The primary cause of tick-borne relapsing fever in western North America is Borrelia hermsii, a rodent-associated spirochete transmitted by the fast-feeding soft tick Ornithodoros hermsii. We describe a patient who had an illness consistent with relapsing fever after exposure in the mountains near Los Angeles, California, USA. The patient’s convalescent-phase serum was seropositive for B. hermsii but negative for several other vector-borne bacterial pathogens. Investigations at the exposure site showed the presence of O. hermsi ticks infected with B. hermsii and the presence of rodents that were seropositive for the spirochete. We determined that this tickborne disease is endemic to the San Gabriel Mountains near the greater Los Angeles metropolitan area. Tick-borne relapsing fever was first observed in California, USA, in 1921 when 2 persons were infected in a cabin in Nevada County, north of Lake Tahoe (1). Another 251 cases of relapsing fever were reported in this state through 1941. Most persons who became ill had exposures at high elevations in various mountain locations (2). During this time, tick-borne relapsing fever was found to b
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