Tickborne Relapsing Fever Endemic to Western United States

Jennifer Garcia

January 30, 2015

A new report from the US Centers for Disease Control and Prevention (CDC) finds that tickborne relapsing fever (TBRF) is endemic to the mountainous regions of the western United States, particularly among nonresident male visitors. These results were published in the January 30 issue of the Morbidity and Mortality Weekly Report.

Researchers led by Joseph D. Forrester, MD, from the Epidemic Intelligence Service, CDC and the Division of Vector-Borne Disease, CDC, evaluated data from 504 TBRF cases reported between 1990 and 2011. The cases were reported from 12 western states, with California, Washington, and Colorado accounting for approximately 70% of cases.

TBRF is most often caused by caused by Borrelia hermsii and transmitted by Ornithodoros hermsi ticks. There is no standard case definition, so "for the purpose of this report, a TBRF case was defined as a clinically compatible illness with laboratory confirmation of infection or a clinically compatible illness epidemiologically linked to a laboratory-confirmed case." TBRF was most often diagnosed during the summer months among males (57%) visiting areas where TBRF is endemic. The median age was 38 years, with peaks among those aged 10 to 14 years and 40 to 44 years. The study authors suggest the bimodal age distribution may reflect healthcare-seeking behavior or differences in clinical manifestations.

The researchers note that febrile episodes among patients with TBRF typically last 3 to 5 days, with relapses occurring 5 to 7 days after apparent recovery. Although clinical signs are typically mild and easily treated with antibiotics, complications including acute respiratory distress syndrome and spontaneous abortion have been reported. In the acute setting, diagnosis can be confirmed by identifying spirochetes in a blood smear obtained during a febrile episode.

"Because tick-infested buildings can serve as a source of infection for years, it is important to investigate all TBRF cases to identify the likely location of exposure and guide remediation of rodent and tick infestations," write Dr Forrester and colleagues.

The authors acknowledge that the lack of clear case definition and standard surveillance for TBRF may lead to reporting bias and underestimation of disease incidence.

"Clinicians need to consider TBRF as a cause of febrile illness in visitors to, and persons living in, areas where TBRF is endemic," the study authors write. The researchers also suggest that "[e]ducational outreach would further public health objectives to increase awareness of TBRF prevention measures and clinical signs and symptoms of disease."

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. Published online January 30, 2015. Full text

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