Western blacklegged tick “Deer tick”

Ixodes pacificus

Photo by CDC-Western Blacklegged Tick

This hard tick is not known to occur in Colorado. It is documented primarily along the Pacific coast of the U.S., particularly northern California and inland to Eastern Oregon, western Utah and Arizona. It is known to transmit Lyme disease, Anaplasmosis, Bartonella; and may carry Erhlichiosos (HME), but transmission capability is still in question. Nymphs often feed on lizards, small animals as well as birds; adults on large mammals, commonly deer, canids, and horses. 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 adults. Many people that are bitten do not recall a bite due to their small size at the nymph stage. It is a common human biter.

Visit University of Rhode Island’s TickEncounter Resource Center for additional photos to help in the identification of ticks.


CDC Distribution Map 2019: This map provides general insight into the expected distribution of these human-biting ticks in the contiguous United States. Populations of ticks may be found outside noted areas. Naturally occurring populations of the Western Blacklegged tick are not known to occur in Alaska or Hawaii.