Lyme disease occurs primarily in 3 distinct geographic regions of the United States. Most cases occur in southern New England and in the eastern mid-Atlantic states. The disease also occurs, but with lower frequency, in the upper Midwest, especially Wisconsin and Minnesota, and less commonly on the West Coast, especially northern California. The occurrence of cases in the United States correlates with the distribution and frequency of infected tick vectors-Ixodes scapularis in the east and Midwest and Ixodes pacificus in the west. Reported cases from states without known enzootic risks may have been acquired in states with endemic infection or may be misdiagnoses resulting from false-positive serologic test results. Rash similar to erythema migrans has been reported in states without endemic infection; however, the Etiology of this condition remains unknown. Most cases of early disease occur between April and October; more than 50% of cases occur during June and July. People of all ages may be affected, but incidence in the United States is highest among children 5 through 9 years of age and adults 45 through 54 years of age.
The incubation period from tick bite to appearance of single or multiple erythema migrans lesions ranges from 1 to 32 days with a median of 11 days. Late manifestations can occur months to years after the tick bite.
Endemic Lyme disease transmitted by ixodid ticks occurs in Canada, Europe, states of the former Soviet Union, China, and Japan. The primary tick vector in Europe is Ixodes ricinus , and the primary tick vector in Asia is Ixodes persulcatus . Clinical Manifestations of infection vary somewhat from manifestations seen in the United States, probably because of different genomospecies of Borrelia .
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