Ticks

Ticks are essentially large mites that are covered with a leathery integument. A tick’s sole purpose is sucking blood from mammals, birds and reptiles, and then reproducing to provide the next generation. Ticks are not insects, but arachnids. An adult will have eight legs and three body segments. As arachnids, ticks are related to spiders, chiggers, scorpions and mites.

Ticks are capable of causing direct harm to the host such as anemia, tick paralysis, and skin wounds that may result in secondary infections. Ticks also have a sideline business; they transmit a wide variety of infectious diseases as they feed on their host. Ticks require large blood meals and as a result are more likely than other ectoparasites to ingest infectious agents along with their host’s blood. There are over 850 tick species, of which, about one hundred are capable of transmitting disease. Ticks may also carry multiple parasites.

There are three Families of ticks, two of which are significant in the U.S. Argasidae ticks are known as the soft ticks and are a small family of 140 species. Most of the tick species in this family parasitize birds. A few other of these species feed on bats, reptiles, wild mammals and man. As a group, these ticks typically parasitize only one type of host. This group of ticks feeds repeatedly and females lay a clutch of eggs after each blood meal. These ticks hide in cracks or crevices in the daytime and feed at night.

Ixodid ticks are known as hard ticks and are more than 650 species of large ticks. Greater than 600 of these species are three-host ticks. The term three-host refers to the feature that each stage of tick development, nymph and larvae, feed on a different host and then drop off the host to molt. Once these ticks have molted, they find an additional host, usually different from the first. The number of hosts is an important factor in disease transmission and in the implementation of a tick control plan. Three-host ticks of this family spend 90% of their lifetime off the host.

As a group, these ticks have a shield or scutum that covers the entire dorsal surface of the male but only part of the dorsal surface of the female. The scutum gets proportionately smaller in the female because as the tick feeds, she becomes engorged and therefore her body enlarges while the scutum does not. The female Ixodid tick may increase her weight an astounding 100 times her original size after she mates. The larvae, nymph and adult stages of these ticks feed only once and several days are required to complete engorgement. These ticks lay only a single batch of eggs, usually in batches of 1,000 to 4,000, but over 12,000 eggs have been recorded. The female dies after laying her eggs.

Ticks can survive several years of starvation until the right host is contacted. Ticks require a habitat with very high humidity (greater than 80%), so they are seldom found living indoors. Ticks cannot move quickly but are very good climbers. They are usually found outdoors in grassy or forested areas from ground level to three feet above the ground.

References:

Kahn, Cynthia: “The Merck Veterinary Manual”. 9th Edition. pp. 749-764.
Faust, Ernest, Paul Russell and Rodney Jung. “Clinical Parasitology”. 8th Edition. Lea and Febiger. Philadelphia 1976. pp. 598- 603.
Georgi, Jay and Marion Georgi. “Parasitology for Veterinarians”. 5th Edition. WB Saunders. 1990. pp. 46 – 56.
Eberts, Matt. “Tick-borne Disease: A Case of Coinfection”. DVM. April 2006.