Tick basics awaiting entomology signoff
Western blacklegged tick
Ixodes pacificus
Treat this as the West Coast blacklegged tick: visually similar to I. scapularis, but range and official local surveillance are the practical clue.
I
approx. 3-4 mm unfed
Size by life stage
- Adult female
- approx. 3-4 mm unfed source caveated editorial
- Adult male
- approx. 2-3 mm unfed source caveated editorial
- Nymph
- approx. 1-2 mm; reader copy should emphasize "very small" rather than exact measurement source caveated editorial
- Larva
- approx. 0.5-1 mm; larvae have 6 legs source caveated editorial
How to tell it apart
- Scutum: Plain dark scutum similar to blacklegged tick; no ornate white pattern.
- Color (unfed): Adult female can show reddish-brown body behind a dark scutum; adult males look darker overall.
- Color (engorged): Engorged body can become larger, pale gray, tan, or bluish; western/eastern ID from a swollen body alone is unreliable.
Where it lives
Region: Established primarily on the Pacific Coast, especially California, Oregon, and Washington.
Habitat preference: Wooded, brushy, and grassy edge habitats in western coastal states; local risk varies strongly by habitat and county.
Hosts: humans, dogs, lizards, rodents, deer, birds, wildlife
Where this species shows up in our state guides
Primary species in: California, Washington
Disease associations
- can transmit Lyme disease
- associated with anaplasmosis in some western contexts
Look-alikes
- blacklegged deer tick
- american dog tick
- small dark nymphs
- skin specks in low resolution photos
Sources
- CDC Lyme Disease Causes
- CDC Western Blacklegged Tick Surveillance
- Oregon Health Authority Ticks
- PA Tick Research Lab Western Blacklegged Tick