Tick Fever

Basics

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.

TBRF
- 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.
CTF
- 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.

TBRF
- 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.
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 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.

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