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Western Black-legged Tick

(Ixodes pacificus)

To the uneducated layperson's eye the western black-legged tick, Ixodes pacificus, is practically identical to its eastern cousin, Ixodes scapularis. It is the primary vector to humans and domestic animals of the disease agents causing Lyme disease and granulocytic ehrlichiosis (HGE) on the West Coast.

Unique enzootic cycles have been identified on the West Coast. Western fence lizards (Sceloporus occidentalis) and Columbian Black-tailed deer (Odocoileus hemionus columbianus) are major hosts of I. pacificus. Antibodies to Borrelia burgdorferi  have been detected in 12 species of wildlife in California. Species exhibiting the highest seropositivity rates include brush rabbits and black-tailed jackrabbits (100% and 90% respectively). Dusky-footed woodrats (Neotoma fuscipes) and California kangaroo rats (Dipodomys californicus) serve as reservoirs of the Lyme disease spirochete.
Like I. scapularis, I. pacificus has catholic feeding habits, but one of its favorite hosts - the western fence lizard - is reservoir-incompetent for Borrelia burgdorferi sensu lato (s.l.). In one study at least 94% of the lizards were infested by ticks, many by 15 or more. This is probably the primary reason for the relatively low prevalence of infection with B. burgdorferi in I. pacificus nymphs and adults.
The tick burden and spirochete infection of birds, especially ground-dwelling birds, in the Sierra Nevada foothills suggests that they may be involved in a local B. burgdorferi enzootic cycle and likely participate in the transport of ticks and spirochetes to other locations.
I. pacificus takes a minimum of 3 years to complete its life cycle in northwestern California. Adults lay their eggs after mating on deer. After larvae hatch out they feed mainly on small rodents and lizards. Nymphs, the stage associated with most cases of human disease, are found primarily in leaf litter under deciduous trees. Favoring cooler, moister climatic conditions, they typically become active by mid-March, peak by early May, and are absent by late July to mid-August when the west coast is in the middle of its hot, dry season. Nymphs may be found on the trunks of trees and on downed logs during spring and summer.
Distribution of Bartonella among ixodid ticks, both adult and nymphal stages, appears widespread in California. The rickettsial agent of human monocytic (HME) ehrlichiosis has also been found.
California's climate ranges from desert to temperate rain forest. Ticks are active year-round except in extremely dry areas.

Reprint courtesy of Professor Robert S. Lane, Phyllis Mervine, et al.

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