Lyme disease, caused by the bacterium Borrelia burgdorferi, is an infection transmitted to humans through the bite of an infected tick. While it is more commonly associated with the Northeastern United States, the disease is confirmed to be present in California. The risk profile and specific biological factors in California differ from those on the East Coast, requiring residents and visitors to have localized knowledge. This information aims to clarify the presence, vector, symptoms, and necessary precautions for Lyme disease specific to the California environment.
Confirmed Presence and Geographic Hotspots
Lyme disease is endemic in California, though the overall risk is lower than in high-incidence states in the Northeast and upper Midwest. The California Department of Public Health (CDPH) data shows that the average annual incidence rate is low statewide, but the risk is not evenly distributed. The vast majority of locally acquired cases are concentrated in Northern California, specifically the northern coastal regions and the Sierra Nevada foothills. Counties such as Humboldt, Santa Cruz, and Sonoma report the state’s highest average annual incidence rates. Risk areas typically involve wooded habitats, chaparral, and brushy terrain where the tick vector thrives. While cases are fewer in Southern California, isolated high-risk areas can be found.
The California Lyme Disease Vector
The specific tick responsible for transmitting the Lyme disease bacteria in California is the Western Black-legged Tick, Ixodes pacificus. This tick is found in 56 of the state’s 58 counties and attaches to humans more frequently than any other local tick species. The infection rate for Borrelia burgdorferi in this tick is significantly lower than in the Eastern Black-legged Tick (Ixodes scapularis).
The nymphal stage poses the greatest threat to humans because of its tiny size, comparable to a poppy seed, making it difficult to detect. The nymphal stage is most active in the spring and early summer, typically between April and June, which is when most human infections occur. Adult ticks are more commonly encountered from late fall through early summer.
Identifying Symptoms and When to Seek Testing
Early symptoms of Lyme disease appear within three to 30 days following a bite from an infected tick. These initial signs are often flu-like and include fatigue, fever, chills, headache, and muscle or joint pain. The most recognizable sign is the Erythema Migrans (EM) rash, often described as a “bullseye,” which is an expanding red lesion.
In California, the characteristic bullseye rash occurs in fewer than 50% of confirmed cases, making diagnosis challenging. The EM rash may appear simply as a uniformly red or blue-red expanding patch without the classic central clearing. If untreated, the infection may progress to cause severe issues, such as facial palsy, arthritis, or heart palpitations. Anyone experiencing these symptoms after spending time in known tick habitats should inform their physician of potential tick exposure.
Effective Prevention and Post-Exposure Steps
Preventing Lyme disease begins with minimizing tick exposure when recreating in wooded or brushy areas, especially during spring and summer. Wearing light-colored clothing makes it easier to spot ticks before they attach. Tucking pants into socks and shirts into pants creates a physical barrier. The use of an Environmental Protection Agency (EPA)-registered insect repellent containing ingredients like DEET or Picaridin is highly effective. After returning indoors, perform a thorough tick check, and showering within two hours can help wash off unattached ticks.
If a tick is found attached, remove it immediately using fine-tipped tweezers. Grasp the tick as close to the skin surface as possible and pull upward with steady, even pressure. Transmission typically requires the tick to be attached for 24 to 36 hours or more, so prompt removal is crucial. For a high-risk bite (an Ixodes tick attached for at least 36 hours in an endemic area), a single dose of prophylactic antibiotics may be recommended; consult a medical professional immediately.