Transmission

People become infected with Colorado tick fever (CTF) virus from the bite of an infected Rocky Mountain wood tick (Dermacentor andersoni). Rocky mountain wood ticks are found in the western United States and western Canada at 4,000–10,000 feet above sea level.

Ticks become infected with CTF virus when they feed on the blood of a reservoir animal (an animal that has the virus circulating in its bloodstream). The most important reservoirs for CTF virus are small rodents such as squirrels, chipmunks, and mice. After the tick becomes infected, it passes the virus to other hosts (animal or human) while it feeds.

The virus is not transmitted from person to person, except in rare instances by blood transfusion. Since CTF virus can remain in red blood cells for several months, blood and bone marrow should not be donated for 6 months following infection.

Ecology of Colorado Tick Fever Virus - 1. Small rodents such as squirrels, chipmunks, and mice are infected with CTF virus through tick bites. These animals usually do not become ill but they can pass the virus to other ticks that feed on them. 2. CTF virus is also passed from one stage of the tick life cycle to the next - from larvae to nymph to adult. 3. People are infected with CTF virus through the bite of infected ticks. People who work or play outdoors are most likely to be exposed to ticks. 4. Other animals such as elk, marmots, and deer also can be infected with CTF virus through tick bites. However, these animals probably do not play an important role in passing the virus to other ticks.

Page last reviewed: February 26, 2021

Content source: Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Vector-Borne Diseases (DVBD)