Erythema migrans is a unique cutaneous eruption that characterizes the localized or generalized early stage of Lyme disease (caused by Borrelia burgdorferi) (Figure 6–18) (see also Chapter 34). Three to 32 days (median: 7 days) after a tick bite, there is gradual expansion of redness around the papule representing the bite site. The advancing border is usually slightly raised, warm, red to bluish-red, and free of any scale. Centrally, the site of the bite may clear, leaving only a rim of peripheral erythema, or it may become indurated, vesicular, or necrotic. The annular erythema usually grows to a median diameter of 15 cm (range: 3–68 cm, but virtually always greater than 5 cm). It is accompanied by a burning sensation in half of patients; rarely, it is pruritic or painful. Multiple secondary annular lesions similar in appearance to the primary lesion but without indurated centers and generally of smaller size will develop in 20% of patients but may be even more common in European patients. In the southeastern United States, similar lesions are seen in patients who are not as ill and who tend to have classic central clearing of their lesions. These patients have negative Lyme serology tests. This condition has been called Southern tick-associated rash illness (STARI). This illness is transmitted by the lone star tick Amblyomma americanum and some cases have been shown to be caused by Borrelia lonestari, for which the white tail deer is the animal reservoir. Systemic symptoms are uncommon in STARI and the skin lesions respond to the same antibiotic agents used for Lyme disease, suggesting that a spirochete (probably as yet unidentified Borrelia species) is causative in all these cases.
Erythema migrans on trunk. Annular plaque with central clearing and central puncta from the bite. (Reproduced, with permission, from Soutor, Hordinsky MK. Clinical Dermatology. The McGraw-Hill Companies; 2013.)
Without treatment, erythema migrans and the secondary lesions fade in a median of 28 days, though some may persist for months. Ten percent of untreated patients experience recurrences over the ensuing months. Treatment with systemic antibiotics (see Table 34–4) is necessary to prevent systemic involvement. However, only 60–70% of those with systemic involvement have experienced erythema migrans.