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Microbiology · Spirochetes

Borrelia recurrentis and tick-borne relapsing fever borreliae !

  • Bacterium
  • Does not Gram-stain
  • For completeness

Borreliae spread by lice or soft ticks that cause fevers that come and go for weeks.

Updated 2026-11-01

Also called: relapsing fever

At a glance

Gram
Does not Gram-stain
Shape
Large spirochete with loose coils
Oxygen
Microaerophilic
Family
Borreliaceae
Stain
Visible on Giemsa- or Wright-stained blood smear during fever

Morphology

  • Spirochetes between red cells on the blood smear
  • Change their main surface protein at each relapse

Transmission

  • B. recurrentis: body louse, crushed into broken skin (not by the bite)
  • Other species (such as B. hermsii): brief bite of Ornithodoros soft ticks, often at night in cabins
  • Some hard ticks transmit B. miyamotoi

Reservoir: B. recurrentis: humans only; Tick-borne species: wild rodents

Vector: Body louse (Pediculus humanus corporis) or Ornithodoros soft ticks

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Antigenic variation: each new variant causes a new fever spike
  • High blood load during fever

Diseases it causes

Clinical picture

  • Bouts of high fever, headache and myalgia lasting days, separated by symptom-free days
  • Hepatosplenomegaly, jaundice and neurological involvement may occur
  • The louse-borne form is more severe and appears with crowding, war and refugee camps

Diagnosis

  • Peripheral blood smear taken during fever (thick or thin film)
  • PCR when available

Treatment

Tetracyclines (doxycycline) or macrolides (erythromycin); penicillin or ceftriaxone with neurological involvement. The Jarisch-Herxheimer reaction is common and intense when the antibiotic starts and requires monitoring.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Louse control: personal hygiene, washing clothes in hot water
  • Rodent-proof cabins in soft-tick areas

Test yourself

Think of the answer, then tap the question to see it.

Why does relapsing fever cause repeated fever bouts?

Because the bacterium changes its surface proteins (antigenic variation) and escapes antibodies.

What is the simplest way to diagnose relapsing fever?

A Giemsa- or Wright-stained blood smear taken during a fever spike.

Which vector transmits B. recurrentis?

The body louse; the bacterium enters when the louse is crushed on the skin.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: About Tick and Louse-borne Relapsing Fevers (Centers for Disease Control and Prevention (CDC), 2024); Typhus Fevers Basics | Typhus Fevers | CDC (Centers for Disease Control and Prevention (CDC), 2024).

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Representative model, for educational purposes only.