What bacterium causes Lyme disease?

Causative Agent of Lyme Disease

Lyme disease is caused by the spirochete bacteria Borrelia burgdorferi in North America, while in Europe and Asia it is primarily caused by Borrelia afzelii or Borrelia garinii. 1

Bacterial Characteristics and Transmission

• Borrelia burgdorferi sensu lato (sl) is a spirochete bacterium that is helically shaped, motile, and measures 20-30 μm in length and 0.2-0.3 μm in width 1
• In the United States, Lyme disease is specifically caused by Borrelia burgdorferi sensu stricto and more recently identified Borrelia mayonii 1
• The bacteria are transmitted to humans through the bite of infected ticks belonging to the Ixodes genus 1
• The primary vectors in the United States are Ixodes scapularis (blacklegged tick) in the northeastern and midwestern regions and Ixodes pacificus (western blacklegged tick) along the West Coast 1, 2
• In Europe, the primary vector is Ixodes ricinus 1

Geographic Distribution and Epidemiology

• Lyme disease is the most common vector-borne disease in both North America and Europe 1
• In the United States, Lyme disease causes an estimated 300,000 illnesses annually, with cases occurring primarily in the northeast and upper midwest regions 1
• Only Borrelia burgdorferi sensu stricto is found in North America, while Europe has all three pathogenic species (B. burgdorferi sensu stricto, B. afzelii, and B. garinii) 1
• The geographic distribution of Lyme disease is expanding due to ecological and environmental changes 1

Clinical Manifestations

• Lyme disease typically progresses through three stages if untreated: early localized, early disseminated, and late disseminated infection 1
• The classic sign of early localized infection is erythema migrans (EM), an expanding annular lesion >5 cm in diameter that occurs in 70-80% of infected persons 1
• Early disseminated Lyme disease can manifest as multiple EM skin lesions, facial palsy, meningitis, or carditis 1
• Late disseminated disease typically presents as recurrent large-joint arthritis, and less commonly as neurologic manifestations 1
• Clinical manifestations can vary between North America and Europe due to the different Borrelia species involved 3

Diagnosis

• For patients with erythema migrans who live in or have traveled to Lyme-endemic areas, diagnosis can be made clinically without laboratory testing 1
• For all other manifestations, laboratory confirmation is necessary, with the recommended approach being two-tiered serologic testing 1
• The two-tiered approach consists of an enzyme-linked immunoassay (EIA or ELISA) or immunofluorescence assay (IFA), followed by a reflex Western immunoblot if the first test is positive or equivocal 1
• Sensitivity of two-tiered testing is low (30-40%) during early infection but increases to 70-100% for disseminated disease 1
• Specificity is high (>95%) during all stages of disease 1

Clinical Pitfalls to Avoid

• Relying on laboratory testing alone for diagnosis of early Lyme disease with erythema migrans, as serologic tests have low sensitivity during this stage 1
• Failing to consider co-infections, as ticks can transmit multiple pathogens simultaneously (such as Anaplasma phagocytophilum and Babesia microti) 1, 2
• Misinterpreting serologic test results, particularly during the early window period when antibody response is still developing 1
• Overlooking the geographic and seasonal context when evaluating patients with suspected Lyme disease 2