Relapsing fever: An acute infection with recurrent episodes of fever caused by spirochetes of the genus Borrelia which are borne by ticks or lice. The relapsing nature of the fever is associated with the presence of antigenic variants. As an immune response develops to the predominant antigenic strain, variant strains multiply and cause a recrudescent infection.
The characteristic features are fever, chills, headache, nausea and vomiting, and muscle and joint pain. Complications may include meningeal inflammation (meningitis), pneumonia, inflammation of the heart muscle (myocarditis) and liver (hepatitis), and bleeding (hemorrhage).
In the western US, relapsing fever is transmitted principally by the bites of the soft-bodied ticks Ornithodoros hermsi and O. turicata. These ticks normally inhabit the burrows and nests of rodents in which the natural infection cycle proceeds without apparent disease in the rodents. Humans are incidental hosts when bitten by an infected tick. The tick vectors are reclusive night feeders and bites often go unnoticed or are thought due to a mosquito bite. Cabins in wilderness areas are attractive nesting sites for potentially infected rodents, particularly when food is made available by cabin users. The disease can occur anytime but is most common in the summer.
The most rapid diagnostic test is identification of the spirochetes on a blood smear obtained during an episode of fever. Treatment with tetracycline, erythromycin, or penicillin is effective in terminating this infection.
Prevention of relapsing fever consists of avoiding likely tick habitats or, when this is not possible, reducing the risk of tick bites by the use of repellents. Additional measures include fumigating nesting sites in human habitations, "rodent-proofing" buildings in endemic areas, and eliminating rodent access to unnatural food sources.