Lyme disease is the most commonly reported vector-borne illness in the United States. It is caused by the spirochete Borrelia burgdorferi, and is transmitted to humans through the bite of an Ixodes tick.
PATHOGENESIS AND EPIDEMIOLOGY
B burgdorferi is transmitted into the skin while the tick is feeding on its host. Once in the skin, the organism does not produce toxin but is able to produce disease primarily through an immunologic host response. Lyme disease cannot be transmitted either from person-to-person or by direct contact with infected animals. There have been no documented cases of congenital Lyme disease. Transmission in breast milk has not been documented. Although B burgdorferi can survive in stored blood for several weeks, there also have not been any documented cases of transmission via blood transfusion.
Since it became a nationally notifiable disease in 1991, the annual number of reported cases of Lyme disease in the United States has more than doubled, with approximately 25,000 new cases reported annually in recent years. More than 90% of the cases of Lyme disease are reported from 10 states along the northeastern and mid-Atlantic seaboard and from Wisconsin. The highest incidence of Lyme disease is in children 5 to 9 years of age. The principal risk factor for acquiring Lyme disease in endemic areas is residence in suburban or rural areas that are wooded or overgrown with brush and are infested by infected vector ticks. The ticks that transmit Lyme disease (Ixodes scapularis, the black-legged tick or the deer tick, in the eastern and the midwestern United States, and Ixodes pacificus, the western black-legged tick, in the western United States) are found in wooded areas, high grasses, marshes, and gardens. B burgdorferi is transmitted to humans while the tick is taking a blood meal.
The clinical manifestations of Lyme disease depend on the stage of the illness—early, localized disease; early, disseminated disease; or late disease. The most common manifestation of early, localized Lyme disease, erythema migrans (EM), appears 3 to 30 days (but typically within 7–14 days) after and at the site of a tick bite (although most often, the bite is not recognized). EM begins as a red macule or papule and usually expands over days to weeks to form a large, annular, erythematous lesion that is at least 5 cm and as much as 70 cm in diameter (median of 15 cm). The rash most often is uniformly erythematous, but it may appear as a “bull’s eye” lesion with variable degrees of central clearing (Fig. 262-1). It can vary greatly in shape and, occasionally, may have vesicular or necrotic areas in the center. EM is usually asymptomatic, but may be pruritic or painful and may be accompanied by nonspecific systemic symptoms such as fever, malaise, headache, myalgia, or arthralgia.