Erythema chronicum migrans

The lesion begins as a red macule or papule at the bite site and expands over days to weeks, commonly reaching more than five centimetres, with a flat advancing border; central clearing occurs in a minority, so absence of a bull's-eye does not exclude the diagnosis. It is typically warm but not painful or pruritic, and favours the axilla, groin, and popliteal fossa. Lyme disease is caused by Borrelia burgdorferi transmitted by Ixodes scapularis in the north-eastern and upper midwestern United States and Ixodes pacificus on the west coast, with the white-footed mouse as reservoir and deer as the reproductive host; transmission usually requires more than thirty-six hours of attachment. Because serology is often negative in the first weeks, erythema migrans is a clinical diagnosis, treated with doxycycline, or amoxicillin or cefuroxime in children under eight and in pregnancy. Untreated, disease progresses to early disseminated stage with multiple secondary lesions, facial nerve palsy which may be bilateral, and AV block, and to late Lyme arthritis of the knee.