Western Deer Tick

Ixodes pacificus


The Western deer tick (Ixodes pacificus), actually called the Western Blacklegged tick, can transmit the organisms responsible for causing anaplasmosis and Lyme disease in humans. Wild rodents (such as white-footed mice) and other mammals are likely reservoirs of these pathogens. This tick is distributed along the Pacific coast of the United States. Larvae and nymphs feed on birds and small rodents, while adult ticks feed on deer and other mammals. Both adult and nymphal ticks are known to transmit disease to humans.

Adult Male Adult Female Engorged Female

Hosts of Blacklegged Ticks


This three-host tick tends to have a serial host preference. Larvae feed primarily on small rodents (especially white-footed mice), birds, and reptiles. Nymphs feed on various small rodents, birds, cats, and humans. Adults feed on larger mammals including cats, dogs, and of course, humans.


Most commonly found in the habitats of white-footed mice and white-tailed deer, deer ticks are predominantly found in the leaf litter and taller grasses and shrubs of deciduous forests (e.g., maple and oak) and surrounding grassy areas.


The life cycle of the deer tick takes approximately two years to complete. Their development is dependent on environment and the availability of hosts. Under favorable conditions, they may be capable of developing in less than one year.

All three of the deer tick’s development stages require blood meals from hosts. Deer ticks attach themselves to and feed on one host during the larval stage, another during the nymphal stage and a third during their adult stage. Deer tick larvae and nymphs both molt after feeding.

After laying eggs, female deer ticks die. However, one female is capable of laying up to 3,000 eggs. Six-legged larvae emerge from these eggs and begin to search for a host. Larvae feed for approximately four days before dropping to the ground to molt into nymphs. Resulting nymphs have eight legs and search again for hosts. They, too, will feed and molt into adults.

Disease Transmission

The blacklegged tick is the primary vector of Lyme disease across the northern part of its range. This tick also transmits babesiosis to humans in the northeastern and north-central US, and is known to be one of the tick vectors involved in transmitting human anaplasmosis; formerly known as human granulocytic ehrlichiosis. It is also thought to transmit bovine anaplasmosis to cattle.

It is important to note that the pathogen that causes Lyme disease is maintained by wild rodent and other small mammal reservoirs, and is not transmitted everywhere that the western blacklegged tick lives. In some regions, particularly in the southern US, the tick has very different feeding habits that make it an unlikely vector in the spread of human disease.

Range Distribution

Western Blacklegged ticks are only found on the western coast of the U.S.; throughout Washington, Oregon, and California. Colonies have also been found in Arizona, Utah, and Nevada.

Range Map - Western Blacklegged Deer Tick