What is the primary cause of Relapsing Fever?

Primary Cause of Relapsing Fever

Relapsing fever is primarily caused by various Borrelia species that are transmitted to humans through the bite of infected ticks, predominantly soft ticks of the genus Ornithodoros (Alectorobius). 1

Causative Organisms and Transmission

Tick-borne relapsing fever (TBRF) is caused by at least 16 different Borrelia species that have specific relationships with their tick vectors:

• Primary vector: Soft ticks of the genus Ornithodoros (Alectorobius) 1
• Disease severity: TBRF is a serious disease with an untreated mortality rate of up to 5% 1
• Vector-pathogen specificity: Each Borrelia species associated with relapsing fever appears to be specific to its tick vector 1

While most relapsing fever is transmitted by soft ticks, there are two notable exceptions:

• Borrelia recurrentis is transmitted by the human body louse (louse-borne relapsing fever) 2
• Borrelia miyamotoi is transmitted by hard-bodied ixodid ticks 2

Geographic Distribution

TBRF has a complex global distribution pattern:

• Worldwide presence: TBRF has been reported globally except in Antarctica and Australia 3
• North America: Most cases occur in western United States, southern British Columbia, with some cases in Mexico 4
• Europe: In southern Europe, recurrent fever caused by Borrelia hispanica has been reported in the Iberian Peninsula, Greece, and Cyprus 1
• Latin America: Historical and emerging evidence of TBRF exists in Mexico, Panama, Colombia, Venezuela, Peru, Argentina, Bolivia, Brazil, and Chile 5

Clinical Presentation

The hallmark of relapsing fever is the eponymous pattern of recurring febrile episodes:

• Defining feature: Recurring episodes of high fever with septicemic signs and symptoms 1, 3
• Most common symptom: Relapsing fever pattern (present in 100% of cases) 3
• Laboratory findings: Thrombocytopenia is the most suggestive laboratory finding 3
• Complications: Neurological complications are frequent 3

Diagnosis

Diagnosis of TBRF relies on several methods:

• Gold standard: Demonstration of borreliae in peripheral blood of febrile patients 1
• Microscopy: Blood smears examined by dark-field microscopy or stained with Giemsa or Wright stain (70% sensitivity) 1
• Molecular detection: Nested PCRs targeting fragments of 16S rRNA-encoding gene or flagellin gene can detect TBRF borreliae in human blood and ticks 1
• Serological limitations: Serological assays have limited diagnostic value due to antigenic variation shown by TBRF borreliae 1

Treatment and Prognosis

Treatment options include:

• Antibiotics: Beta-lactams, tetracyclines, or macrolides 3
• Jarisch-Herxheimer reaction: Risk appears lower in TBRF (19.3%) compared to louse-borne relapsing fever (55.8%) 3
• Mortality: Overall case fatality rate of TBRF is approximately 6.5% 3

Clinical Pearls and Pitfalls

• Diagnostic challenge: TBRF may be confused with other febrile illnesses like malaria, typhoid, or dengue, leading to misdiagnosis 5
• Exposure history: Consider TBRF in patients with recurring fevers who have history of exposure to rustic cabins, caves, or rural areas with known tick presence 4, 6
• Seasonal variation: While TBRF can occur year-round, it may have seasonal peaks (e.g., summer and autumn as reported in some regions) 6
• Perinatal risk: Unlike louse-borne relapsing fever where perinatal fatalities are primarily due to abortion, TBRF-related perinatal fatalities primarily affect newborns 3

Understanding the causative organisms and transmission patterns of relapsing fever is essential for proper diagnosis, treatment, and prevention of this potentially serious infection.