Erythema chronicum migrans

Erythema chronicum migrans

Erythema chronicum migrans refers to the rash often (though not always) seen in the early stage of Lyme disease. It can appear anywhere from one day to one month after a tick bite. This rash does not represent an allergic reaction to the bite, but rather an actual skin infection with the Lyme bacteria, Borrelia burgdorferi sensu lato. It is a pathognomonic sign: a physician-identified rash warrants an instant diagnosis of Lyme disease and immediate treatment without further testing, even by the strict criteria of the Centers for Disease Control. These rashes are characteristic of Borrelia infections and no other pathogens are known that cause this form of rash.

It is also sometimes called erythema migrans (without the "chronicum") or "EM". However, this phrase is also used to describe geographic tongue.


In a 1909 meeting of the Swedish Society of Dermatology, Arvid Afzelius first presented research about an expanding, ring-like lesion he had observed. Afzelius published his work 12 years later and speculated that the rash came from the bite of an Ixodes tick, meningitic symptoms and signs in a number of cases and that both sexes were affected. This rash is now known as erythema migrans, the skin rash found in early-stage Lyme disease.

In the 1920s, French physicians Garin and Bujadoux described a patient with meningoencephalitis, painful sensory radiculitis, and erythema migrans following a tick bite, and they postulated the symptoms were due to a spirochetal infection. In the 1940s, German neurologist Alfred Bannwarth described several cases of chronic lymphocytic meningitis and polyradiculoneuritis, some of which were accompanied by erythematous skin lesions.


The erythema migrans EM rash is classically 5 to 6.8 cm in diameter appearing as an annular homogenous erythema (59%), central erythema (30%), central clearing (9%), or central purpura (2%). Because of the "bull's-eye" description to describe the Lyme disease rash, the condition commonly called ringworm is sometimes confused with Lyme disease. Uncommonly, erythema migrans may be less than 5 cm in diameter. Multiple painless EM rashes may occur, indicating disseminated infection.

The EM rash occurs, according to sources, in 80% to 90% of those infected with Borrelia. A systematic review of the medical literature shows that 80% of patients have an expanding EM rash, at the site of the tick bite, although some patients with EM do not recall a tick bite. In endemic areas of the United States homogeneously red rashes are more frequent.

A small group of practitioners disputes the generally accepted incidence of the rash, claiming that it occurs in less than 50% of infections. These practitioners suggest that a condition they call "chronic Lyme" (resembling chronic fatigue syndrome or fibromyalgia) exists in the absence of evidence for Borrelia infection. Their proposed treatment of patients with months or years of antibiotics is opposed by the wider medical community's scientific consensus, since these treatments are potentially dangerous, are not based on diagnoses with objective evidence, and have been shown in clinical trials to be ineffective even when evidence of infection is at hand.

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