An Ornithodoros soft tick. Image from CDC's page
Soft tick relapsing fever (STRF) — also called tickborne relapsing fever — is caused by Borrelia bacteria such as B. hermsii and B. turicatae, spread by the bite of soft-bodied ticks of the genus Ornithodoros. The name sets it apart from relapsing fever spread by hard ticks, caused by B. miyamotoi and possibly other species. In the United States, most people are infected while sleeping overnight in rodent-infested cabins.
Symptoms
| Common | fever, headache, muscle aches, chills |
|---|---|
| During fever episodes | can be severe — shaking, drenching sweats, low blood pressure |
| Other | dizziness, joint pain, nausea and vomiting, loss of appetite; rarely facial paralysis, eye pain or redness, or vision changes |
Symptoms start 4 to 21 days after exposure and relapse in a telltale pattern: about 3 days of fever, about 7 days without, then fever again — repeating several times without antibiotics.
The general timeline of STRF symptoms. Image from CDC's page
In pregnancy, STRF can cause pregnancy loss, premature birth, and severe infection or death in newborns.
How it spreads
Each Borrelia species usually goes with a particular tick species. Soft tick bites are brief — usually under half an hour — and painless, so most people never know they were bitten.
| Setting (U.S.) | Tick and bacterium | Notes |
|---|---|---|
| Rodent-infested cabins in western mountains — the most common | Ornithodoros hermsi, carrying B. hermsii | ticks come out at night and feed while people sleep; most common in summer, but also in winter, when a fire warms the cabin and wakes ticks in the walls and woodwork |
| Caves, less often — mainly in parts of Texas | O. turicata, carrying B. turicatae |
An O. hermsi tick before and after feeding. Image from CDC's page
STRF cases in the United States, 2012–2021. Image from CDC's page
Worldwide, STRF has been found on every continent except Australia and Antarctica, and is likely under-recognized. It's most common in East Africa (B. duttonii, spread by O. moubata), West Africa (B. crocidurae, O. sonrai), the Mediterranean (B. hispanica, O. erraticus) and the Middle East (B. persica, O. tholozani).
Prevention
Soft ticks can live for decades, feed many times at each life stage, and some pass Borrelia to their eggs. Once a cabin is infested with rodents and soft ticks, it can stay that way until the nests and ticks are removed.
- Avoid sleeping in rodent-infested buildings. Signs include droppings, nests of shredded paper, fabric or dried plants, gnaw marks and a stale smell.
- Keep ticks off your bed: move beds away from walls, don't let sheets touch the floor, and don't sleep on the floor.
- Tell the owner if a cabin you're visiting has rodents.
- Cabin owners: keep rodents — especially chipmunks and squirrels — out, and have a licensed pest control professional remove nests from walls, attics, crawl spaces and floors, treat cracks and crevices with pesticide, and set up a plan to keep rodents out.
- Don't remove rodent nests yourself: contact with deer mouse droppings, urine, saliva or nests can spread diseases such as potentially fatal hantavirus pulmonary syndrome.
- In caves or near rodent burrows where STRF occurs, wear long sleeves and pants and keep skin off cave floors and burrows.
- Prevent tick bites with EPA-registered repellent, or treat clothing and gear with 0.5% permethrin.
CDC also has a fact sheet for pest control professionals.
Sources
Based on "About Soft Tick Relapsing Fever (STRF)," Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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