Tick-borne relapsing fever is a disease caused by one of several spirochetes from the Borrelia genus. This infection is not encountered in Australia, but it is otherwise encountered worldwide and should be considered in a patient with recent travel. In the United States, the most common organism responsible for this infection is Borrelia hermsii, transmitted by the soft-body tick Ornithodoros hermsii primarily found at higher elevations in the western United States.

The infectious organism is transmitted to humans while the tick is taking a blood meal at night. All stages of the tick can transmit the infection, and the blood meal (and transmission of the infectious organism) is usually complete in 20 minutes. The bite of the tick is painless, and the patient may not recall a bite. A travel and exposure history should be obtained to determine if the patient has recently been camping or has had any known or possible exposure to rodents or other animals known to be reservoirs of the infection including rabbits.

The first febrile episode usually begins after an incubation period of about 1-2 weeks. The fever can be quite high (up to 105° F) and usually culminates with a "crisis" with rigors, rising temperature, tachycardia, and hypertension followed by profuse diaphoresis and hypotension. Following the first fever, associated symptoms may include headache, muscle and joint aches, and nausea, with recurrence of fever at least twice. A nonspecific rash may also be present. Central nervous system involvement is estimated to occur in 9% of cases and may manifest as lymphocytic meningitis or altered mental status. Uncommon manifestations of the infection include iritis, acute respiratory distress syndrome, and myocarditis.

The first febrile episode lasts about 3 days and corresponds to periods when spirochetes are most likely to be seen in the blood. Laboratory evaluation may reveal leukocytosis, thrombocytopenia, and elevated liver transaminases. The patient is then typically afebrile for about 1 week before a recurrence of symptoms occurs. Antigenic shift of outer surface proteins of the organism is responsible for the relapsing symptoms.

As opposed to patients infected with louse-borne relapsing fever who typically have only 1 or 2 episodes of relapsing symptoms, patients with tick-borne relapsing fever can have many relapses.

Mortality occurs in approximately 2%-5% of cases, which is a lower rate than for louse-borne relapsing fever. Death due to the infection is rare in the United States. Infection during pregnancy has been associated with spontaneous abortion.

Treatment is with antibiotics. Jarisch-Herxheimer reaction may occur after antibiotic therapy is started.