revision of the topic, we found a convergence in the type of immune response that mammals display against both venom and tick saliva. Type 2 immune responses are mediated by lymphocytes T helper type 2 (Th2), IgE and IgG1 antibodies, but also by eosinophils, mast cells, basophils and, alternatively, by activated macrophages. This Th2 immune response encompasses a wider concept, namely allergies . In mammals, venoms can induce allergic sensitization and development of specific IgE [37, 38, 51]; tick feeding also induces a Th2 polarization , specific IgE [52, 53], and causes allergic sensitization . The complex association between allergen IgE recognition with histamine secretion by mast cells and basophils that subsequently provoke uncomfortable reactions in animals has been highlighted . This association goes beyond a specific neutralizing IgE antibodies response to a more complex detection of sensory stimuli by the olfactory, gustatory and visual systems that, surprisingly, may eventually result in developing aversive behaviors to specific locations or foods . This suggests that the evolution of a differentiated pattern of immunity against venoms, including tick saliva, may have yet unexplored ecological implications. Another example of immune response convergence against venoms is that mast cells can be activated by the venom of scorpions without the concomitant presence of specific IgE , suggesting that the protective activities of mast cells is independent to the high affinity binding of IgE to the IgE receptor (FcϵRI) present on mast cells. Wada and colleagues  recently showed that the protective role of mast cells in resistant mice to the tick Haemaphysalis longicornis was also independent of FcϵRI. The above referenced studies show that (i) immune modulation may be a major function of venoms and (ii) the type of immune response elicited against the venom of ticks and other venomous taxa undergo similar immune pathways, thus tick saliva may possess venom-like molecules.
Tick saliva; or, the structural convergence of venomous proteins with venomous functions
The types of toxicoses induced by tick bites (ranging from lethal paralysis to local hypersensitivity) is not limited to the presence of lethal toxins but also to the presence of specific tick salivary protein families common among other venomous taxa. Recent advances in sequencing technologies have revealed an amazing body of information from the salivary glands of both hard and soft ticks [56–64]. From these high-throughput investigations, several protein families have been identified that are involved in tick-host interactions. Such protein families are found in the venoms of several other Metazoan species . Examples of such venomous protein families found in tick saliva are defensins , lectins , cystatins , lipocalins [21, 68–71], hyaluronidase , phospholipase A2 , Kunitz-like peptides [56, 74, 75], metalloproteases , AVIT , CAP proteins (Cysteine-Rich Secretion Proteins, Antigen 5, and Pathogenesis-Related)  and sphingomyelinase D .
Not only are these protein families present in tick saliva, but they also possess major functions described in conventional venomous systems. These functions include inhibition of thrombin, fXa, fVII/tissue factor system, platelet aggregation (i.e., collagen-induced, ADP-induced), act as a GPIIb/IIIa receptor antagonist, or affect fibrino(geno)lytic activity (revised in ). At the molecular level, venomous agents display common characteristics, despite their numerous biochemical activities and sequence variability, such as (i) possessing a signal peptide, (ii) displaying functional versatility within a protein family, (iii) targeting short-term physiological processes and, (iv) stabilizing their tertiary structures via disulfide bonds. Finally, after being recruited as a functionally stable venomous agent, (v) duplication events occur to reinforce its adaptation (for a thorough description of said characteristics see ). An exception to this last property (gene duplication) is seen in platypus venom . In the following sections we show the structural convergence between tick Kunitz peptides, cystatins, defensins, lipocalins, lectins and phospholipase A2 and their conventional venomous counterparts.
To date, only a few salivary secreted tick Kunitz peptides have been structurally resolved; however, these few reports reveal the venomous nature of these salivary peptides compared with other Kunitz structures from venomous animals. Figure 2A shows that the archetypal Kunitz fold is highly conserved for these tick salivary peptides and that they are structurally similar to HWTX-XI and LmKKT-1a. These structurally resolved tick salivary peptides show a structural conservation in their disulfide bridges (indicated by roman numerals), β-hairpin and the C-terminus α-helix. The only deviant from the archetypical Kunitz tertiary structure is Ra-KLP, since it is missing the second (II) disulfide bridge and possess a modified apex due to two atypical disulfide bridges (1 and 2; Figure 2A). Figure 2A also shows that the electrostatic potential of HWTX-XI and LmKKT-1a is strikingly similar to both TdPI and Ra-KLP, both from the salivary glands of R. appendiculatus. Ra-KLP has been reported as an ion channel modulator  like LmKKT-1a and HWTX-XI with no protease activity. It remains to be tested, however, if and how TdPI affects ion channels. Figure 2B shows a Cα backbone protein structural alignment of the represented Kunitz peptides. The root mean square deviation compared with HWTX-XI does not exceed 3 Å (TAP = 3 Å; ornithodorin = 2.4 Å; boophilin = 1.7 Å; TdPI = 2.8 Å; Ra-KLP = 2.8 Å); the structural difference with LmKKT-1a slightly varies from these deviations, but does not exceed 3.3 Å. Regardless of the conservative nature in the Kunitz fold, these tick salivary peptides display functional versatility and target different short-term physiological processes [85–89]. Therefore, as one of the most abundant tick salivary protein families , we consider Kunitz peptides as a typical example of a venomous agent that fit all five properties (i-v) referred above and described by Fry and colleagues .
