(Lyme borreliosis) is the most common vector-borne disease in the United States. The causative organism is the spirochete Borrelia burgdorferi, which is transmitted to humans through the bite of tick species including the deer tick (Ixodes scapularis). White-footed mice and deer are important in the life cycle of the tick. Distribution of Lyme disease correlates with the distribution of these hosts. Most cases in the United States are clustered in the northeast from Massachusetts to Maryland, in the midwest in Wisconsin and Minnesota, or in California. There have been cases reported from all states and also in Canada, Europe, China, Japan, and Russia. Humans are most likely to be infected in June, July, and August. The incubation period from tick bite to the appearance of skin lesion(s) ranges from 1-32 days with a median of 11 days.

The clinical manifestations of Lyme disease may be divided into three stages: In the early localized stage, an annular, erythematous, nonpruritic rash known as erythema chronicum migrans presents at site of a tick bite, usually within 1 to 2 weeks. The early localized stage may also present with multiple erythema migrans lesions, fever, myalgia, and arthralgia. Patients with early disseminated disease may present with multiple erythema migrans lesions, neurologic involvement (meningitis, cranial nerve palsy, and peripheral radiculopathy), and carditis (atrioventricular block and myocardial dysfunction). Late disease manifests as recurrent pauciarticular arthritis, peripheral neuropathy, and cognitive impairment.

Early case reports and case series confirmed that transplacental transmission of B. burgdorferi was possible and raised concerns about a congenital Lyme disease syndrome analogous to that seen with other spirochetal infections such as syphilis. A wide variety of clinical manifestations were noted, with most initial concerns being focused on congenital cardiac malformations and fetal death. However, epidemiologic studies have not supported an association between congenital infection and adverse fetal or neonatal outcomes. A prospective study of 2,014 pregnant women showed no association between seropositivity or history of tick bite and congenital malformations, low birth weight, and fetal death. A report by the same authors compared 2,504 infants born in an endemic region to 2,507 delivered in a nonendemic region. This study showed a significant increase in the rate of congenital cardiac malformations in the endemic compared with the nonendemic region, but notably no association within the endemic region between seropositivity and cardiac malformation. Similarly, in a retrospective case-control study of 796 patients with congenital heart disease and 704 control infants, there was no association between cardiac anomalies and clinical evidence of Lyme disease during pregnancy. Although these studies were limited by the low prevalence of Lyme disease, it appears from available evidence that any increased risk for adverse neonatal effects of prenatal Lyme borreliosis are likely to be small.