Erythema Migrans

Erythema migrans is a characteristic skin lesion associated with Lyme disease, a tick-borne disease caused by infection with the spirochete Borrelia burgdorferi; a tick must be attached for 2 to 3 days to transmit the spirochete. Lyme disease has three stages, localized, disseminated and persistent. Erythema migrans is a symptom of the first stage. The rash begins days to weeks after the initial tick bite. Many patients do not recall being bitten.

Erythema migrans begins as an erythematous macule or papule, which expands over a period of days to weeks, eventuating in a large lesion (at least 5 cm, and averaging 16 cm), often with central clearing (one third of US cases and two thirds of European cases). The very center may be discolored or scaly. The lesions is most likely an inflammatory response in the skin to the presence of the spirochete. The rash may be asymptomatic, or pruritus and burning may be present. There may be associated systemic symptoms that may be described as "flu-like"

With what can it be confused?

The differential diagnosis includes cellulitis, tinea corporis, fixed drug eruption, insect bite (other than tick), and granuloma annulare. Cellulitis is warm and tender. Tinea corporis has scale and will exhibit hyphae on a KOH preparation. Fixed drug eruptions do not exhibit central clearing and will recur rapidly following exposure to the triggering drug. Insect bites usually do not reach the size of erythema migrans, and they usually clear more quickly. Granuloma annulare is a granulomatous lesions with an elevated border. Unlike erythema migrans, granuloma annulare is persistent.

How is it diagnosed?

Most affected patients can recall a history of exposure to wooded areas where ticks might be encounters. Some can recall a tick bite, though this is rare. In the acute stage, many will report systemic flu-like symptoms. The skin lesions must be at least 5 cm and will usually be noted to be "expanding". The morphology of the lesions is variable. It may have a raised or a macular border; the center may be clear, bright red, ulcerated, necrotic or vesciular, and it may have a central punctum; and multiple annular rings may be present. The lesion is usually located at an area accessible to a tick bite, such as the waistband and extremities.

Skin biopsy is usually not necessary. Culturing Borrelia burgdorferi from biopsies is not usually practical. Antibody testing is not recommended while the rash of erythema migrans is present, as it may be too early for antibodies to have developed. At later time points, if suggested by symptoms and an appropriate exposure history, the CDC recommends the following laboratory tests for Lyme disease:

If symptoms consistent with Lyme disease are present, an antibody titer should be performed (either total or separate IgG and IgM).

Positive results are confirmed by Western blot.

If a patient has a low likelihood of having been exposed to ticks (i.e. has not been in an endemic area), then these laboratory tests are probably of low positive predictive value.

How is it treated?

If erythema migrans is present, the patient requires antibiotics active against Borrelia burgdorferi. Doxycycline, 100 mg orally twice daily, for 30 days is the recommended regimen for isolated skin manifestations in Lyme disease. Intravenous antibiotics are recommended for neurological or cardiac involvement.

What is the prognosis?

Untreated, the rash of erythema migrans resolves in 2-3 weeks. Approximately 20% of patients will have a recurrence. Approximately two thirds of patients with erythema migrans will develop other symptoms of Lyme disease. Without treatment of the localized stage of Lyme disease, patients may progress to the disseminated and persistent stages. In disseminated disease, the spirochete spreads hematogenously to distant sites. Multiple lesions of erythema migrans may develop, which are usually smaller than the first one. Systemic symptoms are frequently present. Multiple organ symptoms may be involved, resulting in the protean manifestations of Lyme disease. arthritis, carditis, heart block, Bell's palsy (seventh cranial nerve palsy), and meningitis are among the more common of many possible symptoms. Persistent Lyme disease manifests as persistent arthritis and, often, persistent neurological disorders.

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