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- Tick-borne relapsing fever (TBRF) and Colorado tick fever (CTF) is discussed in this chapter.
- TBRF is a vector-borne infection characterized by recurrent fevers caused by several species of spirochetes of the genus Borrelia. In the United States, the vector for TBRF is the soft-bodied tick of the genus Ornithodoros.
- CTF is an acute, febrile, usually benign systemic illness caused by a coltivirus in the family Reoviridae and transmitted by a tick bite. Although the primary reservoir for infection is the Dermacentor andersoni tick (wood tick), the causative organism has been isolated from many other ticks.
- Reported in almost all western states up to and including Texas with the highest incidence reported in the states of California, Washington, and Colorado
- Ornithodoros ticks inhabit the burrows and nests of rodents living in caves and coniferous forests at elevations between 1,500 and 8,000 feet.
- The most important risk factor for human infection is sleeping in a rodent-infested cabin or house in endemic, forested areas.
- Most cases present during June through September.
- Approximately 20 cases were reported annually in the United States between 1990 and 2011.
- Age distribution is bimodal, with peaks among persons aged 10 to 14 years and 40 to 44 years, with a male preponderance.
- Human infections typically occur in areas where D. andersoni is found: western United States and southwestern Canada at elevations of 4,000 to 10,000 feet.
- The U.S. states with the highest incidence include Wyoming, Montana, and Utah.
- <15 cases are reported annually in the United States, typically between May and July when adult ticks are most active.
- Infection is more common in males and people aged ≥40 years, but 15% of cases occur in those <20 years.
- Transfusion-related and laboratory-associated infection are rare but have been reported.
- Both of these infections can be prevented by avoidance or protection from ticks.
- Light-colored, long-sleeved shirts and pants should be worn when tick-infested areas cannot be avoided.
- Permethrin should be applied to clothing and N,N-Diethyl-meta-toluamide (DEET) and picaridin applied to exposed skin to help repel ticks.
- Persons who enter endemic areas should inspect themselves and each other frequently for adherent ticks.
- Avoid rodent-infested homes in endemic areas. If necessary, rodent-nesting materials should be removed with protective gloves.
- Other measures include the application of acaricides to nesting sites in human habitations in endemic areas and eliminating rodent access to unnatural food sources.
- Confirmed cases should be reported to health authorities so that control measures can be instituted.
- When an Ornithodoros tick feeds on a natural host (e.g., squirrels, chipmunks, and rodents), Borrelia subsequently invade all tissues of the tick including the salivary glands. Ticks in the larval stage are unlikely to be infectious.
- Borrelia is often transmitted to humans through a painless and brief (<30 minutes) nocturnal tick bite. Transmission may occur within minutes of the start of a blood meal. After transmission, spirochetemia develops, resulting in systemic symptoms.
- Between episodes of spirochetemia, organisms likely persist in the CNS, bone marrow, liver, and spleen.
- The relapsing fever pattern is attributed to antigenic variation which allows the spirochete to temporarily evade host immunity and rebound.
- Pathologic findings in humans include petechial hemorrhages on visceral surfaces, hepatosplenomegaly, and a histiocytic myocarditis.
- Ticks are infected during their larval stage when they feed on viremic, intermediate hosts such as chipmunks, ground squirrels, and porcupines.
- Once infected, ticks remain infected for life (as long as 3 years).
- Human infection typically occurs during outdoor recreational (camping, hiking) or occupational activities when the adult D. andersoni wood tick attaches and ingests a blood meal from an incidental human host.
- CTF virus is thought to infect hematopoietic cells, causing leukopenia and prolonged viremia for up to 3 to 4 months.
- TBRF is caused by several species of spirochetes in the genus Borrelia. Borrelia hermsii, Borrelia turicatae, and Borrelia parkeri are the most common species found in the United States.
- CTF is caused by CTF virus, a double-stranded RNA coltivirus in the family Reoviridae.