Ticks are obligate ectoparasites of most types of terrestrial vertebrates. They are large mites and thus are arachnids, members of the subclass Acari. They are more closely related to spiders than to insects. The ~850 described species are exclusively bloodsucking in all feeding stages.
Ticks transmit a greater variety of infectious organisms than any other group of arthropods and, worldwide, are second only to mosquitoes in terms of their public health and veterinary importance.
Ticks can harm their hosts directly by inducing toxicosis (eg, sweating sickness, Tick Paralysis caused by salivary fluids containing toxins), skin wounds susceptible to secondary bacterial infections and screwworm infestations, and anemia and death.
International movement of animals infected with the tick-transmitted blood parasites Theileria, Babesia, and Anaplasma spp and Ehrlichia(Cowdria) ruminantium is widely restricted.
Two of the three families of ticks parasitize canine: the Argasidae (argasids, “soft ticks”) and the Ixodidae (ixodids, “hard ticks”). Although they share certain basic properties, argasids and ixodids differ in many structural, behavioral, physiologic, ecologic, feeding, and reproductive patterns. Tropical and subtropical species may undergo one, two, or rarely three complete life cycles annually.
In temperate zones, there is often one annual cycle; in northern regions and at higher elevations in temperate regions, at least 2–4 year are required by most species. There are four developmental stages: egg, larva, nymph, and adult. All larvae have three pairs of legs; all nymphs and adults have four. Adults have a distinctive genital and anal area on the ventral body surface. The foreleg tarsi of all ticks bear a unique sensory apparatus—Haller’s organ—to sense carbon dioxide, chemical stimuli (odor), temperature, humidity, etc. Pheromones stimulate group assembly, species recognition, mating, and host selection.
Certain tick species that parasitize livestock can survive several months, and occasionally a few years, without food if environmental conditions permit. Tick host preferences are usually limited to a particular genus, family, or order of vertebrates; however, certain ticks are exceptionally adaptable to a variety of hosts, so each species must be evaluated separately. The larvae and nymphs of most ixodids that parasitize livestock feed on small wildlife such as birds, rodents, small carnivores, or even lizards.
In the Argasidae, the leathery dorsal surface lacks a hard plate (scutum). Male and female argasids appear to be much alike, except for the larger size of the female and differences in external genitalia. The argasid capitulum (mouthparts) arises from the anterior of the body in larvae but from the ventral body surface in nymphs and adults.
In the Ixodidae, the male dorsal surface is covered by a scutum. The scutum of the ixodid female, nymph, and larva covers only the anterior half of the dorsal surface. The ixodid capitulum arises from the anterior end of the body in each developmental stage.
Source: Merck Veterinary Manual
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