The American Dog Tick

Common Name:
American Dog Tick
Scientific Name:
Dermacentor variabilis (Say)


This tick's common name comes from the fact that it is only found in North America and that domestic dogs are the favorite host of the adults. Although not a structural pest, it is commonly found on dogs and readily attacks humans. It is of medical importance because it vectors the causal organisms of Rocky Mountain spotted fever and tularemia, and also causes tick paralysis. It is found throughout the United States except for the area of the Rocky Mountains, and in Canada and Mexico.


Unengorged adult female about 3/16" (5 mm) long, male slightly smaller (about 1/8"/3.6 mm); engorged female up to about 5/8" (15 mm) long, 3/8" (10 mm) wide. Body oval, dorsoventrally flattened (top to bottom). Color brown with whitish to grayish markings often with silvery hue (ornamentation) on scutum (dorsal shield). Scutum (dorsal shield just behind mouthparts) restricted to front half of dorsum in female, almost completely covers dorsum in male except for festoons. Eyes on margin of scutum. Capitutum (mouthparts and their base) visible from above; basis capituli (bass for mouthparts) rectangular with sides not laterally producedlangular; about as long as mouthparts; 2nd segment of palpi about as long as wide, not laterally produced. Abdominal festoons (rectangular areas divided by grooves along posterior margin) 1 1 in number; anal grove present, posterior to anus. Spiracular plate (ventral/boftom, near margin just behind 4th coxae, 1 pair) broad, usually with blunt process reaching dorsum, goblets (round structures) very small and numerous.

Both larvae (6 legs) and nymphs (8 legs) with red markings near eyes and lack white on scutum; unengorged lst instar larvae about 1/64" (0.59-0.64 mm) long, yellow, becoming gray to black when engorged; unengorged 2nd instar nymphs about 1/32" (0.9 mm) long, pale yellowish brown, becoming slate gray when engorged.


  1. Rocky Mountain wood tick (Dermacentor andersont) with spiracular plate goblets (round structures) moderate in size and number, found in Rocky Mountain area only.

  2. Winter tick (Dermacentor albipictus) and brown winter tick (Dermacentor nigrolineatus) with spiracular plate oval, lacking prolongation, and goblets few and large; in addition, brown winter tick scutum (dorsal shield just behind mouthparts) with little or no whitish/grayish markings.

  3. Pacific Coast tick (Dermacentor occidentalis) with posterior/rear lateral extensions of basis capituli (base for mouthparts) long, length equal to or greater than width.

  4. Tropical horse tick (Anocentor nitens) with 7 abdominal festoons (rectangular areas divided by grooves along posterior margin) and hypostome (middle mouthpart) with denticies (teeth on venter) in 8 rows (vs. 6 rows for Dermacentor spp.).


The engorged female drops off the host and seeks a sheltered place to lay her eggs. Over 14-32 days she lays egg masses totaling 4,000-6,500 yellowish-brown eggs, and then dies. Egg hatch usually occurs in 36-57 days. Unfed larvae actively crawl about seeking a host. They can survive for up to 540 days unfed. Larvae require about 4 days (range 3-13 days) to become engorged, then drop off the host and seek shelter for molting purposes. Usually 10+ days (range 6-247 days) are required from drop to nymphal emergence. Unfed nymphs actively crawl about seeking a host. Engorgement usually requires about 6 days (range 3-12) but they can survive for up to 584 days unfed. After feeding, they drop off the host and seek shelter in which to molt. Molting usually requires about 24 days (range 24-291). Adults crawl up on grass or other low vegetation and wait for a host to pass. After both sexes have fed, females are completely engorged in about 10.5 days (range 5-27 days), mating occurs on the host. Males continue to feed but females drop off to lay their eggs. Females require a 3-58 day preovipostion or waiting period before egg laying begins. Unfed adults can survive for about 2-3 years (up to 1,053 days). The entire life cycle (egg to egg) requires from 3 months to more than one year, and both larvae and nymphs can overwinter. In the northern states, a 2-year life cycle may be more common.

American dog ticks are the primary vector of Rocky Mountain spotted fever in the eastern United States, which they transmit from small animals. This is a severe, acute, infectious disease of the small peripheral blood vessels caused by a rickettsiae organism whose characteristic symptom is a rash which develops in 2-5 days, starting with the wrists and ankles and then spreads all over the body. Mortality in humans is 20% or more. Fortunately, attachment for 2 hours or more is required for transmission.

These ticks also transmit tularemia which is caused by a bacillus and is transmitted from rabbits, meadow mice, ground squirrels, sheep, beavers, coyotes, and various game birds. Symptoms include chills and fever, prostration, an ulcer at the tick-bite site, and tender, swollen lymph nodes.

In addition, American dog ticks can cause tick paralysis when they attach on the back of the neck or at the base of the skull and feed for at least 5-6 days. Paralytic symptoms usually start in the extremities and become evident as unsteadiness and loss of reflex actions. If the tick is not removed, death may result from respiratory failure; children are particularly susceptible. If the tick is removed, recovery is rapid and usually within 24-72 hours.

Deticking dogs is an important way that Rocky Mountain spotted fever is spread. Handpicking is dangerous because infected tick secretions on the hands can be transmitted via contact with eyes, mucous membranes, etc.; use forceps for removal.


The American dog tick does not survive well indoors. If found indoors, it was probably carried in on a dog and dropped off when fully engorged to seek a suitable place for egg laying. This is a 3-host tick, with each stage requiring a different host. Both larvae and nymphs actively crawl about seeking a small mammalian host, primarily rodents; hosts include the white-footed mouse (Peromyscus), meadow mouse (Microtus), cotton rat (Sigmodon), cottontail and swamp rabbits (Sylvilagus), muskrat (Ondatra), Norway rat (Rattus norvegicus Berkenhout)), squirrel, and cat. Larvae alone are known from house mouse (Mus musculus (Linnaeus)), jack rabbit (Lepus), and mole (Scalopus aquaticus (Linnaeus)). Nymphs alone are known from wood rat (Neotoma), sheep, cattle, and dog. Because of this kind of host seeking activity, neither larvae nor nymphs are picked up on tick drags.

Adults crawl up grass or other low vegetation, cling to it with their 3rd pair of legs, and wave their other legs about ready to grasp onto any passing host; this is called their "waiting position." They prefer larger mammals as hosts and these include the preferred dog and others such as man, cattle, opossum, coyote, hog, horse, raccoon, wild cat, squirrel, sheep, skunk, deer, fox, domestic cat, mule, rabbit, Norway rat, ground squirrel, donkey/burro, weasel, and woodchuck.

American dog ticks are attracted by the scent of animals and are therefore most numerous along roads, paths, and trails. The concentration is further increased along such travel routes by the dropping of engorged ticks from their host animal. Larval and nymphal activity usually starts about the end of March, representing those which overwintered, and continues to mid-July. Nymphal activity predominates from June to early September. Adults become active about mid-April, peak in June, and decline until mid-September.