Ticks are ectoparasitic arthropods (Phylum Arthropoda, Class Arachnida, Order Acarina) of animals and humans, of considerable socioeconomic and medical importance. There are two main families of ticks, Argasidae (the so-called soft ticks) and Ixodidae (the so-called hard ticks) which, at our latitudes, infest respectively birds and mammals.
Of the hundreds of existing species of ticks, around forty have been reported in Italy. The most important genera which infest companion animals (and, in some cases, also humans) are Ixodes, Rhipicephalus, Haemaphysalis, Boophilus, Hyalomma, Dermacentor, among the hard ticks, and Ornithodorus andArgas, among the soft ticks.
The main characteristics of hard ticks, which are of greater epidemiological and clinical importance than soft ticks, are described below.
MORPHOLOGY
The different genera and species of Ixodidae ticks are characterized by numerous morphological features. Ticks have an oval or pyriform shaped body, which is divided into two main parts, an anterior part, the gnathosoma, and a posterior part, the idiosoma. The size of ticks varies, in the order of a few millimetres, depending on the species and on the biological and physiological stage. The gnathosoma, which has different shapes and lengths depending on the species, comprises the buccal apparatus of the tick, called capitulum, or rostrum, which is made up, in a latero-medial direction, by paired organs, the palps and the chelicerae, and by a single midline organ, the hypostome. In brevirostrate ticks (e.g. Rhipicephalus spp., Dermacentor spp., Haemaphysalis spp., Boophilus spp.) the rostrum is shorter than it is wide, while in longirostrate ticks (Ixodes spp., Hyalomma spp.) the rostrum is longer than it is wide. The palps, which have numerous sensory structures, protect the chelicerae, rigid appendages provided with hooks which, in turn, cover the hypostome, which is located ventrally. The hypostome, with its retroverted teeth, is the organ that the tick uses to anchor to the host.
The idiosoma bears the legs (four pairs in adult stages and nymphs, three pairs in larvae) and, in hard ticks, a scutum or shield on the dorsal surface. This scutum is a chitin plate of variable colour, usually brownish-red, or ornamented (“enamelled”) in ticks of the genus Dermacentor. The scutum covers the whole of the dorsal surface of males while in females it is present only in the anterior portion, in order to allow the tick’s volume to expand during a blood meal (see later). The scutum is limited to the anterior portion of the dorsum also in larvae and nymphs (immature stages).
Ventrally the idiosoma bears the genital (visible only in adults) and anal openings. The latter is surrounded posteriorly or anteriorly by a sulcus.
In addition to the digestive and reproductive organs, the idiosoma contains the salivary glands, made up of units (acini) organized in clusters, which occupy most of the available space. The salivary glands are fundamental both for osmoregulation and for the production of some anaesthetic, anticoagulant and cytolytic substances which are necessary for the blood meal. The saliva produced by these glands contains antigens capable of stimulating the host’s immune response and is the vehicle via which toxins and various pathogenic organisms can be transmitted.
BIOLOGY AND PATHOGENIC ROLE
Many of the peculiar biological characteristics of ticks (e.g. hematophagy, resistance, longevity, telotropism – see later) are closely related to their capacity of exerting, in the different stages of their biological cycle, major pathogenic actions against animals and man.
The stages of the life cycle of ticks are the egg, larva, nymph and adult. The immature stages (larvae and nymphs), the females and, to a lesser extent, the males, are haematophagous and consequently consume a blood meal on the host. The moulting from larva to nymph and from nymph to adult takes place either on a host or in the external environment. Specifically, most tick species, in the course of their biological cycle, infest one (monophasic ticks), or three (triphasic ticks) hosts. In the former case the tick moults (from larva to nymph and from nymph to adult) on the same host, while in the latter case each biological stage occurs on a different host and the two moults take place in the external environment.
The larva and nymph of some species of ticks feed on the same host, while the adult stages parasitize a second host (biphasic ticks). In such cases the first moult (from larva to nymph) takes place on the first animal, while the second moult (from nymph to adult) takes place in the external environment.
If the hosts which are parasitized belong to the same animal species the ticks are called monotropic, while if two different animal species are parasitized the ticks are called ditropic. If the ticks consume their blood meal on three different species of animals they are called telotropic. These differences in tropism are not necessarily rigid as some species, such as the dog tick Rhipicephalus sanguineus, may be both monotropic and telotropic, depending on environmental conditions and on the availability of the host species.
