On the Pacific Coast, the bacteria that causes Lyme disease are transmitted to humans by the western black-legged tick, Ixodes pacificus. The bite of these ticks spreads the bacterium that causes Lyme disease. These ticks, which normally feed on the white-footed mouse, deer, other mammals, and birds, transmit Lyme disease to humans.
Ticks in the young stage (nymph) are the main source of transmitting Lyme disease bacteria to humans. Since they are tiny (less than 2mm) and rarely noticed, nymphs have the necessary time to feed and transmit the bacteria, typically after feeding for 18 hours or more. Also, nymphal ticks feed during the Spring and Summer months when people spend the most time outdoors.
Adult ticks can also transmit Lyme disease bacteria. Because adult ticks are larger and more noticeable, they are more likely to be removed from a person’s body within a few hours and therefore are less likely to have had enough time to transmit the bacteria. Moreover, adult Ixodes ticks are most active during the cooler months of the year when people spend less time outdoors and wear more clothing which provides added protection.