Erythema Migrans: Not Always a Bulls-eye

Erythema migrans (EM) is the characteristic skin lesion in Lyme disease, occurring in either early localized disease (most often 7 to 14 days after the tick bite, but ranging from 3 to 30 days) or early disseminated disease (weeks to months after the bite). 

  • Unlike EM, early erythema at the tick bite site (including immediately after removing the tick) represents an allergic reaction to antigens in tick saliva. This reaction typically appears and expands over hours (usually to about the size of a quarter) then disappears within a couple of days. In contrast, EM lesions are larger and expand more slowly before disappearing over weeks.
  • EM occurs in approximately 80% of adult patients with Lyme disease, but only about 25% of patients with EM recall the tick bite.

EM lesion appearance: 

  • EM can occur anywhere on the body. In early cutaneous disease, EM occurs at the site of the tick bite (with rare exceptions). Multiple EM lesions, which can be seen in patients with early disseminated disease, are a sign of spirochetemia, not multiple tick bites.
    • In adults with early cutaneous disease, EM is most often found near the axilla, inguinal region, popliteal fossa, or at the belt line. In children, the most common areas are the head and neck (especially younger children), arms and legs, and back. 
  • EM lesions are most often patches of uniform erythema. The classic bulls-eye target lesion only occurs about one third of the time owing to the need for considerable expansion to occur before central clearing appears (i.e., usually not yet present in the first days of illness).
    • In patients with darkly pigmented skin, the erythema of an EM lesion may be faint or appear as hyperpigmentation.
  • EM lesions typically expand slowly over the course of 7-10 days, averaging around 15 cm (and almost always exceeding 5 cm) and sometimes even much larger.
  • EM lesions are not particularly painful, but may occasionally burn or itch. Lesions are often warm to the touch.
  • EM lesions can rarely have vesicular or necrotic centers, occurring in 5% of patients. 

Other symptoms that frequently coincide with EM in patients with Lyme disease:

  • Fatigue
  • Subjective fever and/or chills
  • Anorexia
  • Headache
  • Neck stiffness
  • Myalgias
  • Arthralgias
  • Regional lymphadenopathy

Last, it’s important to recognize that serologic testing for Lyme disease is of little use in patients with EM. Two-tier serologic testing for antibodies to B. burgdorferi (typically ELISA followed by a confirmatory Western blot) has poor sensitivity in patients with EM during the acute phase of infection (positive results occur in only 25-40% of patients without evidence of dissemination). As such, the diagnosis of EM should be made on clinical grounds alone when at least one characteristic EM lesion is present in a patient who lives in or has recently traveled to an endemic area (like New England!)

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