Most of the several dozen hard ticks in the genus Dermacentor have a 3-host life cycle. Dermacentor ticks are found on all continents except Australia and Antarctica. In Eurasia, D. marginalis and D. reticulatus are well known as ectoparasites of livestock and other domestic animals, whereas in North America D. variabilis and D. andersoni parasitize livestock and dogs and may sometimes feed on humans as well. Dermacentor ticks are not significant pests of livestock in Africa. Dermacentor reticulatus is a vector for Rickettsia sibirica, which causes Siberian tick typhus in the former Soviet Union. Dermacentor variabilis and D. andersoni are vectors for Rickettsia rickettsia bacteria, which cause Rocky Mountain spotted fever, a life-threatening but treatable disease that is endemic to much of the United States and beyond, as well as tularemia (caused by the bacterium Francisella tularensis); these ticks are not known to transmit Lyme disease. (Jongejan and Uilenberg 2004; U.S. Center for Disease Control and Prevention Tick Website)
