Figure 1. Colorado tick fever virus, Florio strain (originally isolated by Carl Eklund, Rocky Mountain Laboratory, Hamilton Mt.), passaged in BHK-21 cells. Virions, partially purified by differential ultracentrifugation and stained with potassium silicotungstate, are 80 nm in diameter and composed of three capsid layers. In this image the outer thin, indistinct capsid layer is missing, showing the icosahedral structure of the middle capsid. (Courtesy of Dr Frederick A. Murphy, University of California, Davis.)

Figure 2. Dermacentor andersoni (Rocky Mountain wood tick), the principal vector of Colorado tick fever virus. It is also responsible for transmission of Rickettsia spp. (causing Rocky Mountain spotted fever), and Francisella tularensis, the bacterium that causes tularaemia (Hunters disease). Females lay about 4000 eggs in masses at a single location. Adults and nymphs can be found from March to mid-summer. Larvae feed on small mammals such as ground squirrels and chipmunks, whereas adults focus on large mammals, especially deer, canids, livestock and humans. They can be collected primarily in the upper eastern part of the state, from Modoc county down to the eastern range of the northern Sierra as well as British Columbia, Alberta and Saskatchewan in Canada. (Courtesy of Webmedic4u Infectious Disease Information Center: http://home.earthlink.net/~webmedic4u/)