Clinical Guidance for Soft Tick Relapsing Fever (STRF)

Key points

  • Serologic and molecular tests for STRF are currently available through a limited number of laboratories and CDC.
  • Clinical and epidemiological features, as well as travel and exposure history, are important to guide interpretation of STRF test results.
  • Report STRF cases to public health authorities to help prevent additional cases.
Ornithodoros soft tick.

General information

For information on transmission, symptoms, and prevention, visit: About STRF.

Diagnosis and testing

Spirochetes may be present in high concentrations in the blood of febrile patients (≥106 spirochetes/ml). Spirochetes are most readily detected by microscopy in symptomatic, untreated patients early in the course of infection. Direct visualization by microscopy using dark field or stained peripheral blood smears is generally adequate to confirm the diagnosis.

Molecular testing, such as polymerase chain reaction (PCR), is available from some commercial laboratories or through CDC. PCR is more sensitive than microscopy and may also be used during asymptomatic periods or soon after treatment initiation. The preferred specimen type for PCR testing is whole blood. Some PCR methods do not distinguish between the agents of STRF and hard tick relapsing fever (HTRF).

Serologic testing is available from some labs to diagnose STRF. Serologic assay results are most sensitive when specimens are collected at least 14 days after symptom onset. Serum taken early during infection may yield negative results. Currently available serologic assays may not distinguish between STRF and HTRF. Patients with relapsing fevers might have false positive serologic tests for Lyme disease.

Because some PCR and serologic tests can cross-react with other Borrelia species, clinical and epidemiologic features, such as travel and exposure history, are important to guide interpretation of test results. Consider a diagnosis of STRF for patients with positive Lyme disease or HTRF serology who have not been in areas endemic for these diseases.

Culturing Borrelia species is challenging and typically not performed outside specialized laboratories.

Tests for STRF are currently available through a limited number of laboratories. CDC provides molecular and serologic testing for STRF at the request of state health departments.

General laboratory findings in patients with STRF may include thrombocytopenia, increased white blood cell count, mildly increased serum bilirubin level, elevated erythrocyte sedimentation rate (ESR), and slightly prolonged prothrombin time (PT) and partial thromboplastin time (PTT).

Relapsing fever bacteria

Borrelia bacteria that cause relapsing fever have a unique process of DNA rearrangement. This process, called antigenic variation, allows the spirochete to evade the host immune system and cause relapsing episodes of fever and other symptoms.


Data to inform treatment for patients with STRF are limited. Consider the following regimens for patients who are not pregnant and do not have neurologic complications.

For patients who are pregnant or when neurologic involvement is present, initial parenteral therapy with a beta-lactam antibiotic is advised. Treatment should be continued for 10 to 14 days with close monitoring given the potential for severe complications.

Age category Drug Dosage Duration (days)
Adults Doxycycline, intravenous or oral (preferred) 100 mg every 12 hours 10
Azithromycin, oral 500 mg daily 10
Penicillin G, intravenous 4,000,000 units every 6 hours 10
Ceftriaxone, intravenous 2 g daily 10
Children Doxycycline, intravenous or oral (preferred) 2.2 mg/kg per dose, every 12 hours, maximum 100 mg dose 10
Azithromycin, oral 10 mg/kg daily, maximum 500 mg/day 10
Penicillin G, intravenous 50,000-100,000 units/kg every 6 hours, maximum 4,000,000 units/dose 10
Ceftriaxone, intravenous 50-75 mg/kg daily, maximum 2 g/day 10


Observe patients during the first 4 hours of antibiotic therapy for Jarisch-Herxheimer reaction. The reaction, a worsening of symptoms with rigors, hypotension, and high fever, occurs in up to 50% of cases and may be difficult to distinguish from a febrile crisis.

Acute respiratory distress syndrome (ARDS) is a rare complication that has been associated with STRF. The onset of ARDS has been described to occur after initiating treatment in some cases.

In Israel, postexposure prophylaxis (PEP) with doxycycline has been associated with decreased rates of infection following recognized tick bites or high-risk exposures (Binenbaum et al., 2020). However, PEP with doxycycline to prevent STRF has not been studied in the United States.

Disease reporting

STRF is reportable in some western states. Report STRF cases to public health authorities to help prevent additional cases. Contact your local or state health department with questions about disease reporting.