TBRF is caused by several species of Borrelia carried in ticks. B. hermsii is the most common cause of TBRF in the United States (US). B. hermsii is transmitted by the soft tick, Onithodoros hermsi. B. parkerii and B. turicatae can also cause relapsing fever in the US.
Transmission occurs through a brief nighttime feed of less than 30 minutes and the tick bite is painless. Most people are unaware they have been bitten. Many patients who are infected report an overnight stay in rodent-infected dwellings at greater than 2,000 feet elevation. O. hermsi prefers coniferous forests at altitudes of 1,500 to 8,000 feet altitude. These ticks do not search for prey in tall grasses or brush, but live in rodent burrows or cabins. The ticks normally feed on tree squirrels and chipmunks.
Most cases occur in summer when vacationers sleep in rodent-infested cabins. TBRF can also occur in winter when fires warm cabins sufficiently to activate the ticks. 70% of all reported TBRF cases are in California (33%), Washington (25%), and Colorado (11%).
Another species, O. turicata, is found at lower altitudes in the Southwest US. Cattle, rodents, pigs, snakes, tortoises, and possibly coyotes are hosts for this tick.