Tick-borne relapsing fever occurs in the western United States and is usually linked to sleeping in rustic, rodent-infested cabins in mountainous areas. Louse-borne relapsing fever is transmitted by the human body louse and is generally restricted to refugee settings in developing regions of the world.
Prevention
Keep rodents, especially chipmunks and squirrels, out of homes and cabins.
To prevent infection:

• Avoid sleeping in rodent-infested buildings.
• Prevent tick bites by using insect repellent containing DEET (on skin or clothing) or permethrin (applied to clothing or equipment).
• Rodent-proof buildings in areas where the disease is known to occur.
• Consult a licensed pest control specialist who can safely:
• Identify and remove any rodent nesting material from walls, attics, crawl spaces, and floors.
• Treat "cracks and crevices" in the walls with pesticide.
• Provide additional pesticide treatments as necessary to effectively rid the building of the soft ticks.

Symptoms

Peripheral blood smear. The TBRF bacteria are long and spiral-shaped. The circular objects are red blood cells. The irregular purple object in the top right corner is a white blood cell.

Relapsing Fever Bacteria

Relapsing fever is caused by certain species of Borrelia,
a gram negative bacteria 0.2 to 0.5 microns in width and 5 to 20 microns in length. They are visible with light microscopy and have the cork-screw shape typical of all spirochetes. Relapsing fever spirochetes have a unique process of DNA rearrangement that allows them to periodically change the molecules on their outer surface. This process, called antigenic variation, allows the spirochete to evade the host immune system and cause relapsing episodes of fever and other symptoms. Three species cause TBRF in the United States: Borrelia hermsii, B. parkerii, and B. turicatae. The most common cause is cause is B. hermsii.

Clinical Disease

Relapsing fever is characterized by episodes of fever lasting several days, followed by an interval without fever, followed by another episode of fever. This process can recur from 1 to 4 times. Along with fever, patients may experience generalized body aches, muscle pain, joint pain, headache, nausea, vomiting, anorexia, dry cough, light sensitivity, rash, neck pain, eye pain, confusion, and dizziness.  If you suspect that you have relapsing fever, see your health care provider.
Transmission--Soft Ticks Transmit TBRF
O. hermsi tick, before and after feeding. Photo taken by Gary Hettrick RML, NIAID.
Borrelia bacteria that cause TBRF are transmitted to humans through the bite of infected "soft ticks" of the genus Ornithodoros. Soft ticks differ in two important ways from the more familiar "hard ticks" (e.g., the dog tick and the deer tick). First, the bite of soft ticks is brief, usually lasting less than half an hour. Second, soft ticks do not search for prey in tall grass or brush. Instead, they live within rodent burrows, feeding as needed on the rodent as it sleeps.
Humans typically come into contact with soft ticks when they sleep in rodent-infested cabins. The ticks emerge at night and feed briefly while the person is sleeping. The bites are painless, and most people are unaware that they have been bitten. Between meals, the ticks may return to the nesting materials in their host burrows.
There are several Borrelia species that cause TBRF, and these are usually associated with specific species of ticks. For instance, B. hermsii is transmitted by O. hermsi ticks, B. parkerii by O. parkeri ticks,and B. turicatae by O. turicata ticks. Each tick species has a preferred habitat and preferred set of hosts:

• Ornithodoros hermsi tends to be found at higher altitudes (1500 to 8000 feet) where it is associated primarily with ground or tree squirrels and chipmunks.
• Ornithodoros parkeri occurs at lower altitudes, where they inhabit caves and the burrows of ground squirrels and prairie dogs, as well as those of burrowing owls.
• Ornithodoros turicata occurs in caves and ground squirrel or prairie dog burrows in the plains regions of the Southwest, feeding off these animals and occasionally burrowing owls or other burrow- or cave-dwelling animals.

Soft ticks can live up to 10 years; in certain parts of the Russia the same tick has been found to live almost 20 years. Individual ticks will take many blood meals during each stage of their life cycle, and some species can pass the infection along through their eggs to their offspring. The long life span of soft ticks means that once a cabin or homestead is infested, it may remain infested unless steps are taken to find and remove the rodent nest.
Distribution
O. hermsi tick.
TBRF is found in discrete areas throughout the world, including mountainous areas of North America, plateau regions of Mexico, Central and South America, the Mediterranean, Central Asia, and much of Africa.
In the United States, TBRF occurs most commonly in 14 western states: Arizona, California, Colorado, Idaho, Kansas, Montana, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, Washington, and Wyoming.
TBRF is spread by multiple tick species, each of which has a preferred habitat and set of hosts. Ornithodoros hermsi, the tick responsible for most cases in the United States, prefers coniferous forests at altitudes of 1500 and 8000 feet where it feeds on tree squirrels and chipmunks. The two other U.S. tick species that transmit TBRF, O. parkeri and O. turicata, are generally found at lower altitudes in the Southwest, where they inhabit caves and the burrows of ground squirrels, prairie dogs, and burrowing owls.
Most TBRF cases occur in the summer months when more people vacationing and sleeping in rodent-infested cabins. Nevertheless, TBRF can also occur in the winter months. Fires started to warm a cabin are sufficient to activate ticks resting in the walls and woodwork.