Erythema migrans (EM) 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, in varying 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.

lyme rash
Typical EM rash (photo source: NY Medical College)

Late Lyme Disease:

Some symptoms and signs of Lyme disease may not appear until weeks, months, or years after a tick bite:

Arthritis is most likely to appear as brief bouts of pain and swelling, usually in one or more large joints, especially the knees.
Nervous system abnormalities can include numbness, pain, nerve paralysis (often of the facial muscles, usually on one side), and meningitis (fever, stiff neck, and severe headache).
Rarely, irregularities of the heart rhythm may occur.

In some persons, the bull’s eye rash never appears; in some, the first and only sign is arthritis, and in others, nervous system problems are the only evidence of Lyme disease.

Lyme Disease and Pregnancy:

Rarely, Lyme disease acquired during pregnancy may lead to infection of the placenta and possibly to stillbirth. However, studies of women infected during pregnancy have found no adverse effects on the fetus when the mother received appropriate antibiotic treatment for her Lyme disease.

Diagnosis:

Many 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. On the other hand, other infections, arthritis, or neurologic diseases can be misdiagnosed as Lyme disease.

Diagnosis of Lyme disease should take into account the following factors:

Patient’s history of possible exposure to ticks in areas where Lyme disease is known to occur.
Symptoms and signs of the illness.
The results of blood tests used to detect whether the patient has antibodies to the Lyme disease bacterium (B. burgdorferi).

Laboratory tests for Lyme disease must be interpreted in relation to the patient’s clinical assessment. 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. In 1995, recommendations were made to standardize testing for Lyme disease. Two tests that measure the body’s production of antibodies to the Lyme disease bacterium are recommended: (1) an enzyme-linked immunosorbent assay (ELISA) or indirect immunofluorescence assay (IFA), followed by (2) a Western immunoblot of samples that tested positive or equivocal by ELISA or IFA. 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.

SOURCE: Center for Disease Control and Prevention http://www.cdc.gov

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