Erythema migrans is a red circular patch that appears at the site of the tick bite, usually within 3 days to 1 month after the bite of an infected tick. The patch then grows larger. Sometimes many patches appear, varying in shapes and sizes. Common sites are the thighs, groin, trunk, and armpits. The center of the rash may clear as it enlarges, resulting in a “bull’s-eye” appearance. The rash may be warm, but it usually is not painful. Not all rashes that occur at the site of a tick bite are due to Lyme disease, however. An allergic reaction to tick saliva often occurs at the site of a tick bite and may be confused with the “bull’s-eye” rash of Lyme disease. Allergic reactions to tick saliva usually appear within hours to a few days after the tick bite, usually do not expand, and disappear within a few days.
Late Lyme Disease: Some signs and symptoms of Lyme disease may not appear until weeks, months, or years after a tick bite:
Different people exhibit different signs and symptoms of Lyme disease. Some people never develop a bull’s eye rash. Some people only develop arthritis, and for others nervous system problems are the only symptom of Lyme disease.
Many of the symptoms of Lyme disease are similar to those of other diseases. The fever, muscle aches, and fatigue of Lyme disease can be mistaken for viral infections, such as influenza or infectious mononucleosis. Joint pain can be mistaken for other types of arthritis, such as rheumatoid arthritis, and neurologic signs can mimic those caused by other conditions, such as multiple sclerosis. Other infections, arthritis, or neurologic diseases can also be misdiagnosed as Lyme disease.
Diagnosis of Lyme disease should take into account the following factors:
Laboratory tests for Lyme disease must be interpreted in relation to a person’s recent medical history, and signs and symptoms. Both false-positive (the test results read positive, but the patient is not infected with Lyme disease-causing bacteria) and false-negative test results (the results read negative, but the patient is infected with Lyme disease-causing bacteria) may occur.
Currently, a two-stage testing process to measure the body’s production of antibodies to the Lyme disease bacterium is recommended:
These tests do not detect an infection until the body begins to produce measurable levels of antibodies to the Lyme disease bacterium, usually 2-4 weeks after the bite of an infected tick. Therefore, while these tests are good for identifying Lyme disease in patients with late-stage symptoms such as arthritis, they may be falsely negative in patients with erythema migrans (rash).