In about half of those infected with Borrelia bacteria, the body will be able to adequately defend itself against the infection and the person will not become ill. When the disease does develop, erythema migrans is the most common symptom of illness (estimated at about 95% of manifestations) occurring 2 to 30 days after the tick bite (generally 7 to 14 days). It concerns a red circle on the skin that gradually enlarges from the site of the tick bite while the center often becomes lighter in color. An erythema migrans is usually 5 to 15 cm in size. The redness (erythema) is not painful and disappears on its own after a few weeks. It can therefore go unnoticed. Other symptoms may accompany this redness, such as fever, fatigue, muscle pain and headaches. An erythema migrans should be distinguished from a local reaction to the tick bite, in which the redness occurs very quickly, usually remains limited in size and does not spread further. When an erythema migrans is diagnosed, further examination (e.g. via a blood test) is not indicated and the (family) physician will prescribe antibiotic treatment.
Once present in the body and without treatment, the bacteria can spread to other parts of the body, especially to the joints and central nervous system and, less frequently, to the heart. In some cases, it then causes late symptoms, weeks to months after the bite, such as arthritis (painful, red, warm and swollen joint, often the knee) or neurological disorders (e.g., inflammation of a nerve root).