Colorado Tick Fever in Central Oregon, 2018: A CDC Investigation

In early summer 2018, four residents of central Oregon were reported with Colorado tick fever (CTF), confirmed by CDC's reverse transcription–polymerase chain reaction (RT-PCR) test. Over the previous decade, the Oregon Health Authority had reported an average of less than one case a year.

About the disease

• Cause: a coltivirus spread by infected Rocky Mountain wood ticks (Dermacentor andersoni), found across the western United States and Canada, typically at 4,000–10,000 feet (1,219–3,048 meters) in grassy areas near sagebrush.
• Illness: an acute fever with nonspecific symptoms. Deaths are rare, but up to 30% of patients are hospitalized.
• Treatment: none specific — care is supportive.
• Telling it apart from other tick- and insect-borne infections: a biphasic illness, leukopenia (low white blood cell count), no rash, and where the person was exposed.
• Reporting: reportable in six states, including Oregon, but not nationally notifiable.

The four confirmed cases

CDC and Oregon health officials reviewed medical records and interviewed patients.

How they were exposed

• All spent ≥5 hours a day outdoors, including work in wooded or brushy areas, and all recalled a tick bite in the 2 weeks before falling ill.
• Three knew where: in two of the counties, at 3,200–4,500 feet (975–1,372 meters).
• No cluster: those exposure sites were spread across about 540 square miles (1,399 square kilometers).
• All wore long sleeves and pants outdoors, but none used insect repellent.

Looking for more cases

Investigators searched the same health system's electronic records for diagnosis codes for fever and leukopenia. A suspected case was fever and leukopenia with no other explanation in a patient seen April 15–July 31, 2018.

• Three suspected cases turned up: two children and one adult.
• The adult, a man in his 60s, had a blood sample positive for CTF virus–specific neutralizing antibodies. He had been bitten by a tick while hunting in the same county where two confirmed patients were exposed.

Conclusions

• Oregon saw more CTF in 2018 than in earlier years — possibly from more tick activity, or from greater awareness and testing by providers.
• With no common exposure site, the virus was likely circulating in several areas of central Oregon in spring 2018.
• Health departments should reinforce tick prevention, including EPA-registered insect repellents, and aim messages at people whose outdoor activities carry a high risk of tick exposure.

Sources

Based on "Notes from the Field: Investigation of Colorado Tick Fever Virus Disease Cases — Oregon, 2018," by Emily McDonald and others, of CDC, the Deschutes County Health Department, the Oregon Health Authority, Bend Memorial Clinic and St. Charles Health System, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention, citing Lambert and others, Marfin and Campbell, Romero and Simonsen, Yendell and others, Staples and Fischer, and Brackney and others; a work of the United States government in the public domain.