About Relapsing Fever
Relapsing fever is an illness caused by several species of Borrelia bacteria, which can be transmitted by some species of ticks and lice. The main symptoms of relapsing fever are one or more episodes of high fever, headaches, and muscle and joint aches. In the United States, cases of tick-borne relapsing fever are categorized into two groups: soft tick relapsing fever and hard tick relapsing fever. There are differences between the bacteria that cause these infections, the ticks that carry them, and where a person may be exposed.
Soft tick relapsing fever (STRF) is caused by Borrelia hermsii, Borrelia turicatae, and some other Borrelia species and transmitted by the bite of infected soft ticks of the genus Ornithodoros.
Hard tick relapsing fever (HTRF) is caused by Borrelia miyamotoi and transmitted by the bite of infected hard ticks: the black-legged tick, Ixodes scapularis, in the eastern United States and the western black-legged tick, Ixodes pacificus, in the western United States.
A third type of relapsing fever, louse-borne relapsing fever, is spread by the human body louse.
Where Relapsing Fever is Found
In the United States, most soft tick relapsing fever (STRF) cases are associated with overnight stays in rural, rodent-infested cabins in mountainous areas of the western states. In Washington state, STRF is transmitted by the soft tick Ornithodoros hermsi, which is associated with rodents found in coniferous forests at higher elevations (usually 1500-8000 feet). STRF is the most commonly-reported tick-borne disease acquired in Washington state. The majority of cases are reported from the eastern slopes of the Cascades and the northeastern corner of the state (Selkirks). The other species of tick that transmits STRF in the US, O. turicata, is usually found at lower altitudes in the Southwest. STRF infections typically occur in the summer months between May and September, but people can become infected year-round and sometimes within their primary homes, not just rustic cabins.
Hard tick relapsing fever occurs mainly in the upper midwestern, northeastern, and mid-Atlantic US, although cases have also been identified in California. In Washington State, Ixodes pacificus ticks infected with Borrelia miyamotoi have been identified, but no human cases have been reported to-date.
Louse-borne relapsing fever occurs in regions of Africa so risk of infection in the United States is very low.
How Relapsing Fever Spreads
People can only get relapsing fever if they are bitten by an infected tick or louse. Relapsing fever is not spread directly to humans from animals or from one person to another.
The ticks that can transmit soft tick relapsing fever (STRF) are called "soft ticks" and are different from "hard ticks" that transmit other diseases such as Lyme disease, Rocky Mountain spotted fever, and hard tick relapsing fever (HTRF). Soft ticks bite and feed for much shorter periods of time, typically less than 30 minutes, and usually at night. They live within rodent nests and burrows, rather than grassy or bushy areas. Soft ticks prefer to feed on rodents but may seek out humans if rodents are scarce. Their bites are painless and most people don't realize that they have been bitten.
The soft ticks that can transmit STRF are within the Ornithodoros genus. They become infected with Borrelia bacteria by feeding on infected wild rodents. Once infected, the ticks remain infected for the rest of their lifespan, which can be up to 10 years.
In contrast, HTRF is spread by hard ticks (Ixodes pacificus in Pacific coastal states, including Washington). Hard ticks embed in the skin and feed for longer periods of time (up to seven days) compared to soft ticks. Ixodes pacificus ticks are common throughout western Washington and on the eastern slopes of the Cascades.
Symptoms of Relapsing Fever
Relapsing fever starts with symptoms of fever, chills, headache, muscle and joint aches, and may include nausea or vomiting. Soft tick relapsing fever (STRF) characteristically cycles between 2-7 days of fever and 4-14 days without fever. This cycle may repeat many times, over several weeks, if the person is not treated. In contrast, hard tick relapsing fever (HTRF) cases rarely experience multiple cycles of fever; relapsing fever has only been documented in approximately 10% of cases.
If you or your family member develops symptoms of relapsing fever, visit your healthcare provider or contact your local health department. Untreated STRF can cause serious complications. STRF can affect a fetus during pregnancy and can cause pregnancy loss.
How to Prevent Relapsing Fever
Prevention recommendations vary for soft tick relapsing fever and hard tick relapsing fever.
Prevent Soft Tick Relapsing Fever (STRF)
The most important action you can take is to educate yourself about where and how you are likely to be exposed to STRF. Be aware of the potential risks when visiting or camping in mountainous areas of the western US:
- Avoid sleeping in rodent-infested buildings whenever possible. Check sleeping areas for evidence of rodents: holes in the floor or walls, shredded material from mattresses, and rodent droppings on counters or in cupboards. Avoid vacuuming, sweeping, or stirring up dust in and around rodent-contaminated areas to prevent hantavirus pulmonary syndrome, a rare, but potentially fatal respiratory disease. If the dwelling has been unoccupied, change and wash all bedding before use.
- If you cannot avoid sleeping in a rodent-infested building, pull beds away so that they are not touching any walls. Avoid sleeping on the floor.
- If you are renting a cabin and notice a rodent infestation, contact the owner to alert them.
- If you own a cabin or other structure with a rodent infestation, consult a licensed pest control professional who can safely:
- Identify and remove any rodent nests from walls, attics, crawl spaces, and floors.
- Treat cracks and crevices in the walls with pesticide to remove ticks.
- Establish a pest control plan to keep rodents out.
- Caulk cracks and crevices to keep rodents and pests out.
- Use EPA-registered insect repellent to prevent tick bites.
- Rodent-proof cabins, homes, and other buildings in areas where the disease is known to occur. Learn more about sealing up your home, trapping, and cleaning up after rodents.
Pesticide control of ticks in an infested building may be necessary for comprehensive prevention of STRF. Eliminating rodents without tick control may increase the risk of people becoming infected, because hungry ticks will seek out other mammals (including humans) on which to feed if rodents are unavailable. Contact a licensed professional exterminator or pest controller to discuss appropriate tick control measures for your dwelling.
Prevent Hard Tick Relapsing Fever (HTRF)
- Use EPA-registered insect repellent to prevent tick bites.
- Wear long-legged pants and shirts when in tick habitat.
- Avoid wooded and brushy areas with high grass and leaf litter.
- Check your body, clothing, and pets for ticks after being outdoors.
- Shower within two hours after being in tick habitat.
More Resources
For the Public
Ticks - Species found in Washington, preventing disease, and submitting a tick for identification.
Tick-borne Relapsing Fever Fact Sheet (PDF)
Hard Tick Relapsing Fever Fact Sheet (PDF)
Soft Tick Relapsing Fever Fact Sheet (PDF)
Tick and Louse-born Relapsing Fever, CDC
For Healthcare Providers and Local Health
Relapsing Fever - Notifiable Condition - Includes case definition, incident rate, reporting guideline, and reporting form.