To allow ovodeposition the female tick must first mate, on the ground or on the host, and then eat a blood meal. Having completed the meal the tick detaches from the animal on which it has fed, and falls into the external environment where, usually in dark and protected areas, it lays its eggs in single bunches. As R. sanguineus females tend to clamber up surfaces, the eggs may also be found in cobwebs or in cracks in walls.
Having laid a variable number of eggs, from a few hundred to a few thousand, depending on the species, the tick dies.
The eggs, pinkish brown in colour and visible to the naked eye, are covered by a waxy substance which makes them impermeable and keeps them bound together. After 2-4 weeks, depending especially on the external temperature, the eggs hatch intolarvae which, after a short period of inactivity, start searching for a host on which to feed. Once the larvae have eaten a blood meal they moult into nymphs, on the same host or on the ground, depending on the species and on the number of hosts infested, and the cycle continues. The nymph consumes its blood meal and subsequently moults into an adult.
In general, the different species of ticks exploit the same group of stimuli (e.g. olfactory, gustatory, mechanical, thermal) to search for and approach a host to infest, although some relevant differences do exist. For example the R. sanguineus tick actively searches for a host attracted by carbon dioxide, while the typical wood tick Ixodes ricinus positions itself at the tips of vegetation and moves waving its first pair of limbs until it becomes hooked to the first receptive host that passes by.
The tick climbing on the body of the host and its movements are usually not perceived by the host, thanks to the presence of cushion-like pads (“pulvilli”) at the extremities of the limbs of the arthropod. Having found the vertebrate animal to parasitize the tick, via sensory organs on its palps, searches for an anatomical site to anchor its buccal apparatus and feed. Ticks usually prefer areas of the body from which it is difficult for the host to eliminate them by scratching or licking. The feeding site also varies depending on the species of tick. Brevirostrate ticks chose auricular (Fig. 1), periauricular or perianal sites, or in any case areas where the skin is thinner, while longirostrate ticks tend to feed where the skin is thicker, such as the groin or neck.
The tick that is most widely spread throughout the world is R. sanguineus (Fig. 2), which typically parasitizes dogs but which can also be found on ovine, bovine, equine and swine species, but only rarely on cats. Although the biological cycle of this tick can be extremely long (up to 3 years) and the different stages may survive without feeding for up to 1.5 years, particularly favourable conditions may allow the completion of the biological cycle in about 2 months, meaning the presence of more generations within a single solar year. This species is characterized by a considerable biological plasticity, meaning that it can survive in climatic conditions incompatible with the survival of other species of hard ticks, and that it can search for hosts to parasitize in extended areas. It has adapted perfectly to cities and can easily reproduce within a home environment. In cities, infected dogs can transport ticks to public areas, where suitable conditions may be found for the development of various generations of ticks; in the absence of their preferential host the immature stages represent the most severe risk of infestation for human beings. In these conditions if the level of infestation is particularly high, any crevice, any cracks in the plaster of walls, gaps in pavements, edges of roads or steps, may become ideal micro-habitats for the survival and reproduction of R. sanguineus. This species, which is triphasic and telotropic in rural and woodland settings (in extra-urban or peri-urban areas, in small vegetable gardens, uncultivated fields, greenhouses, ruins, brushwood), becomes monotropic within cities and in domestic environments, and all of its stages can infest dogs. The major infectious pathogens transmitted by R. sanguineus are Ehrlichia canis, Babesia canis, Babesia caballi, Theileria equi, Hepatozoon canis, Rickettsia conorii andCoxiella burnetii.
Another important species in terms of epidemiology and medical implications is I.ricinus (Fig. 3). It is an extremely common triphasic and teletropic tick which prefers rural and woodland environments, where ruminants and wild or domestic dogs are present, these animals being the main hosts for the adult stages of the arthropod. In the immature stages, instead, I.ricinus feeds on small mammals such as hedgehogs and rodents, or reptiles and sparrows. In these settings I. ricinus survives well in areas with lawns and dry leaves, heaths and pastures, where it can find nourishment and the ideal temperature and humidity, factors which have a major impact on their spread (e.g. fairly rainy and damp climates favour their biological cycle). This species may expand into peri-urban or suburban areas where the preferential hosts may become dogs, cats and even humans. The biological cycle of I.ricinus can last for up to 3 years, even though the various stages may feed for only a few days. It is an extremely sturdy tick, capable of fasting for many months: the larvae can fast for up to a year and a half and the adults for up to two years and a half. This species is the major vector for Borrelia burgdorferi although one should not forget its potential for transmitting also Babesia divergens, Babesia bovis, Anaplasma marginale as well as other infectious agents that can cause fever and encephalitis. Furthermore, like R. sanguineus, I.ricinus can cause tick paralysis.
