tick

tick, (order Ixodida), any of about 970 species of invertebrates in the order Ixodida that are obligate, blood-feeding parasites. Ticks are serious pests of large wild and domestic animals and are significant carriers of serious diseases. Indeed, ticks surpass all other arthropods, except for mosquitoes, in the number of diseases they transmit to humans. Although no species is primarily a human parasite, some attack humans.

The group has a worldwide distribution, and all species are assigned to three families: Argasidae, comprising the soft ticks, and Ixodidae, comprising the hard ticks. The family Nuttalliellidae is represented by one rare African species (Nuttalliella namaqua).

Physical description

Ticks are arachnids and have eight legs as nymphs and adults; the young larval stage has only six legs. Ticks’ bodies form a single region, unlike the segmented bodies of their spider cousins. Adults of some species can grow up to 30 mm (slightly more than 1 inch) when engorged on blood, but most species are 15 mm (0.6 inch) or less. Soft ticks have only a few hardened plates, or shields, on their bodies, while hard ticks are strongly sclerotized with numerous hardened plates. Ticks may be distinguished from their close relatives, mites, by a sensory pit (Haller’s organ) on the end segment of their first of four pairs of legs. Eyes may be present or absent.

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Hard ticks

The majority of ticks are hard ticks of the family Ixodidae, with more than 770 species. These animals attach to their hosts and feed continuously on blood for several days during each life stage. When an adult female has obtained a blood meal, she mates while still on the host or after having dropped off, finds a suitable site off the host to lay her eggs in a mass, and dies. Six-legged larvae hatch from the eggs, move up blades of grass, and wait for a suitable host (usually a mammal) to pass by. The odor of butyric acid, emanated by all mammals, or carbon dioxide stimulates the larvae to drop onto and attach to a host. After filling themselves with the host’s blood, the larvae detach and molt, becoming eight-legged nymphs. Nymphs also wait for, and board, a suitable host in the same way as larvae. After they have found a host and engorged, they also fall off and molt into adult males or females. Adults may wait for a host for as long as three years.

(Read Britannica’s essay “How Long Can Ticks Live Without Food?”)

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Most hard ticks live in fields and woods, but a few, such as the brown dog tick (Rhipicephalus sanguineus), are household pests.

Soft ticks

There are about 200 species of soft ticks, which are members of the family Argasidae, though the group’s taxonomy is contentious. They differ from hard ticks by feeding intermittently, laying several batches of eggs, passing through several nymphal stages, and carrying on their developmental cycles in the home or nest of the host rather than in fields. They also typically feed more quickly—in a few minutes rather than the days-long feeds of hard ticks.

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Diseases

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Ticks harm their hosts not only by drawing substantial amounts of blood—even large animals like moose can be weakened by a heavy tick infestation—but also by secreting neurotoxins that sometimes cause paralysis or even death in certain animals. Tick bites often go unnoticed because tick saliva has active compounds that alter or block pain, itch, and immune responses and modulate blood coagulation and wound healing.

Both hard and soft ticks are disease vectors, transmitting a wide range of pathogens—including bacteria, viruses, and protozoan parasites—to the animals on which they feed. When a tick feeds on an infected host, it takes in blood that carries the causative pathogen. These pathogens live and multiply in the tick, sometimes remaining in its gut or migrating to its salivary glands or other tissues. Many of these pathogens can survive through molts (such as when a tick moves from larva to nymph, or nymph to adult) in a process called transstadial transmission. Some pathogens, such as certain rickettsia and Crimean-Congo hemorrhagic fever virus, can even be passed from a female tick to her eggs and subsequent larvae, a phenomenon known as transovarial transmission. Because ticks often feed on multiple host species over their lifetime, they can spread diseases among wildlife, domestic animals, and humans.

Prevention

In recent years the incidence of tick-borne diseases has risen in association with anthropogenic climate change. As temperatures rise, tick habitats are expanding, tick activity seasons are lengthening, and tick populations are growing, pushing diseases into new regions and populations. Reducing exposure to ticks and preventing tick bites are key defenses against the diseases they can transmit. Protective measures include avoiding contact with brush and tall grass and walking in the middle of trails whenever possible. Clothing and gear can be treated with the insecticide permethrin, and other insect repellents can be applied to skin and clothing. Wearing light-colored clothing with long sleeves and tucking long pants into tall socks can reduce the amount of skin vulnerable to tick bites and help someone spot a tick before it attaches. It is recommended that people check their skin and clothing for ticks after being outside in tick-prone areas.