Over 80 cystatins have been reported in the salivary glands of hard and soft tick species [57, 61, 63, 64, 77, 94]. For a full description on the physiological role of tick cystatins refer to . In general, tick cystatins are potent inhibitors of papain-like cysteine proteases and play important roles during tick feeding. Tick salivary cystatins have been shown to serve as host immune modulators but their basic functions in tick saliva are unknown. A secreted cystatin has also been identified in the tick gut of H. longicornis that increases in expression during feeding on its host (Hlcyst-2; Figure 3A) . Three crystal structures of cystatins secreted by tick salivary glands of I. scapularis (sialostatinL and sialostatinL2) and O. moubata (OmC2) have been resolved. Although the binding of these tick cystatins remain elusive, an in silico study showed that these inhibitory loop regions for sialostatinL2 are conserved (Figure 3) . A recent study showed that several tick cystatins were constantly expressed during a 5-day feeding period; among these was the cystatin ACX53922 . Compared with the other five cystatins in Figure 3A, ACX53922 displays a more positive electrostatic potential throughout its all-atom landscape while still maintaining the archetypal tertiary backbone structure (Figure 3B; all structures have <3.0 Å root mean square deviation compared with JZTX-75).
A single experimentally induced genetic deletion or mutation transforms a non-toxic defensin into a neurotoxin , thus, reinforcing the concept that toxic molecules are recruited from ancestral proteins possessing a non-toxic physiological function . This also suggests the evolutionary steps necessary for recruiting defensins in the venom of venomous animals . In agreement with the functional diversity of defensins, it is evident, from reported crystal structures, that the tertiary structure is highly divergent (Figure 4B; all have ~3.5 Å root mean square deviation compared with crotamine). Defensins in ticks show a diverse expression pattern, thus they have been isolated from tick haemocytes, gut, intestine, ovaries, malpighian tubules and fat body [110, 111]. Nevertheless, some of these defensins are exclusively expressed in tick salivary glands . The most widely reported defensin structure in both hard and soft tick species is the CSαβ . The only function assigned to the majority of characterized tick defensins, thus far, is AMP [65, 110–112]; however, haemolytic activity was also recently reported for I. ricinus and H. longicornis defensins [111, 114]. This obviously does not exclude the possibility that tick defensins may have other toxic functions in the vertebrate host. Other types of cysteine-rich AMPs from ticks were found to inhibit serine proteases , specifically chymotrypsin and elastase . Furthermore, some tick defensins have secondary and tertiary structures similar to membrane potential modulators, such as scorpion neurotoxins, snake safaratoxins and plant γ-thionins  suggesting a toxic role for these tick defensins. Another example of evolutionary convergence between ticks defensins and toxic defensins are the recently discovered multigenic defensin-like peptides, scasin and scapularisin, from the toxicoses-inducing tick I. scapularis (scapularisin is shown in Figure 4A) . Of these functionally uncharacterized novel defensin-like peptides, scasins show a strong positive selection acting on the whole molecule , an evolutionary pattern observed before in conotoxins from the molluscs of the genera Conus  that act as ion channel modulators . Nevertheless, further studies should clarify whether scapularisin and scasins are ion channel effectors, or not. As Figure 4A shows, however, tick defensins have similar electrostatic potentials as those found in snake and scorpion venom.
Lipocalins are multifunctional proteins with a β-barrel structure that share three conserved domains in their primary structure, namely, motifs 1–3. Lipocalins have been implicated in development, regeneration, and pathological processes, but their specific roles are not known . In reptiles and other venomous taxa, venom systems are enriched through gene duplication [2, 121], thereby increasing its functional divergence to develop a new function or neofunctionalization . Neofunctionalization in tick lipocalins is a good example of functional diversification found in the venom of several venomous taxa .
Due to a recent European upsurge of allergic reactions caused by the pigeon tick A. reflexus (e.g. ) a major allergen was identified (Arg r1) that is homologous to toxic lipocalins from O. savignyi[127, 128]. Histamine-binding proteins were also described as lipocalins from the tick saliva of R. appendiculatus and recently, a novel group of lipocalins was reported from metastriate tick saliva possessing a modulatory activity on dendritic cells . Another lipocalin was isolated from A. monolakensis (AM-33) that binds to cysteinyl leokotrienes with high affinity, avoiding endothelial permeability and formation of edema, thus ensuring the tick to replete an erythrocyte-rich meal .
