Key points
- Colorado tick fever is an uncommon disease spread by the bite of an infected Rocky Mountain wood tick found in the western United States and western Canada.
- Colorado tick fever is not a nationally notifiable disease; however, several states require that cases be reported to the state health department.
About Colorado tick fever
Colorado tick fever occurs in people who live in or visit areas where there are infected Dermacentor andersoni ticks. These ticks are found in the western United States or western Canada at elevations of 4,000-10,000 feet above sea level. Most cases of Colorado tick fever occur during spring or summer months when ticks are most active.
In the United States, a total of 223 Colorado tick fever cases were reported to CDC from 2003 through 2022. During this time frame, there was a median of 7 cases (range 3 to 37) reported annually.
Colorado tick fever is not a nationally notifiable disease; however, several states require that cases be reported to the state health department. As of 2023, Colorado tick fever was reportable in ten states: Arizona, Colorado, Idaho, Montana, New Mexico, Oregon, South Dakota, Utah, Washington, and Wyoming. All state health departments are encouraged to report Colorado tick fever cases to CDC on a voluntary basis.
*Derived from James AM, Freier JE, Keirans JE, Durden LA, Mertins JW, Schlater JL. Distribution, seasonality, and hosts of the Rocky Mountain wood tick in the United States. J Med Entomol. 2006;43(1):17-24.
†24 cases had no information on county of residence