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Relapsing fever is caused by the bacteria Borrelia. In the United States it typically occurs in mountainous areas of the western states. It is transmitted to humans by the bites of argasid (soft) ticks that become infected when feeding on infected rodents such as squirrels and chipmunks. These ticks are found where rodents burrow and nest, often in older buildings. The ticks typically feed only at night and, unlike the ticks that cause Lyme disease, do not remain attached for prolonged times. They can survive for long periods between blood meals, and typically do not leave a noticeable bite wound. In the western United States and British Columbia, exposure commonly occurs in older buildings and cabins located in higher elevations.


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Purpose of surveillance:

  • To identify common source outbreaks
  • To identify and eliminate sources of transmission
  • To facilitate environmental clean-up
  • To facilitate appropriate treatment of infected persons

Relapsing fever case data

Local epidemiology:

In 2015, two cases of tick-borne relapsing fever were reported as part of a cluster associated with staying in an infested cabin in north-central Washington, where relapsing fever has been a documented problem in recent years. Neither case required hospitalization. Public health provided guidance on how to eliminate both ticks and rodents from the property.

Typically, one to three cases of relapsing fever are reported each year in King County. Each year in Washington state between two and eight cases are reported.