Lyme disease is a tickborne disease caused by the bacteria Borrelia burgdorferi.  It was first identified in California in 1978 from a Sonoma County hiker.  While incidence of Lyme disease (number of cases adjusted per population) is highest in the northwestern part of the state, human cases have been reported from many regions of California.

In California, the western black-legged tick, Ixodes pacificus, is the only tick that transmits Lyme disease to people.  This tick has three life stages, a larval, nymphal and adult stage.  People become infected with Lyme disease via the bite of an adult female or a nymphal western-black legged tick.  Infection prevalences of B. burgdorferi in the western black-legged tick are approximately 1-2% in adults, 2-15% in nymphs, and 0% in larvae.  People are at highest risk of acquiring Lyme disease from nymphal ticks due to their small size and higher infection prevalence. Lyme disease is most commonly reported in spring and early summer in California, when nymphal ticks are most abundant.  Adult western black-legged ticks are most active in fall and winter.

For more information on Lyme disease, please visit the California Department of Public Health website at: