Blood samples from four patients who had returned home to Seattle were sent to RML. There Dr. Schwan and his group isolated and identified the cause of the illness as Borrelia hermsii, a spiral-shaped bacterium or spirochete. Drs. Schwan and Damrow then visited the cabin where the individuals had become infected. In the attic, they discovered the species of tick that is known to transmit this bacterium in other regions of the western United States.
The bacteria that cause relapsing fever are related to those bacteria that cause Lyme disease. The illnesses, however, are quite different, and the ticks that transmit relapsing fever spirochetes have a strikingly different lifestyle compared with the ticks that transmit Lyme disease spirochetes.
For example, while Lyme disease is usually acquired from ticks in an outdoor setting, most people who develop relapsing fever become infected while sleeping in rodent- and tick-infested cabins. Because the ticks feed quickly and only at night, most people are not aware of having been bitten. In nature, the bacteria--which infect rodents such as pine squirrels and chipmunks--are transmitted among these animals by the ticks that live in or near their nests.
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NIH/National Institute of Allergy and Infectious Diseases