Infectious Disease and Microbiology – Borrelia Species

Overview

Borrelia species are spirochetal bacteria responsible for several vector-borne infections. The species B. recurrentis, B. hispanica, B. mazzottii, and B. venezuelensis are particularly associated with relapsing fever, an illness characterized by recurrent episodes of high fever separated by periods of clinical improvement.

Microbiology

Borrelia organisms are long, thin, helically shaped spirochetes rather than typical rod-shaped bacilli. They possess an outer membrane with structural features resembling those of Gram-negative bacteria, although they are not reliably visualized with a routine Gram stain.

Because of their relatively large size among spirochetes, organisms causing relapsing fever may be visible directly in peripheral blood during periods of high spirochetemia.

Important Species

Several Borrelia species are associated with relapsing fever, including B. recurrentis, B. hispanica, B. mazzottii, and B. venezuelensis. Other species can produce similar tick-borne infections in different geographic regions.

B. recurrentis is particularly important because it causes louse-borne relapsing fever, whereas several other Borrelia species are transmitted by ticks.

Incubation Period

Symptoms generally develop 4–18 days after exposure, with an average incubation period of approximately 7 days.

Epidemiology and Transmission

Relapsing-fever Borrelia infections occur in different regions worldwide. Transmission depends on the particular species.

Louse-borne relapsing fever is primarily caused by B. recurrentis and is transmitted by the human body louse.

Tick-borne relapsing fever is transmitted primarily by infected soft ticks, particularly Ornithodoros species. Rodents and other small mammals frequently serve as reservoirs for tick-borne organisms.

Crowding, poor hygienic conditions, and situations that promote body-louse infestation increase the risk of louse-borne disease.

Clinical Infection

The principal clinical syndrome is relapsing fever. Patients typically develop an abrupt episode of high fever accompanied by chills, severe headache, myalgias, arthralgias, and generalized weakness.

The initial febrile episode resolves, but fever subsequently returns. These alternating periods of fever and improvement result from changes in the organism’s surface antigens, allowing Borrelia to temporarily escape the host immune response.

Multiple relapses are particularly characteristic of tick-borne disease.

Diagnosis

Diagnosis is most useful during a febrile episode, when large numbers of spirochetes may circulate in the bloodstream.

Peripheral blood smears stained with Giemsa or Wright stain may demonstrate the characteristic spirochetes. Dark-field microscopy can also visualize the organisms, although this technique is not routinely available in many laboratories.

PCR provides another method for detecting and identifying Borrelia and may be especially useful when microscopy is negative or species identification is required.

Serologic testing can be performed, but its usefulness is limited by imperfect sensitivity and specificity.

Treatment

Treatment differs somewhat between louse-borne and tick-borne relapsing fever.

For louse-borne relapsing fever, the traditional source regimen includes a single oral dose of tetracycline 500 mg or doxycycline 100 mg.

For tick-borne relapsing fever, treatment generally requires a longer course. The source regimen lists tetracycline 500 mg orally every 6 hours or doxycycline 100 mg orally every 12 hours for 7 days.

Current antibiotic choice and duration should be individualized according to the specific Borrelia infection, patient characteristics, disease severity, and contemporary guidelines.

Additional Treatment

Erythromycin may be used as an alternative in selected circumstances.

When infection is complicated by meningitis or encephalitis, an antibiotic with reliable central nervous system penetration is required. Intravenous penicillin G or ceftriaxone may be used.

Important Treatment Reaction

A major clinical consideration is the Jarisch–Herxheimer reaction, which may develop shortly after antibiotic therapy is started.

Rapid destruction of spirochetes can trigger an acute inflammatory response characterized by fever, chills, rigors, headache, myalgias, tachycardia, and hypotension. Patients, particularly those with severe infection, should therefore be monitored after the first antimicrobial dose.

High-Yield Clinical Pattern

Think of relapsing-fever Borrelia when a patient has repeated episodes of abrupt high fever separated by afebrile periods, particularly when there is a history of body-louse exposure or soft-tick exposure.

Demonstration of spirochetes on a peripheral blood smear obtained during a febrile episode strongly supports the diagnosis.

Exam Essentials

Genus: Borrelia

Organism type: Spirochete

Major infection: Relapsing fever

Important species: B. recurrentis

Incubation: Approximately 4–18 days

Louse-borne disease: Primarily B. recurrentis

Tick-borne disease: Several Borrelia species transmitted by soft ticks

Diagnosis: Giemsa/Wright-stained peripheral blood smear, PCR; serology has limitations

Treatment: Usually doxycycline or tetracycline

CNS disease: Penicillin G or ceftriaxone may be required

Classic treatment complication: Jarisch–Herxheimer reaction

Key clue: Recurrent febrile episodes separated by periods of improvement.