Lyme disease is an infection caused by a bacterium called Borrelia burgdorferi. In the western United States it is transmitted to humans through the bite of the Western Black-Legged Tick (Ixodes pacificus). The disease is named after the town of Old Lyme in Connecticut, where some of the first human cases were detected in the early 1970's. It is not new to the U.S., it had simply never been diagnosed in people with symptoms, which had been attributed to something else. The bacterium has been found in humans, dogs, horses, cattle and in wild animals such as mice, squirrels and other small mammals, but rarely in cats. Ticks pick up the disease when they feed on mice and woodrats, which serve as hosts or reservoirs for the bacterium and do not become ill. In this way, ticks spread the disease from animal to animal and to people by biting and feeding.
Lyme disease was first reported in California in 1978. It is now the most common tick-borne disease in California and the United States and is the most commonly reported vectorborne illness in the U.S. In 2014, it was the fifth most common nationally notifiable disease in the U.S. However, the disease does not occur nationwide and is concentrated most heavily in the northeast and upper Midwest, which account for over 96% of cases reported to the Centers for Disease Control (CDC). Studies by the CDC estimate that around 30,000 people are diagnosed with Lyme disease each year in the U.S.
Western Black-Legged Ticks have been found in 56 out of the 58 counties in California. It can be found in the Sierra Nevada foothills and the coastal range, mostly above elevations of 500-700 feet. In Yuba County, these ticks are commonly dispersed throughout the foothills. In Sutter County, they can be found in the Sutter Buttes, but only at the highest elevations. In some areas of the U.S. where Lyme disease is more common (mainly the northeast), between 15% and 25% of deer ticks (Ixodes scapularis, which do not occur in California) can carry the bacterium. In California, only about 1% to 2% of adult Ixodes pacificus ticks and 2% to 15% of the nymphal ticks, on average, are infected with Borrelia burgdorferi. Nymphal ticks are responsible for most Lyme disease transmission. In some areas north of the San Francisco Bay area, infection rates in ticks can be as high as 5%. In foothill areas of eastern Butte County, up to 1 in 4 collected ticks have been found to be infected with the bacterium. Some areas can have very high rates of infected ticks, such as Mendocino County with 41% of nymphal ticks infected. Infection rates will vary somewhat from year to year.
Early recognition of Lyme disease is very important. Ticks need to be feeding and attached to your skin for 24 hours or more before the bacterium will leave the tick and enter your body. If you can remove the tick within 24 hours of attachment, it may reduce your chances of contracting Lyme disease. Sometimes tick bites go unnoticed, so it is important to watch for early symptoms. One of the first symptoms is a rash that slowly enlarges, usually at the site of the bite, from about the size of a half dollar or larger to a large circular rash with a clear area in the center that may have a bull's-eye appearance. It may appear in 7 to 10 days or sometimes up to 30 days after the tick bite, and will only appear in about 60% of cases. Some people with Lyme disease never report seeing any kind of rash. This rash should not be confused with redness or an allergic reaction to the bite, which may appear within minutes or a few hours at the site of the bite. This is usually not a sign of Lyme disease. In addition to the rash, flu-like symptoms such as fever, body aches, vomiting and fatigue may be experienced. If you have been to an area where ticks occur or have found a tick attached to your skin, or believe you have been bitten by a tick and have any symptoms of Lyme disease, you should visit your physician immediately.
Early recognition of Lyme disease is very important, especially in relation to treatment options. Most Lyme disease cases can be successfully treated with antibiotics if treatment is started early. If the disease goes untreated, later stages can be difficult to treat. As time goes by, migratory pain in the joints, tendons, muscles and bones may occur. Long term complications may develop weeks to months later and can include disorders of the heart and nervous system. Months to years after contracting Lyme disease, intermittent joint pain (Lyme arthritis) may become chronic, causing erosion of cartilage and bone.
Other tick-borne diseases such as Human Granulocytic Ehrlichiosis, Babesiosis and Rocky Mountain Spotted Fever occur in California and the U.S. but are much less prevalent than Lyme disease, especially in California.