Closing the Brief Case: Tick-Borne Relapsing Fever in a Returned Traveler

ANSWERS TO SELF-ASSESSMENT QUESTIONS

1. Which of the following tests is not recommended for detection of acute tick-borne relapsing fever (TBRF)?

   • a.
     Serology
   • b.
     PCR
   • c.
     Giemsa smear of peripheral blood
   • d.
     Dark-field microscopy of peripheral blood

Answer: a. Relapsing fever Borrelia (RFB)-specific serology is only intended for retrospective diagnosis or epidemiological studies. Lyme disease (LD) serology will often be positive in RFB patients, and that testing should only be ordered if it is clinically indicated. A thorough patient history noting the frequency and duration of febrile episodes, recent travel, place of residence, and animal/vector exposure can be helpful in the selection of appropriate diagnostic tests for acute TBRF.

1. What is the typical morphology of RFB on Giemsa-stained thick smears?

   • a.
     Short, curved rods
   • b.
     Long (>10 μm), sinusoidal spirochetes
   • c.
     Long (>10 μm), irregularly curved or “stringy” spirochetes
   • d.
     Long (>10 μm), straight, boxy rods

Answer: c. RFB cells typically lose their predictable sinusoidal morphology on thick smears and will appear as irregularly curved or “stringy” spirochetes. The sinusoidal morphology seen in Fig. 1A in the Brief Case is typical of the morphology seen on thin smears, while Fig. 1B illustrates the irregular morphology observed on thick smears.

1. What is the most sensitive method for detection of acute phase TBRF from blood specimens?

   • a.
     Culture of peripheral blood
   • b.
     PCR of peripheral blood
   • c.
     Giemsa smear of peripheral blood
   • d.
     Dark-field microscopy of peripheral blood

Answer: b. PCR is the most sensitive laboratory method for detection of RFB in blood specimens. It is, however, extremely important to understand the inclusivity of the specific PCR test that is ordered. While the analytical sensitivity of the assay for the intended species may be very high, the analytical specificity may be prohibitively narrow for rare or nonendemic species of RFB. Examination of peripheral blood smears would be recommended for such cases.

TAKE-HOME POINTS

• •Tick-borne relapsing fever is endemic to many parts of the world and is attributable to many different species of Borrelia.
• •PCR is the most sensitive method for detecting relapsing fever Borrelia, but one assay may not reliably detect all causative species. Collecting a thorough travel history is critical for determining appropriateness of PCR.
• •Blood smears are capable of detecting any species of relapsing fever Borrelia, but are only 80% sensitive on average. If travel history is unclear, a peripheral smear should always be ordered regardless of whether PCR is also ordered.
• •Serology specific for TBRF is only indicated for retrospective diagnosis or epidemiological studies and should not be used for acute diagnosis.
• •TBRF typically results in the generation of crossreactive antibodies that can be detected by commercial Lyme disease (LD) enzyme-linked immunosorbent assays (ELISAs), which can lead to clinical confusion. Care should be taken when ordering LD serology in patients suspected of TBRF.

See https://doi.org/10.1128/JCM.01621-19 in this issue for case presentation and discussion.