Because nymphal feeding precedes larval feeding, the enzootic transmission of B. burgdorferi is highly efficient. Thus, the majority of mice become infected with Lyme disease spirochetes in the spring before serving as hosts to the larvae later that summer. Spirochetes overwinter in the fed larvae, in the unfed nymphs or in the host animal. The patch then expands, often to a large size. Sometimes many patches appear, varying in shape, depending on their location.
Common sites are the thigh, groin, trunk, and the armpits.
The center of the rash may clear as it enlarges, resulting in a bulls-eye appearance. The rash may be warm, but it usually is not painful. Allergic reactions to tick saliva usually occur within a few hours to a few days following the tick bite, but usually do not expand and normally disappear within a few days. Less frequently, irregularities of the heart rhythm occur. In rare cases, Lyme disease acquired during pregnancy may have possibly lead to infection of the fetus and to stillbirth, but adverse effects to the fetus have not been conclusively documented.