American dog tick (Dermacentor variabilis)

How to identify it

• Reddish-brown, rounded body marked with ornate silvery-gray to whitish patterning on the scutum (dorsal shield) — the single most reliable field clue, unlike the solid-colored blacklegged tick or 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 and darkens/expands as she feeds.
• Adult male: the mottled silvery-gray-and-brown pattern covers almost the entire back (the scutum extends over the whole dorsal surface).
• Short, stubby mouthparts and a roughly rectangular base of the head (Dermacentor), versus the long mouthparts of the Lone Star and Gulf Coast ticks (Amblyomma).
• Adults are the human-biting stage; the tiny larvae and nymphs feed almost entirely on small rodents and are rarely found on people.

Size: Larvae are about 0.6 mm and nymphs about 0.9 mm (both rarely bite humans). An unfed adult female is roughly 5 mm (about the size of a sesame-to-apple seed) and an unfed male about 3.6 mm; a fully engorged female can swell to around 15 mm long and 10 mm wide (grape-like).

Where it lives & when

Widely distributed across the United States east of the Rocky Mountains — from the Great Plains eastward to the Atlantic and throughout the Southeast and Midwest — with more limited populations reported west of the Rockies, including along the Pacific coast. (Per CDC, many of these western/Pacific-coast populations were identified in 2021 as a separate species, Dermacentor similis, whose disease role is still under study.) This is a much broader east-of-Rockies distribution than the Lone Star tick's mainly southeastern/south-central range.

Season: Adults are the biting stage and are most active in spring and summer, roughly April through August with a peak in the warmer months (about May–July); risk tapers off by early fall.

• Grassy fields, meadows, and overgrown pastures
• Roadsides, trail edges, and fence lines with low vegetation
• Shrubby areas and the borders where lawns or paths meet taller grass
• Open, sunnier sites frequented by dogs and medium-to-large mammals — often less deeply wooded than blacklegged-tick habitat

Diseases the american dog tick 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 shares the same ornate silvery-gray scutum; the two are separated mainly by range (D. andersoni in the Rocky Mountain states) and, in overlap zones, only by microscopic examination. The recently split western/Pacific-coast species Dermacentor similis is likewise near-identical and distinguished chiefly by range and laboratory examination. The brown dog tick is similarly reddish-brown but lacks the ornate silver markings and is more uniform. The Gulf Coast tick (Amblyomma maculatum) also has an ornate patterned shield but has long mouthparts. The Lone Star female is distinguished by a single central white dot rather than the American dog tick's diffuse silvery patterning.