The disease is the most frequently diagnosed tick-transmitted illness in the United States. The three major geographic loci of Lyme disease in the United States are the Northeast and middle Atlantic coastal regions, the upper Midwest, and the Pacific Northwest. The disease is found in Europe, Australia, the former U.S.S.R., China, Japan, and several African countries. The vector of Lyme disease is the tick, Ixodes dammini, or related Ixodes ticks such as pa-cificus, scapularis, or ricinus. B. burgdorferi has been found in other ticks such as the Dermacentor variabilis, Amblyomma. americanum, and Haema-physalis leporispalustris; however, transmission has not been proved.

B. burgdorferi has been found in horseflies, deer-flies, and mosquitoes, but proof that these insects are possible secondary vectors has not been established. The reservoirs of B. burgdorferi occur in animals parasitized by infected ticks. The Ixodes tick is a three-host tick with a life cycle of 2 years. Adult ticks mate and feed primarily on deer in late fall; the female deposits eggs on the ground, which produce larvae that are active late the following summer. The tiny larvae obtain a blood meal from infected rodents such as white-footed mice, shrews, chipmunks, and squirrels, which are primary reservoirs for B. burgdorferi. Ground foraging birds are also important hosts for the larvae and nymphs. After a blood meal, the larvae molt to a nymphal form, which is active the following spring and early to midsummer. It seeks an animal host, obtains a blood meal, and molts to an adult stage to complete the 2-year life cycle. The animal hosts can include humans, dogs, deer, cows, horses, raccoons, cats, skunks, black bears, and Virginia opossums.

Each developmental stage of the tick requires feeding once and may take several days. B. burgdorferi is transmitted to the host during the blood meal. The longer the time of attachment of the infected tick to the host, the greater probability of transmission.

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