The phospholipase A2 (PLA2) superfamily are ubiquitously found throughout the animal kingdom to catalyze the hydrolysis of ester bonds in a variety of different phospholipids producing lysophospholipids and free fatty acids that play important physiological roles . The PLA2 superfamily includes five distinct enzyme types that are composed of 15 groups with many subgroups depending if they are secreted, cytosolic, calcium-independent or based on their specific target . The PLA2 superfamily has also been recruited via convergent evolution into the toxic arsenal of cephalopods, cnidarians, insects and arachnids . In the venom of reptiles, PLA2 appears as an antiplatelet aggregation factor , a myotoxin and a neurotoxin .
A comparison of salivary proteins from humans and ticks
Considerations on the most recent common ancestor of Parasitiformes and Pseudoscorpiones
Given the evidence provided in the previous sections, we find it necessary to reconsider ticks as venomous ectoparasites due its salivary properties and its evolutionary implications. Arthropods represent a major component of the biodiversity of life on Earth and venom systems, as an evolutionary adaptation, appear many times in Arthropoda (e.g., spiders, scorpions, wasp, bees and flies). With a few exceptions in Hexapoda (i.e., bees and some wasps), the main function of venomous molecular systems amongst Arthropods is to assist in predation . Ticks are haematophagous Arachnids and, based on morphological characteristics, it has been argued that Holothyrida is the sister taxa of Ixodida . This evolutionary relationship with Holotryrida, free-living mites that mainly feed on the body fluids of dead arthropods , has lead to inferences regarding the feeding habits of the most recent common ancestor (MRCA) of ticks, which has been described as a saprophytic organism  or a scavenger in which haematophagy evolved subsequently . Entomophagy was another type of predatory behavior proposed for the MRCA of ticks and the cannibalism observed in some soft tick species was an argument in favor of this hypothesis .
Pseudoscorpions are venomous members of Chelicerata  that, together with spiders and scorpions , constitute well-known examples of venomous animals amongst Chelicerata. Our working hypothesis is that the MRCA of ticks was a venomous predator of smaller preys that later evolved to feed on larger vertebrates. In agreement with this, the feeding plasticity (as a capacity of feeding on several hosts) of the tick ancestor was recently suggested . The convergence of several protein families between tick saliva and the venom of spiders and scorpions is shown in Figure 9A. This suggests a common origin in the venom systems of these three taxa (pseudoscorpions were not included in the comparison due to a lack of information regarding the venom composition of this species). Additionally, ticks are questing animals that will move actively in order to find their hosts [158–160]. Questing behavior constitute an important trait of tick ecology . The evolution of questing behavior is closely related to the evolution of sensory organs like the Haller’s organ . We found striking similarities in the questing behavior and position of sensory organs of ticks compared to the hunting behavior and position of sensory organs  of venomous spiders (Figure 9B).
The literature has split the saliva of haematophagous animals into those who consider the saliva as venomous and those that consider it as a special kind of saliva or a complex cocktail of bioactive components. Just last year (2013) the Journal of Proteomics published two investigations on the salivary gland components of the haematophagous bat Desmodus rotundus, where one refers to it as venomous  and the other simply as salivary components . In tick research, the literature predominantly considers tick saliva as a complex cocktail of bioactive components and the toxicoses induced by ticks are mainly discussed in the context of paralysis. These claims narrow the ecological implications of the venomous relationship between ticks and their hosts. To classify ticks as mere ectoparasites is limited and underestimates the venomous structure of multigenic protein families in tick saliva. Therefore, we propose to consider ticks as venomous ectoparasites based on the intrinsic properties of tick saliva.
Tertiary protein modeling, structural alignment and electrostatic potential
Predicted tertiary models were generated using the Phyre2 server . All predicted models were then refined via minimization and hydrogen-bond network was optimized by means of side chain sampling using the Schrodinger’s Maestro Protein Preparation Wizard . The structural deviations (root mean square deviation) were calculated using the protein structural alignment tool, from the Maestro software. Electrostatic potentials were calculated using the Poisson-Boltzmann equation also implemented in the Maestro software.
Cysteine-stabilized α-helix and β-sheet
Most recent common ancestor
T helper type 2.
JJV was sponsored by project CZ.1.07/2.3.00/30.0032, co-financed by the European Social Fund and the state budget of the Czech Republic. ACC was supported by a grant from the Ministère de l’Education Supérieure et de la Recherche of France.
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