Lyme disease

Lyme disease is a tick-borne bacterial infection caused by members of the Borrelia burgdorferi species complex and transmitted by infected Ixodes ticks. Early illness can include fever, fatigue and an expanding rash called erythema migrans; later involvement can affect joints, the nervous system or the heart. The rash has varied appearances and need not form a textbook bullseye.^[ref]

The bacterial link emerged from a specific line of evidence. In 1982, Willy Burgdorfer and colleagues reported spirochaetes in ticks from an affected area, cultivation of the organisms, and reactions with antibodies from patients with Lyme disease. This connected a vector-associated organism to the human illness, rather than inferring a cause from an association with outdoor exposure alone.^[ref]

Erythema migrans is an expanding skin lesion associated with early Lyme disease, commonly developing near the site of an infected tick bite. The name describes the outward enlargement of the affected area, not movement of the entire patch across the body. Its appearance varies: central clearing may create rings, but a uniform expanding patch can also occur.^[ref]

The time course is part of the observation. Recording the border and size on successive days provides information that a single photograph cannot supply. An immediate small local reaction to a bite and a lesion that enlarges over subsequent days are different patterns. Erythema migrans is not present in every Lyme disease case, so absence of the familiar bullseye image cannot be equated with absence of infection.^[ref]