During the 1973 summer season, 27 employees and 35 overnight guests at the North Rim, Grand Canyon National Park, Arizona, acquired febrile illnesses compatibel with relapsing fever. Sixteen cases were confirmed by finding Borrelia spirochetes in peripheral blood smears or inoculated Swiss mice. Retrospective surveys of 278 employees and 7247 guests at the park revealed that acquisition of illness was significantly associated with the persons sleeping in rustic log cabins and acquiring bites of "unknown" insects. From rodent nesting materials found in the walls and attics of cabins where cases had occurred, infective Ornithodoros hermsi ticks were recovered. Exceptional activity of ticks in human populations appeared to have resulted from a decreased population of the ticks' usual rodent hosts. Vector control activities consisted of spraying the cabins with residual insecticide, removing nesting materials, and "rodent proofing." This outbreak, the largest yet identified in North America, extends the known range of a principal vector and establishes the North Rim as an endemic source of tick-borne relapsing fever.
In the period August 1-10, 1975, seven cases of leptospirosis occurred in residents of Stewart County, Tennessee. Based on serologic evidence, the infection was caused by leptospires of the Grippotyphosa serogroup. Epidemiologic study showed that the patients apparently acquired their infection while swimming in Cub Creek, a small local stream. Stagnation of the stream resulting from subnormal rainfall may have contributed to the timing of the outbreak. The source of leptospiral contamination for the stream could not be determined.
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