Erythema chronicum migrans (New Latin, literally, "chronic migrating redness") 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. "Erythema migrans is the only manifestation of Lyme disease in the United States that is sufficiently distinctive to allow clinical diagnosis in the absence of laboratory confirmation.". 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 and Prevention. These rashes are characteristic of Borrelia infections and no other pathogens are known that cause this form of rash.
A similar condition called Southern Tick Associated Rash Illness (STARI) produces a similar rash pattern although it develops more quickly and is smaller. The associated infectious agent has not been determined. Antibotic treatment resolves the illness quickly.
This erythema is also sometimes called erythema migrans or EM. However, a less common meaning of the term "erythema migrans" is a mucosal condition of the tongue, also called 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 the rash came from the bite of an Ixodestick, meningitic symptoms and signs in a number of cases and that both sexes were affected. This rash is now known as erythema chronicum 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 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, EM may be less than 5 cm in diameter. Multiple painless EM rashes may occur, indicating disseminated infection.
The EM rash occurs in 80% to 90% of those infected with Borrelia. A systematic review of the medical literature showed 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.
Advocates of a diagnosis called "chronic lyme disease" dispute the generally accepted incidence of the rash, claiming it occurs in less than 50% of infections.
^Wormser GP, Dattwyler RJ, Shapiro ED, et al. (November 2006). "The clinical assessment, treatment, and prevention of Lyme disease, Human Granulocytic Anaplasmosis, and Babesiosis: Clinical Practice Guidelines by the Infectious Diseases Society of America". Clin. Infect. Dis.43 (9): 1089–134. doi:10.1086/508667. PMID17029130. ""pp. 1101–2 Background and Diagnosis of Erythema Migrans"