Tick Fever

Basics

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.

TBRF
- 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.
CTF
- 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.

TBRF
- 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.
CTF
- 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.

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