There are a few ticks that cause tick paralysis in animals, including horses. Foals are most susceptible, but even adult horses can be affected by the presence of forty or more female ticks. Tick paralysis is brought about by a toxin injected into the animal by the adult female tick. It usually starts as a paralysis of the hind legs which moves progressively forward until paralysis of the repiratory and heart muscles occurs. A case of tick paralysis can be treated by searching the horse for and removing all the ticks. If the paralysis has not progressed too far, it will reverse itself when the ticks are removed.
The most important ticks that cause paralysis and the areas where tick paralysis is most common are: Ixodes holocyclus in eastern Australia, Ixodes rubicundus in Southern Africa, and Dermacentor andersoni in northwestern United States and western Canada. There are other ticks which cause paralysis and occasionally the condition is found in other countries including Western Europe. Ticks are most important for their transmission of parasites to horses, particularly the protozoan Babesia.
Ticks have rounded bodies with some simple mouthparts and 3 or 4 pairs of legs. There are 4 life-cycle stages. The adult female tick lays egges from which hatch 6-legged larvae (1/24-/1/12 in.) The next stage is the 8-legged nymph, which is slightly larger than larvae. The nymphs in turn develop into the adult ticks. The ticks pushes its mouthparts through the skin and anchors itself for about 5 to 10 days, or occasionally much longer, and sucks blood. As it feeds, the tick swells and becomes a blood-filled, usually gray or yellow-brown in color.
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The SPINOSE EAR TICK (Otobius megnini) is a pink or blue-gray, leathery tick found in America, Southern Africa and Asia. It will infect the ears of many different animals including the horse. Only the larval and nymphal stages will infect animals. The larvae attach themselves onto a horse, climb to the ears, inside which they feed. The larvae and nymphs feed from 1 to 7 months, and, when feeding, become almost spherical. Eventually nymphs drop off the horse. The adult ticks are not parasitic but lay eggs in sheltered spots such as crack in poles, crevices and walls, under mangers and under bark of trees. Because the females need this type of shelter, spinose ear ticks are found only on stabled horses where parasite and host are in close proximity. They are not found on horses on open range. The ticks feeding in the ears may make the horses sensitive about the head, shaking their heads and rubbing their ears. The ear may droop. A large number of ticks in the ear are easily visible. If only a few are present, then a swab can be rubbed firmly over the skin in the ear canal; some ticks will be dislodged and can be seen on the swab. Infected ears should be treated with an acaricide (a chemical agent that kills mites) once a month. Where horses become heavily infected, the the wood and walls of the stable, etc., should be sprayed with an acaricide.
- Veterinary Notes for Horse Owners. Captain M.Horace Hayes, F.R.C.V.S.