Ticks in California do carry Lyme disease, but the risk profile is notably different from that in the Eastern United States. Lyme disease is a bacterial infection caused by the bacterium Borrelia burgdorferi, transmitted to humans through the bite of certain infected ticks. While California has a widespread tick population, the overall incidence of human Lyme disease cases is significantly lower than on the East Coast. Understanding the specific biology of the California tick and its environment is key to assessing the actual threat level. People contract this illness primarily through the bite of an infected tick that has been attached for an extended period.
The Western Blacklegged Tick and Lyme Presence
The species responsible for transmitting Lyme disease in California is the Western Blacklegged Tick (Ixodes pacificus). This tick carries Borrelia burgdorferi, the same pathogen found in the Eastern U.S., but its ecology provides a natural buffer that reduces human infection rates. The bacteria pass to the tick when it feeds on an infected animal, typically a small rodent, and the tick can then transmit the bacteria to its next host, which might be a human.
Transmission rates to humans are comparatively low in California, partly due to the feeding preferences of the immature tick stages. The tiny nymphal stage of the tick, which causes most human infections because it is often overlooked, frequently feeds on the Western Fence Lizard (Sceloporus occidentalis). This lizard plays a unique role in the transmission cycle, acting as a “cleansing” host.
When an infected nymph feeds on the Western Fence Lizard, a protein in the lizard’s blood effectively kills the Borrelia burgdorferi spirochetes. The tick is thus “cured” of the bacteria and cannot transmit Lyme disease to a subsequent host when it molts into an adult. Since a large percentage of the nymphal tick population feeds on these common lizards, the overall number of infected ticks that survive is significantly diminished. This ecological dynamic is a major reason why the incidence of Lyme disease in California is much lower than in other parts of the country.
Defining Geographic Risk Areas in California
The Western Blacklegged Tick is reported in 56 of California’s 58 counties, but the risk of encountering an infected tick is highly localized. The highest risk areas are primarily found in northwestern California, including humid coastal regions and the foothills of the Sierra Nevada mountains.
Counties in the far northern part of the state, such as Trinity, Humboldt, and Mendocino, consistently show the highest average incidence of Lyme disease. These areas feature dense oak woodlands and chaparral environments that provide the moist, shady conditions ticks require to survive. The risk is generally lower in central and southern California, though localized threats exist.
Risk also varies significantly depending on the season and the tick’s life stage. Nymphal ticks, which are about the size of a poppy seed and hardest to spot, are responsible for the majority of human infections. In northwestern California, these nymphs are most active from late March to July, typically peaking in mid-to-late spring.
Adult ticks are larger and active during the cooler, wetter months, generally from late November until early May. While adults can transmit the bacteria, the tiny nymphs are the greater concern because they are harder to detect. Their peak activity coincides with increased human outdoor recreation, creating the greatest risk window.
Mitigation and Proper Tick Removal
Reducing the risk of a tick bite involves simple, consistent preventative measures, especially when spending time in high-risk environments like woodlands and grassy areas.
- Wear long-sleeved shirts and long pants to create a physical barrier.
- Choose light-colored clothing to make ticks easier to spot before they attach.
- Tuck pant legs into socks and wear closed-toed shoes to prevent ticks from crawling up.
- Apply chemical repellents containing at least 20% DEET to exposed skin and clothing.
- Apply products containing 0.5% permethrin to clothing and gear to kill contacting ticks.
- Shower within two hours of returning from a tick habitat to wash off unattached ticks, and perform a full body check.
If a tick is found attached, prompt and proper removal is the best defense against infection. The tick must generally be attached for 36 to 48 hours or more before the Borrelia burgdorferi bacterium can be transmitted, meaning quick removal significantly reduces the risk. Use fine-tipped tweezers to grasp the tick as close to the skin’s surface as possible, ideally by the mouthparts.
Pull upward with steady, even pressure, avoiding twisting or jerking, which can cause the mouthparts to break off. After removal, the bite area and hands should be thoroughly cleaned with soap and water or rubbing alcohol. Dispose of the tick by flushing it down the toilet or placing it in a sealed container.