American dog tick (Dermacentor variabilis)
The American dog tick (Dermacentor variabilis) is a large hard tick. It lives across the United States east of the Rocky Mountains, with more limited groups reported farther west. Adults are reddish-brown. They have a showy silvery-gray pattern on the shield (scutum) behind the mouthparts. Females are the stage most likely to bite people. It is one of the main US carriers of Rocky Mountain spotted fever, and it can also spread tularemia. It does not spread Lyme disease, which blacklegged (deer) ticks carry.
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How to identify it
- Reddish-brown, rounded body with a showy silvery-gray to whitish pattern on the shield (scutum). This is the most reliable clue in the field. It differs from the plain blacklegged tick and from the Lone Star female's single white dot.
- Adult female: the silvery-gray shield covers only the front third of the back, just behind the mouthparts. The rest of the body is plain reddish-brown. It darkens and swells as she feeds.
- Adult male: the mottled silvery-gray and brown pattern covers almost the whole back. His shield covers it all.
- Short, stubby mouthparts and a roughly rectangular head base (Dermacentor). Lone Star and Gulf Coast ticks (Amblyomma) have long mouthparts.
- Adults are the stage that bites people. The tiny larvae and nymphs feed almost only on small rodents and are rarely found on people.
Size by life stage
Larvae are about 0.6 mm and nymphs about 0.9 mm (both rarely bite people). An unfed adult female is about 5 mm, roughly the size of a sesame or apple seed. An unfed male is about 3.6 mm. A fully fed female can swell to around 15 mm long and 10 mm wide, like a grape.
Where it lives and when
Widespread across the United States east of the Rocky Mountains, from the Great Plains east to the Atlantic and throughout the Southeast and Midwest. More limited groups are reported west of the Rockies, including along the Pacific coast. CDC notes that in 2021, many of these western and Pacific coast groups were found to be a separate species, Dermacentor similis. Its role in disease is still being studied. This range east of the Rockies is much broader than the Lone Star tick's mainly southeastern and south-central range.
Season
Adults are the biting stage. They are most active in spring and summer, roughly April through August, with a peak in the warmer months (about May–July). Risk drops off by early fall.
Where it waits
- Grassy fields, meadows, and overgrown pastures
- Roadsides, trail edges, and fence lines with low plants
- Shrubby areas, and the borders where lawns or paths meet taller grass
- Open, sunnier spots that dogs and medium-to-large mammals visit. They are often less wooded than blacklegged tick habitat
Diseases it can carry
Tick presence is not the same as personal disease risk. Not every bite transmits a pathogen.
Look-alikes
Its closest look-alike is the Rocky Mountain wood tick (Dermacentor andersoni), which has the same silvery-gray shield. The two are told apart mainly by range, since D. andersoni lives in the Rocky Mountain states. Where they overlap, only a microscope can separate them. Dermacentor similis, the western and Pacific coast species that was recently split off, is also nearly identical. It too is told apart mainly by range and lab tests. The brown dog tick is also reddish-brown, but it lacks the silver markings and is more evenly colored. The Gulf Coast tick (Amblyomma maculatum) also has a patterned shield, but it has long mouthparts. The Lone Star female has a single white dot in the center, not the American dog tick's spread-out silvery pattern.
Compare all tick speciesCommon questions
Does the American dog tick cause Lyme disease?
No. Blacklegged (deer) ticks spread Lyme disease, not the American dog tick. The American dog tick carries Rocky Mountain spotted fever and can spread tularemia. See a doctor about any illness after a tick bite.
How do I tell an American dog tick from a Lone Star tick?
Look at the back. An American dog tick has a spread-out silvery-gray pattern on its shield and short mouthparts. A Lone Star female has a single bright white dot and clearly longer mouthparts. Males and nymphs are harder to tell apart, so photograph the tick if you are unsure.
Where in the US do American dog ticks live?
Mostly east of the Rocky Mountains. They are common throughout the Midwest and Southeast and along the East Coast. More limited groups are reported west of the Rockies, including along the Pacific coast. CDC now classes some of these as a separate species, Dermacentor similis.
How soon after a bite could Rocky Mountain spotted fever appear?
Usually within about 2–14 days. RMSF can become life-threatening quickly, so getting checked and treated early matters. Tularemia usually appears in about 3–5 days (range 1–14 days). Only a doctor can diagnose these. Get care quickly for fever or rash after a bite.
Sources
- 1.Where Ticks Live (opens in a new tab) · CDC
The Lone Star tick is widely distributed in the Northeast, South, and Midwest U.S.
- 2.Rocky Mountain Spotted Fever (RMSF) (opens in a new tab) · CDC
Spread by American dog, Rocky Mountain wood, and brown dog ticks. Most US cases are in South-Central states; can be fatal without prompt doxycycline.
- 3.Tularemia (opens in a new tab) · CDC
Bacterial illness that can be spread by tick bites (including the Lone Star tick) and other routes.
- 4.TickEncounter Resource Center (opens in a new tab) · University of Rhode Island
Tick identification, prevention, and reporting resources.
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Educational information only, not medical advice. If you were bitten, use the tick bite tool and contact a licensed clinician if symptoms develop. Read our sources and disclaimer.