Erythema (chronicum) migrans (EM) -- "Bulls Eye" Rash

Erythema Migrans, the characteristic rash spreading from the site of a tick bite, typically starts about two to thirty days after the tick bite and is the direct result of the spirochaete migrating through the skin. (Lesions which start earlier could be due to a local reaction to the tick bite or an acute bacterial infection such as streptococcal or staphylococcal infection.)

The rash can become relatively large - up to 75 centimetres (30 inches) in diameter - with gradual clearing of the erythema (redness) from the centre, so that the skin returns to its normal appearance. This phenomenon has given rise to the term "bulls-eye rash". Some rashes may be very faint, with little or no swelling or raising of the advancing edge, and may be easily missed, especially if the person has dark skin or the bite is in an inconspicuous site. The patient may also experience other symptoms, including a mild 'flu-like' illness and swelling of the lymph nodes (glands) of the area affected by the rash. If these symptoms are severe or accompanied by marked arthralgia (joint pains), myalgia (muscle pains), headache or neck stiffness, it is likely that there has been some secondary spreading of the infection to other parts of the body.

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