Soft tick relapsing fever
2026 Case Definition

Background

Soft tick relapsing fever (STRF), previously called tickborne relapsing fever, is caused by infection with some species of the genus Borrelia, including Borrelia hermsii and Borrelia turicatae. Borrelia spirochetes that cause STRF are transmitted to humans through the bite of infected soft ticks of the genus Ornithodoros. These bacteria are maintained in enzootic cycles involving small rodent hosts and tick vectors. The incubation period for STRF averages 1 week following tick bite. Illness is characterized by periods of fever, often exceeding 38.8°C (102°F), lasting 2–7 days, alternating with afebrile periods of 4–14 days. Febrile periods are often accompanied by shaking chills, sweats, headache, muscle and joint pain, and nausea and vomiting. STRF may be severe or even fatal, and infections contracted during pregnancy can cause spontaneous abortion, premature birth, or neonatal death. In the United States, STRF occurs most commonly in 10 western states: Arizona, California, Colorado, Idaho, Montana, New Mexico, Oregon, Texas, Utah, and Washington. Cases of STRF are usually identified as a single case or small clusters of cases, typically after exposure to soft tick-infested buildings or caves.

Clinical Criteria

An acute illness, with:

• Measured fever ≥38.8°C (102°F) or relapse of fever (two or more episodes of subjective or measured fever, commonly separated by 2–14 days), OR
• Two or more of the following criteria: lower measured fever <38.8°C (102°F) or subjective fever or chills, headache, myalgias or arthralgias, or nausea or vomiting.

Laboratory Criteria

Confirmatory Laboratory Evidence:

• Detection of STRF Borrelia spp. by nucleic acid testing such as polymerase chain reaction (PCR) or sequencing that differentiates STRF Borrelia spp. from other relapsing fever Borrelia spp. (such as those that cause hard tick relapsing fever (HTRF) or louse-borne relapsing fever), in any clinical specimen, OR
• Isolation of Borrelia hermsii, B. turicatae, or other STRF-group Borrelia spp. from any clinical specimen using a Borrelia-specific medium such as Barbour-Stoenner-Kelly (BSK) broth medium.

Presumptive Laboratory Evidence:

• Visualization of spirochetes in blood products, cerebrospinal fluid (CSF), or bone marrow by microscopy, OR
• Serologic evidence of infection by enzyme immunoassay (EIA), immunofluorescence assay (IFA), immunoblot, or another serologic test for relapsing fever Borrelia spp. within 6 months of illness onset, OR
• Detection in any clinical specimen of relapsing fever Borrelia spp. by nucleic acid testing that does not differentiate STRF Borrelia spp. from other relapsing fever Borrelia spp.*

Note: The categorical labels used here to stratify laboratory evidence are intended to support the standardization of case classifications for public health surveillance. The categorical labels should not be used to interpret the utility or validity of any laboratory test methodology.

* This includes PCR tests that are specific to relapsing fever Borrelia spp. but that cannot differentiate soft tick relapsing fever Borrelia spp. from hard-tick and louse-borne relapsing fever Borrelia spp. This does not include pan-Borrelia PCR tests, as they do not differentiate from etiologic agents of Lyme disease.

Epidemiologic Linkage

Within 21 days of illness onset:

Tier I:

• Had a shared exposure site with a confirmed case.

Tier II:

• Spent time in a county where Ornithodoros soft ticks are present or presumed to be present or where a confirmed autochthonous (i.e., locally acquired) case of STRF has been previously reported, AND
• Spent time in possible soft tick habitat^† (e.g., caves, cabins, or other rodent-infected structure), camping, or handling firewood.

† The habitats where relapsing fever-group Borrelia spp. are present overlap with that of their Ornithodoros spp. tick vectors. O. hermsi, the soft tick vector for B. hermsii, is typically found in rodent nests in mountainous areas above 450 m (1,500 ft) elevation where chipmunks or squirrels are present. O. turicata, the soft tick vector for B. turicatae, occurs in caves and in the nests and burrows of prairie dogs and ground squirrels in the plains regions of the southwestern United States.

Criteria to Distinguish a New Case from an Existing Case

• An individual previously reported as a probable or confirmed case-patient may be counted as a new case-patient when there is a new onset of clinically compatible illness with new laboratory evidence (not including serology due to persistence of antibodies), 6 months or more after prior enumeration.

Suspect

• Meets clinical criteria AND meets tier II epidemiologic linkage criteria, with no laboratory testing performed.

Probable

• Meets clinical criteria AND meets presumptive laboratory evidence, OR
• Meets clinical criteria AND meets tier I epidemiologic linkage criteria, OR
• Meets confirmatory laboratory evidence AND:
  • DOES NOT MEET clinical criteria OR epidemiologic linkage criteria, OR
  • No additional information is available.

Confirmed

• Meets clinical criteria AND meets confirmatory laboratory evidence, OR
• Meets clinical criteria AND meets presumptive laboratory evidence AND meets tier I or tier II epidemiologic linkage criteria.

Case Classification Comments

Most serologic and PCR tests do not distinguish between Borrelia spp. that cause STRF and B. miyamotoi, the agent of HTRF. In the absence of confirmatory laboratory evidence, public health agencies should use a combination of available test results, information about the location of possible exposures, clinical manifestations, and the incidence of a particular disease in the geographic area to help determine the appropriate case definition to apply for an individual case. STRF and HTRF can have similar clinical presentations, though available data suggest that recurring febrile episodes are less common in HTRF. Individuals should not be classified as cases of both STRF and HTRF based on presumptive laboratory evidence.