Historical:
Tick-borne relapsing fever was first described in the United States in 1915 (Colorado).
- The first case of louse-borne infection in the United States during the twentieth century was reported in 1976 – imported from Ethiopia.

Time and Place:
Peak incidence is reported during the summer, with 47% of cases from July to August.
- The disease is most common in Arizona, California, Colorado and Oregon.
- Sporadic cases are reported from Idaho, Kansas, Montana, Nebraska, New Mexico, Oklahoma, Texas, Utah, Washington and Wyoming.
- Most cases occur in the Cascade, Rocky Mountain, San Bernadino and Sierra Nevada ranges.
- 40% of cases occur in tourists to endemic areas (1977 to 2000).

Washington and Oregon reported 30 cases during 1965 to 1978.
- Washington reported 24 cases during 1991 to 1994.
- California reported 173 cases during 1964 to 1989; Oregon 73; Washington 27; Arizona 74; Texas 25; Idaho 13; New Mexico 15; Oklahoma 2.
- California reported 3 to 18 cases yearly during 1985 to 2002.
- Texas reported 13 cases during 1990 to the first half of 1994.
- Five cases were reported in the Lake Tahoe region of California in 2005, 3 during the summer of 2006.

Incidence:
- Tick-borne relapsing fever is reportable in Arizona, California, Colorado, Idaho, New Mexico, Nevada, Texas, Utah and Washington.
- 450 cases (300 confirmed and 150 probable) were reported by 11 western states during 1977 to January 2000. Only one fatal case was reported during this period – a neonate born to an infected mother.
- 285 cases were reported by California, Colorado, Idaho, Texas and Washington during 1985 to 1996.
- There is evidence for the existence of Borrelia turicatae in Florida.

An American tourist acquired relapsing fever in Israel (1988 publication).

Infecting species:
- Borrelia hermsii (vector Ornithodoros hermsi) and B. coriaceae in the western region
- B. turicatae (vector O. turicatae) in the southwest
- B. parkeri (vector O. parkeri) in the west
- B. mazzottii (vector O. talaje) in the south.

Notes regarding transmission:
- Ornithodoros hermsi is nocturnal and feeds within 20 to 90 minutes – thus, patients rarely recall the tick bite.
- B. hermsii is maintained in small sylvatic rodents, notably chipmunks (Tamias spp.) and pine squirrels (Tamiascuirus spp.) above 1,000 meters’ elevation.
- Antibodies to Borrelia hermsii are present in 5.4% of big brown bats (Eptesicus fuscus). (Georgia, 2007 publication)
- Borellia turicatae has been detected in dogs in texas. (2007 publication) 10

Notable outbreaks:
All of the following outbreaks were associated with tick (Ornithodoros spp.) infestations of rodent nests in cabins.
1968 – An outbreak was reported in Spokane County, Washington.
1973 – Outbreaks (62 clinical cases, 16 confirmed) were reported in Grand Canyon National Park, Arizona.
1989 – An outbreak (6 cases) was reported in San Bernardino County, California.
1990 to 1991 – Outbreaks (24 cases, 14 confirmed) were reported in Grand Canyon National Park, Arizona. No outbreaks were reported in this area during 1992 to 1999.
1995 – An outbreak (23 family members) was reported among campers in Colorado.
2002 – An outbreak (5 cases) of Borrelia hermsii infection was associated with a cabin in western Montana. Infected ticks (Ornithodoros hermsi) were identified at the time.
2002 – An outbreak was associated with a family gathering in New Mexico.

Update: Quoted in ProMED

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