TREATMENT AND CONTROL
The most effective approach to control tick infestation is to use antiparasitic compounds. Various active principles, registered in different formulations, can be used for this purpose for both pets and for large animals. Leaving aside the now outdated organophosphate derivatives, there are currently many compounds on the market, belonging to various pharmacological classes. Some of the molecules used to control and to treat tick infestations are listed below.
Amitraz, a formamidine pesticide, is extensively used for tick infestations. It acts as an octopaminergic agonist at synapses, generating increased nervous activity, rapid paralysis and death of the parasite. Amitraz must not be used in cats.
Pyrethroids act by interfering with the normal functioning of the cell membrane sodium channels of the nervous system of arthropods and can be found on the market in different formulations, often in combination with molecules having a synergistic action. The most widely used pyrethroids are the third generation ones, such as deltamethrin and permethrin. Apart from being active against an extensive variety of arthropods, ticks included, permethrin is also used in formulations containing other compounds (e.g. imidacloprid), in order to exploit the repellent action of the second compound against other ectoparasites such as mosquitoes and sandflies. In view of the fact that cats do not have the glucuronyl transferase enzyme, pyrethroids should not be used in this species, and cats should not be allowed to come into contact with recently treated dogs.
Fipronil belongs to the class of phenyl pyrazoles, the mechanism of action of which is based on blockade of GABA-dependent chlorine channels and on the induction of hyperexcitation of the arthropod’s nervous system. Fipronil is also available in combination with methoprene, a growth regulator which inhibits the moults of the immature stages of some arthropods, so as to achieve the contemporary control of both ticks and fleas. Pyriprole is another phenyl pyrazole which, like fipronil, controls not only tick infestations, causing the arthropods’ death in 48 hours, but also flea infestations.
TICK REMOVAL
Ticks must be removed mechanically without the aid of liquids or heat. Substances which in the past were considered useful for the spontaneous detachment of the hypostome from the skin of the host, such as ether, acetone, alcohol and flames, have been found to be not only useless but even harmful. This is because ticks breathe very slowly (around 2-15 times an hour) and hence to “suffocate” them, the respiratory stigmata would have to remain occluded for at least 4 hours. Furthermore, the occlusion of stigmata and/or the use of heat induces greater salivation by the arthropod, which may in turn increase the likelihood of transmission of infectious pathogens.
Ticks should more correctly be removed mechanically by means of tweezers, preferably with curved tips. The tweezers should be positioned at the base of the rostrum and as close as possible to the skin, and then slight traction exerted – without pressing the abdomen of the parasite – to detach the hypostome from the skin. Once the tick has been removed it is good practice to ascertain that no fragments of the tick have been left behind in the wound and then to disinfect the skin. Violent or inappropriate removal may leave the hypostome or some of its fragments within the wound, which could lead to the development of granulomas or abscesses or favour the penetration of pathogenic microorganisms.
Suggested readings
- Genchi C., Marinculic A., 2007. Zecche e malattie trasmesse. Supplemento de La Settimana Veterinaria n. 566 del 30 maggio 2007, pp 7-22.
- Genchi C., Venco L., Genchi M., 2007. Pulci e zecche: controllare e prevenire le infestazioni. Supplemento de La Settimana Veterinaria n. 566 del 30 maggio 2007, pp 25-30.
- Giangaspero A. 1999. L’infestazione da zecche nei cani e nei gatti. Obiettivi e Documenti Veterinari n.10: Supplemento: 5-18.
- Giangaspero A., Otranto D. 2010. Ectoparassiti ed artropodi vettori. In: Parassitologia e Malattie Parassitarie degli Animali. A cura di Garippa G., Manfredi M.T., Otranto d. Edizioni Mediche Scientifiche Internazionali, Roma, pp. 679-708.
- Otranto D., Dantas-Torres F. 2010. Canine and feline vector-borne diseases in Italy: current situation and perspectives. Parasites and Vectors, 3: 2.