TICK

any of the roughly 825 species of invertebrates of the suborder Ixodida, within the order Parasitiformes, subclass Acari (sometimes Acarina, or Acarida). Ticks, worldwide in distribution, are assigned to three families: Argasidae (soft ticks) and Nuttalliellidae and Ixodidae (together constituting the hard ticks). The family Nuttalliellidae is represented by one rare African species. Adults range in size up to 30 mm (slightly more than 1 inch), but most species are 15 mm or less. They may be distinguished from their close allies, the mites, by the presence of a sensory pit (Haller's organ) on the end segment of the first of four pairs of legs; eyes may be present or absent. Ticks are important parasites of large wild and domestic animals; they are also significant as carriers of serious human and animal diseases. Although no species is primarily a human parasite, some occasionally attack man. Hard ticks, such as the American dog tick, Dermacentor variabilis, attach to their hosts and feed continuously for several days. When engorgement is complete, the female drops from the host, finds a suitable site to rest, lays her eggs in a mass, and dies. Six-legged larvae hatch from the eggs, move up on blades of grass, and wait for a suitable host (usually a mammal) to pass by. The odour of butyric acid, emanated by all mammals, 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 a suitable host; after they have found one and engorged themselves, they fall off the host and molt into adult males or females. Adults may wait for a host as long as three years. Most hard ticks live in fields and woods, but a few, such as the brown dog tick, Rhipicephalus sanguineus, are household pests. Soft ticks 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. Hard ticks damage the host by drawing large amounts of blood, by secreting neurotoxins (nerve poisons) that sometimes produce paralysis or death, and by transmitting disease. Such diseases include Lyme disease, Texas cattle fever, anaplasmosis, Rocky Mountain spotted fever, Q fever, tularemia, hemorrhagic fever, and a form of encephalitis. Soft ticks also are carriers of diseases.