The spirochetal agent of Lyme disease, Borrelia burgdoferi, is transmitted to humans through a bite of a nymphal stage deer tick Ixodes scapularis (or Ixodes pacificus on the West Coast). The duration of tick attachment and feeding is a key factor in transmission. Proper identification of tick species and feeding duration aids in determining the probability of infection and the risk of developing Lyme disease.

The early symptoms of LD can be mild and easily overlooked. People who are aware of the risk of lyme disease in their communities and who do not ignore the sometimes subtle early symptoms are most likely to seek medical attention and treatment early enough to be assured of a full recovery.

The first symptom is usually an expanding rash called erythema migrans, or EM, in medical terms, which is thought to occur in 80% to 90% of all lyme disease cases. An EM rash generally has the following characteristics:

  • Usually (but not always) radiates from the site of the tick bite


  • Appears either as a solid red expanding rash or blotch, or a central spot surrounded by clear skin that is in turn ringed by an expanding red rash (looks like a bull’s-eye)


  • Appears an average of 1 to 2 weeks (range = 3 to 30 days) after disease transmission


  • Has an average diameter of 5 to 6 inches (range = 2 inches to 2 feet)


  • Persists for about 3 to 5 weeks


  • May or may not be warm to the touch


  • Is usually not painful or itchy

EM rashes appearing on brown-skinned or sun-tanned patients may be more difficult to identify because of decreased contrast between light-skinned tones and the red rash. A dark, bruise-like appearance is more common on dark-skinned patients.



If you think you have lyme disease symptoms you should see your physician immediately. The EM rash, which may occur in up to 90% of the reported cases, is a specific feature of lyme disease, and treatment should begin immediately.

Even in the absence of an EM rash, diagnosis of early lyme diasease should be made on the basis of symptoms and evidence of a tick bite, not blood tests, which can often give false results if performed in the first month after initial infection (later on, the tests are more reliable). If you live in an endemic area, have symptoms consistent with early LD and suspect recent exposure to a tick, present your suspicion to your doctor so that he or she may make a more informed diagnosis.

If early symptoms are undetected or ignored, you may develop more severe symptoms weeks, months or perhaps years after you were infected. In this case, the CDC recommends using the ELISA and Western-blot blood tests to determine whether you are infected. These tests, as noted above, are considered more reliable and accurate when performed at least a month after initial infection, although no test is 100% accurate.

If you have neurological symptoms or swollen joints your doctor may, in addition, recommend a PCR (Polymerase Chain Reaction) test via a spinal tap or withdrawal of synovial fluid from an affected joint. This test amplifies the DNA of the spirochete and will usually indicate its presence.