Borrelia burgdorferi

Borrelia burgdorferi is a spirochete — a long, corkscrew-shaped bacterium — and the causative agent of Lyme disease, the most common vector-borne illness in the United States. It is transmitted by the bite of infected Ixodes ticks (deer ticks), with cases concentrated in the Northeast and upper Midwest. The white-footed mouse serves as a key reservoir, and ticks generally must remain attached for an extended period before transmission occurs.

Like other spirochetes, B. burgdorferi is too thin to be seen reliably on standard Gram stain — a classic exam point. It can be visualized with dark-field microscopy or special stains, but diagnosis in practice is usually clinical plus serologic (ELISA confirmed by Western blot).

Lyme disease progresses in stages. Early localized disease features erythema migrans, the slowly expanding "bull's-eye" rash at the bite site, often with flu-like symptoms. Early disseminated disease can bring multiple rash lesions, facial nerve (Bell) palsy — classically bilateral — AV nodal block and carditis, and meningitis. Late disease is dominated by migratory arthritis, especially of the large joints such as the knee, along with chronic neurologic findings. Lyme arthritis is an important item on the differential for a swollen, painful joint.

The USMLE Step 1 tests B. burgdorferi's morphology, tick vector, stage-by-stage manifestations, and its association with myocarditis and AV block, while the CCMA exam includes Lyme disease among the arthritic and joint disorders medical assistants should recognize.