Lyme disease,tick-bornebacterial disease that is named for thetown in theU.S. state ofConnecticut in which it was first observed. Thedisease was first conclusively identified in 1975 and has since been identified in every region of theUnited States and inEurope, Asia, Africa, and Australia. It is the most common tick-borne disease inNorth America and Europe.

Causes and transmission

Lyme disease is caused by several closely relatedspirochetes (corkscrew-shaped bacteria), including primarilyBorrelia burgdorferi in the United States, whichB. mayonii being a much less frequent cause of the disease there.B. afzelii andB. garinii are the primary causes ofinfection in Europe and Asia. The spirochetes aretransmitted to the human bloodstream by the bite of various species ofticks. In the northeastern United States, the carrier tick is usuallyIxodes scapularis (I. dammini); in the West,I. pacificus; and in Europe,I. ricinus.

Ticks pick up thespirochete by sucking the blood of deer or other infected animals.I. scapularis mainly feeds on white-taileddeer (Odocoileus virginianus) and white-footedmice (Peromyscus leucopus), especially in areas of tall grass, and is most active in summer. The larval and nymphal stages of this tick are more likely to bite humans than are the adult and are therefore more likely to cause human cases of the disease.

Britannica Quiz

44 Questions from Britannica’s Most Popular Health and Medicine Quizzes

Symptoms and disease course

In humans, Lyme disease progresses in three stages, thoughsymptoms and severity of illness vary depending on which type ofBorrelia is involved. InB. burgdorferi infections, the first and mildest stage is characterized by a circularrash in a bull’s-eye pattern that appears anywhere from a few days to a month after the tick bite. The rash is often accompanied by flulike symptoms, such asheadache,fatigue, chills, loss ofappetite,fever, and aching joints or muscles.

The majority of persons who contract Lyme disease experience only these first-stage symptoms and never become seriously ill. A minority, however, will go on to the second stage of the disease, which begins two weeks to three months after infection. This stage is indicated byarthritic pain that migrates from joint to joint and by disturbances ofmemory, vision, or movement or other neurological symptoms. The third stage of Lyme disease, which generally begins within two years of the bite, is marked by cripplingarthritis and by neurological symptoms that resemble those ofmultiple sclerosis. Symptoms vary widely, however, and some persons experience facial paralysis,meningitis, memory loss, mood swings, and an inability to concentrate.

Treatment

Because Lyme disease often mimics other disorders, itsdiagnosis is sometimes difficult, especially when there is no record of the distinctive rash. Early treatment withantibiotics is important in order to prevent progression of the disease to a more serious stage. More powerful antibiotics are used in the latter case, though symptoms may recur periodically thereafter.

Incidence

Cases of Lyme disease have increased significantly in the 21st century. The U.S.Centers for Disease Control and Prevention (CDC) reported more than 63,000 confirmed cases in 2022, a dramatic increase from the early 2000s, when about 20,000 cases were reported annually. Health experts suspect that actual case numbers may be much higher, due to underreporting.

Access for the whole family!

Bundle Britannica Premium and Kids for the ultimate resource destination.

Subscribe

The growing incidence of Lyme disease isattributed to multiple factors. Lyme disease is considered an indicator ofclimate change, owing to the fact that warming temperatures are associated with an expansion in thegeographic range and increased activity of theIxodes ticks that transmit the disease. Populations of white-footed mice have also increased, potentially leading to more ticks carrying the causativebacteria. These factors are suspected to have contributed to a rise in Lyme disease cases particularly in areas where it was previously uncommon. Moreover, changes in human activity, such as increased numbers of people spending time outdoors in areas where ticks are present, increases the risk of bites and exposure.