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Western blacklegged tick

Tick ID

Western blacklegged tick

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Western blacklegged tick 

Ixodes pacificus


This western blacklegged tick is documented primarily along the Pacific coast of the US, particularly northern California and inland to eastern Oregon, western Utah and Arizona. It is not known to have an established population in Colorado as records describe only one distant collection from a human.


Nymph stage of the tick often feed on lizards, small animals as well as birds; adults on large mammals, commonly deer, dogs, horses and humans. Stages most likely to bite humans are nymphs and adult females. There is a growing concern for Lyme disease regarding these ticks and some of their host species, gray squirrels and birds, in urban areas such as city parks in California. The western fence lizard has been found to carry a protein in its blood that kills the Lyme bacterium in infected nymphal ticks when they feed, reducing infection rate in adult ticks. Many people that are bitten do not recall a bite due to their small size at the nymph stage.

Vector Status

This tick is a competent vector for many human pathogens. It may transmit the agents of Lyme disease, B. miyamotoi (TBRF), Anaplasma, Bartonella; it may carry Ehrlichiosis (HME), but transmission capability is still in question.


H Joel Hutcheson, James W Mertins, Boris C Kondratieff, Monica M White, Ticks and Tick-Borne Diseases of Colorado, Including New State Records for Argas radiatus (Ixodida: Argasidae) and Ixodes brunneus (Ixodida: Ixodidae)Journal of Medical Entomology, tjaa232,

Centers for Disease Control and Prevention

Borrelia miyamotoi Infections in Small Mammals, California, USA

Western blacklegged tick

Tick ID
CDC photo, James Gathany. Western blacklegged tick, adult female.

Western blacklegged tick life stages

Western blacklegged tick Life satges-CDC photo
CDC photo, Western blacklegged tick life stages

Western blacklegged tick distribution in US: 2019

Pacific Blacklegged tick distribution Map in US
CDC Distribution Map: This map is not meant to represent risks for a specific tick-borne disease, because disease transmission is influenced by multiple factors beyond mere tick presence. This map has been designed to answer the question “What ticks should I be concerned about at a regional scale?” Please consult a local public health authority or USDA Agricultural Extension Office to determine more specific information at the state, county, or municipal level. Background data for this map is from the US National Atlas. Naturally occurring populations of the Western blacklegged ticks are not known to occur in Alaska or Hawaii.