Last updated on: January 4th, 2023

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Lyme disease is a zoonosis transmitted by the bite of infected ticks of the Ixodes species. It represents the most common reportable vector-borne disease in the United States. Lyme disease is primarily caused by the spirochete Borrelia burgdorferi in the United States, and B. afzelii, B. burgdorferi, and B garinii in Europe and Asia. The common reservoirs for B. burgdorferi are mice, chipmunks, and birds, while deer are important in sustaining the life cycle of the vector ticks. The species Ixodes scapularis is distributed in eastern and north central regions of North America, while Ixodes ricinus resides in Europe and Western Asia.


Transmission occurs when the ticks, which have acquired B. burgdorferi by feeding on an infected animal, spread the infection to a human during a blood meal. The ticks feed once during each of the three stages of their life cycle (larva, nymph, and adult) but transmission is most likely during the smallest stage (nymphal). This happens because nymphal forms tend to feed on people for enough days (two days) to transmit the infectious dose of bacteria, while adult forms rarely remain attached for extended periods of time. Nymphs are abundant in the spring and early summer which corresponds to the peak season incidence.

Transmission can also rarely occur through direct contact with freshly infected blood.

Clinical stages

The clinical manifestations of Lyme disease can be divided into three phases: early localized, early disseminated, and late disease. The clinical features of each stage can overlap, and the late stage may not be preceded by a history of early localized or disseminated Lyme disease.

While the essential characteristics of the disease are the same worldwide, there are some regional differences in the clinical stages of the disease, mainly between the disease found in the United States and that found in Europe and Asia. These regional differences consist mainly of the frequency and manifestation of systemic symptoms and the rate of illness progression.

Early localized disease

Early localized disease usually occurs a few days to one month after the tick bite; it is characterized by the appearance of the characteristic skin lesion, erythema migrans, with or without constitutional symptoms.

Early disseminated disease

Early disseminated disease usually occurs weeks to months after the tick bite. It is characterized by multiple erythema migrans lesions and extracutaneous signs including neurological conditions such as cranial-nerve palsy, aseptic meningitis, peripheral neuropathy, radiculopathy as well as carditis. It is due to hematogenous dissemination of spirochetes.

Late disease

Late disease usually occurs months to years after the tick bite. It is characterized by musculoskeletal symptoms such as intermittent monoarticular or oligoarticular arthritis; less than 10 percent of untreated patients develop persistent monoarthritis, usually affecting the knee. Neurological conditions, such as subtle neuropathies, may occur in this stage.

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