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- Tick-borne relapsing fever (TBRF) and Colorado tick fever (CTF) will be 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 a febrile, usually benign, systemic illness caused by 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
- Sites of high exposure include limestone caves and forested areas.
- Most cases present during June through September; ∼450 cases were reported in the United States between 1977 and 2000.
- Human infections typically occur in areas where D. andersoni is found: Western United States and southwestern Canada at elevations of 4,000–10,000 feet
- Cases usually occur between May and June when adult ticks are most active.
- There are only a very small number of cases reported annually the United States.
- Infection is more common in males and the median age of those infected is 43 years, but 25% of cases occur in those younger than 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 the tick vector.
- Light-colored, long-sleeved shirts and pants should be worn when tick-infested areas cannot be avoided.
- Permethrin should be applied to clothing and diethyltoluamide (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.
- Confirmed cases should be reported to health authorities so that control measures can be instituted.
- Ornithodoros ticks typically feed at night and for short periods.
- 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 transmitted to humans when the tick takes a blood meal and then detaches itself. Transmission is possible 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.
- 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 takes place 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 3–4 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.