LoneStarMapTick species • Field guide
Western blacklegged tick (Ixodes pacificus)
The western blacklegged tick (Ixodes pacificus) is the Pacific Coast counterpart of the eastern blacklegged (deer) tick. It lives along the US West Coast — most abundantly in California's coastal and foothill woodlands — and is the far-western vector of Lyme disease and anaplasmosis. Adults are small and dark, with distinctly black legs against a brownish body and a shield that covers only the front half of the back. Poppy-seed-sized nymphs are easy to miss and cause many of the region's human infections. This page is educational and is not medical advice.
Last reviewed: 2026-07-20 · Educational only — not medical advice.
How to identify it
- Adult female: brownish-black legs contrasting with the body (the 'blacklegged' cue); a dark scutum (shield) that covers only the front half of the back, behind which sits a smooth reddish-orange, teardrop-shaped abdomen.
- Adult male: smaller, oval, and uniformly dark brownish-black, with the scutum covering nearly the whole back and no reddish-orange area.
- Long, forward-projecting mouthparts; no eyes and no festoons (no rectangular scalloping along the rear edge). Like all Ixodes ticks, the anal groove curves in front of (anterior to) the anus.
- No white dot, streaks, or silvery mottling anywhere — plain-colored, which separates it from the Lone Star and the ornate Dermacentor ticks.
- Nymph: a dark plate on the back over a pale, translucent abdomen; about the size of a poppy seed and often overlooked.
Size: Larvae are pinhead-sized (~0.5 mm) with six legs; nymphs are poppy-seed-sized (~1-2 mm) with eight legs. Unfed adult females are about 3 mm (roughly 1/8 inch) and unfed males slightly smaller (~2-2.5 mm). A fully engorged female swells to about 10 mm (3/8 inch) or larger and becomes gray-tan as she fills with blood.
Where it lives & when
Restricted to the US Pacific Coast — this is not the Lone Star tick's southeastern range. CDC describes it as living along the Pacific coast, particularly northern California; established populations span California (coastal counties and the Sierra Nevada foothills are especially productive), western Oregon, and western Washington. Scattered outlier records have been reported from Nevada, Utah, and Arizona. It is the West Coast counterpart to the eastern blacklegged (deer) tick and does not occur across the South-Central or Southeastern states.
Season: Activity is largely the reverse of the summer-peaking eastern deer tick. Adults quest mainly from late fall through spring and are most active during the cool, wet winter months in coastal California. Nymphs are active from late winter into summer, peaking March through July (mid- to late spring); because nymphs are tiny and bite in the warmer months, they drive much of the human infection risk.
- Cool, humid coastal and foothill forests — oak woodland, Pacific madrone, and Douglas fir stands
- Leaf litter and fir-needle duff on the forest floor, where nymphs are most abundant
- Grassy trail margins, low brush, and chaparral that abuts brush or forest, where adults quest by climbing vegetation and logs a few feet off the ground
Diseases the western blacklegged tick can carry
Tick presence is not the same as personal disease risk. Not every bite transmits a pathogen.
Look-alikes
Its nearest look-alike is the eastern blacklegged (deer) tick, Ixodes scapularis — the two are nearly identical and are separated mainly by geography (Pacific Coast vs. eastern and midwestern US). Within its own range it can be confused with the Pacific Coast tick (Dermacentor occidentalis) and the American dog tick (Dermacentor variabilis), but those are larger, have shorter mouthparts, an ornate silvery-mottled shield, and festoons — none of which the western blacklegged tick has. Tiny unfed nymphs are the hardest to identify and are frequently mistaken for a fleck of dirt or a poppy seed.
Compare all tick species →Western blacklegged tick — frequently asked questions
What diseases does the western blacklegged tick spread?+
In the far-western US it is the main tick vector of Lyme disease and anaplasmosis. CDC also lists it as a vector of Borrelia miyamotoi, which causes a hard tick relapsing fever. This is educational information only — see a clinician to evaluate any illness after a tick bite.
How is it different from the deer tick people talk about back east?+
It is the West Coast counterpart of the eastern blacklegged (deer) tick (Ixodes scapularis). The two look almost the same; the practical difference is where they live — Ixodes pacificus is found along the Pacific coast, especially in California.
Does the western blacklegged tick carry Rocky Mountain spotted fever?+
No. In the West, Rocky Mountain spotted fever and related spotted fevers are tied to Dermacentor wood and dog ticks — and, in the Southwest, the brown dog tick — not to Ixodes pacificus.
When am I most likely to be bitten?+
Adults quest in the cool, wet winter and spring, while poppy-seed-sized nymphs bite in spring and summer. Because nymphs are so small and easy to miss, they account for much of the region's human infection risk — do a careful tick check after time in oak woodland or on shaded trails.
Sources used
- Where Ticks LiveCDC
The Lone Star tick is widely distributed in the Northeast, South, and Midwest U.S.
- Lyme DiseaseCDC
Spread by blacklegged (deer) ticks — NOT the Lone Star tick. Early sign is often an expanding erythema migrans rash.
- AnaplasmosisCDC
Spread by blacklegged and western blacklegged ticks; treatable with prompt antibiotics.
- TickEncounter Resource CenterUniversity of Rhode Island
Tick identification, prevention, and reporting resources.
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Western blacklegged tick (Ixodes pacificus)
The western blacklegged tick (Ixodes pacificus) is the Pacific Coast vector of Lyme disease and anaplasmosis. Learn to identify it by its dark legs and half-length back shield, plus its US range, life stages, and look-alikes. Educational, not medical advice.