Causative Agent and Epidemiology of Lyme Disease

Bacterial Characteristics and Transmission

The American College of Physicians and the Infectious Diseases Society of America acknowledge that 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, 2
Borrelia burgdorferi sensu lato is a spirochete bacterium that is helically shaped, motile, and measures 20-30 μm in length and 0.2-0.3 μm in width, as described by the European Society of Clinical Microbiology and Infectious Diseases 3
The Centers for Disease Control and Prevention note that in the United States, Lyme disease is specifically caused by Borrelia burgdorferi sensu stricto and more recently identified Borrelia mayonii 2
The bacteria are transmitted to humans through the bite of infected ticks belonging to the Ixodes genus, according to the Clinical Infectious Diseases journal 1
The primary vectors in the United States are Ixodes scapularis and Ixodes pacificus, as identified by the Emerging Infectious Diseases journal 2

The World Health Organization and the Centers for Disease Control and Prevention report that 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, as estimated by the Emerging Infectious Diseases journal 2
The European Centre for Disease Prevention and Control notes that 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) 3
The geographic distribution of Lyme disease is expanding due to ecological and environmental changes, as reported by the Emerging Infectious Diseases journal 2

The Infectious Diseases Society of America states that Lyme disease typically progresses through three stages if untreated: early localized, early disseminated, and late disseminated infection 2
The classic sign of early localized infection is erythema migrans, an expanding annular lesion that occurs in 70-80% of infected persons, as described by the Emerging Infectious Diseases journal 2
Early disseminated Lyme disease can manifest as multiple skin lesions, facial palsy, meningitis, or carditis, according to the Emerging Infectious Diseases journal 2
Late disseminated disease typically presents as recurrent large-joint arthritis, and less commonly as neurologic manifestations, as reported by the Emerging Infectious Diseases journal 2

The American College of Physicians recommends that for patients with erythema migrans who live in or have traveled to Lyme-endemic areas, diagnosis can be made clinically without laboratory testing 2
For all other manifestations, laboratory confirmation is necessary, with the recommended approach being two-tiered serologic testing, as suggested by the Emerging Infectious Diseases journal 2
The two-tiered approach consists of an enzyme-linked immunoassay or immunofluorescence assay, followed by a reflex Western immunoblot if the first test is positive or equivocal, as described by the Emerging Infectious Diseases journal 2
Sensitivity of two-tiered testing is low during early infection but increases to 70-100% for disseminated disease, as reported by the Emerging Infectious Diseases journal 2
Specificity is high during all stages of disease, according to the Emerging Infectious Diseases journal 2

The Clinical Infectious Diseases journal warns against relying on laboratory testing alone for diagnosis of early Lyme disease with erythema migrans, as serologic tests have low sensitivity during this stage 2
The Emerging Infectious Diseases journal advises against failing to consider co-infections, as ticks can transmit multiple pathogens simultaneously 1, 2
The Infectious Diseases Society of America cautions against misinterpreting serologic test results, particularly during the early window period when antibody response is still developing 2

Ixodes scapularis (black‑legged/deer tick) is the principal vector in the eastern United States. 4
Ixodes pacificus (western black‑legged tick) transmits Lyme disease along the Pacific coast and in California. 4
Ixodes ricinus is the main vector throughout Europe. 5
All Ixodes species that belong to the Borrelia burgdorferi sensu lato complex are capable of transmitting the various genospecies of the spirochete. 5

Tick larvae and nymphs primarily feed on small rodents (e.g., Peromyscus) and birds, whereas adult ticks feed on larger mammals such as deer and domestic animals. 5
The majority of human Lyme disease cases arise from bites by infected nymphs rather than adult ticks. 6

In highly endemic areas of the northeastern United States, approximately 15 %–30 % of questing I. scapularis nymphs carry B. burgdorferi. 6
In the western United States, up to 14 % of I. pacificus nymphs are infected with B. burgdorferi. 6
In the southern United States, infection prevalence in I. scapularis ticks is generally low, ranging from 0 % to 3 %. 6

Ixodes ticks feed for an extended period, often several days to more than one week. 5
Transmission of B. burgdorferi to humans typically requires a minimum of 36–48 hours of tick attachment. 4

The geographic range of I. scapularis is expanding slowly in the northeastern and upper north‑central United States, a trend linked to the repopulation of white‑tailed deer. 6