Soft tick relapsing fever (STRF) is caused by Borrelia bacteria with an unusual trick: they rearrange their DNA — antigenic variation — to slip past the immune system, causing repeated bouts of fever and other symptoms. For transmission, symptoms and prevention, see About STRF.
An Ornithodoros soft tick. CDC
Diagnosis
- Microscopy. Febrile patients can have very high numbers of spirochetes in the blood (a million or more per milliliter). They're easiest to see in symptomatic, untreated patients early on, and a dark-field or stained blood smear is usually enough to confirm the diagnosis.
- PCR. Offered by some commercial labs; more sensitive than microscopy and usable between fevers or soon after treatment starts. Whole blood is preferred. Some PCR tests can't tell STRF from hard tick relapsing fever (HTRF). Molecular testing is available from only a limited number of labs.
- Serology may not distinguish STRF from HTRF, and relapsing fever can cause false-positive Lyme disease tests.
- Culture is difficult and generally done only in specialized labs.
Because tests can cross-react with other Borrelia, use clinical and epidemiologic clues — especially travel and exposure history — to interpret results. Consider STRF in patients with positive Lyme or HTRF serology who haven't been in areas where those diseases occur.
Routine labs may show low platelets, raised white cell count, slightly raised bilirubin, a high erythrocyte sedimentation rate, and slightly prolonged PT and PTT.
Treatment
Evidence is limited. For patients who are not pregnant and have no neurologic complications:
| Drug | Dose | Days | |
|---|---|---|---|
| Adults | Doxycycline, IV or oral (preferred) | 100 mg every 12 hours | 10 |
| Azithromycin, oral | 500 mg daily | 10 | |
| Penicillin G, IV | 4,000,000 units every 6 hours | 10 | |
| Ceftriaxone, IV | 2 g daily | 10 | |
| Children | Doxycycline, IV or oral (preferred) | 2.2 mg/kg every 12 hours, max 100 mg a dose | 10 |
| Azithromycin, oral | 10 mg/kg daily, max 500 mg a day | 10 | |
| Penicillin G, IV | 50,000–100,000 units/kg every 6 hours, max 4,000,000 units a dose | 10 | |
| Ceftriaxone, IV | 50–75 mg/kg daily, max 2 g a day | 10 |
For pregnant patients or those with neurologic involvement, start with an IV beta-lactam antibiotic and treat for 10 to 14 days, with close monitoring for severe complications.
Watch for the Jarisch-Herxheimer reaction during the first 4 hours of antibiotics: a worsening with rigors, low blood pressure and high fever, in up to 50% of cases, which can be hard to tell from a febrile crisis. Acute respiratory distress syndrome (ARDS) is a rare complication, sometimes beginning after treatment starts.
In Israel, preventive doxycycline after recognized tick bites or high-risk exposures has been linked to fewer infections (Binenbaum and others, 2020), but it hasn't been studied in the United States.
Reporting
STRF is reportable in some western states. Reporting cases helps prevent more; contact your local or state health department with questions.
Sources
Based on "Clinical Guidance for Soft Tick Relapsing Fever (STRF)," Centers for Disease Control and Prevention; a work of the United States government in the public domain.
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