Lyme Disease in California

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California Department of Public Health - Vector-Borne Disease Section

What is Lyme Disease?

Transmission of the bacteria, *Borrelia burgdorferi*, that cause Lyme disease in California

Lyme disease is caused by a bacteria (spirochete) called Borrelia burgdorferi.

People get Lyme disease when a tick infected with this bacteria attaches and feeds on them. Lyme disease was first recognized in the northeastern United States in the 1970s.

Lyme disease has been reported from many areas of the country, including California. CDC: Lyme disease in the United States (maps/data)

Lyme Disease Incidence in California

Click on a county below for details on the number of human cases and Lyme disease incidence from 2013-2022. Place cursor over the button on the bottom left of the map to view the legend.

Lyme disease incidence in California from 2013-2022. Cases are reported based on county of residence, which does not necessarily reflect the county of infection.

How Do You Get Lyme Disease?

A person can get Lyme disease if they are bitten by a western blacklegged tick (Ixodes pacificus) that is infected with the Lyme disease bacteria (Borrelia burgdorferi). An infected tick must be attached and feeding for at least 24 hours before it can transmit the bacteria that can cause Lyme disease.

The western blacklegged tick has three life stages: larva, nymph, and adult.

The western blacklegged tick (*Ixodes pacificus*). From left to right: larva, nymph, adult male, adult female.

Western fence lizard (*Sceloporus occidentalis*) wiith nymphal western blacklegged ticks attached. When an infected tick feeds on lizard blood, a substance in the lizard’s blood kills the bacteria in the tick, making the tick uninfected in the next life stage.

Larvae and nymphs feed on the blood of small rodents, rabbits, lizards, birds, and occasionally large mammals.

Adults feed on the blood of large mammals, mainly deer.

Both nymphal and adult western blacklegged ticks can bite humans and transmit the bacteria that causes Lyme disease. Nymphs can pose a greater risk of transmitting Lyme disease bacteria to humans because they are small (<1mm or about the size of a poppy seed) and can be difficult to see.

In some areas of California, a higher percentage of nymphal ticks carry Lyme disease bacteria than adult ticks.

Tick Surveillance in California

Symptoms of Lyme Disease in Humans

Lyme disease can affect many different parts of the body. Lyme disease can start as a mild, flu-like illness and can develop into severe chronic health problems over time.

Early Symptoms of Lyme Disease Can Include:

*Photo courtesy of Centers for Disease Control and Prevention.

Red, expanding skin rash (erythema migrans or EM) that may vary in size and appearance
chills and fever
headache
swollen lymph nodes
muscle and joint pain
weakness of some muscles in the face
heart irregularities

The EM rash appears up to 30 days after the bite of an infected tick. One or more EM rashes can occur, not necessarily at the site of the tick bite. The rash can come before, during, or after flu-like symptoms. The rash may not be noticed in some cases due to skin tone or where on the body the rash is located. Occasionally, an allergic reaction to the tick bite can occur on the skin and may be mistaken for an EM. The allergic reaction is different from an EM rash because it happens within minutes to hours after the tick bite and does not spread.

Later Signs and Symptoms

If left untreated, arthritis or nervous system signs can develop in some Lyme disease patients. Arthritis is most likely to appear as bouts of pain and swelling, usually in one or more large joints, especially the knees. Nervous system abnormalities can include numbness, tingling, or pain in the arms and legs, or difficulties in memory and the ability to concentrate.

Swollen knee from arthritis. *Photo courtesy of Centers for Disease Control and Prevention.

Lyme disease can be successfully treated with antibiotics, especially in the early stages.

Preventing tick bites is the best way to ensure you won't be infected.