The lifecycle of Dermacentor variabilis and Dermacentor andersoni ticks (Family Ixodidae) American dog tick (Dermacentor variabilis) range<unk><unk>  wood tick (Dermacentor andersoni) range Ticks are the natural hosts of the disease, serving as both reservoirs and vectors of R. rickettsii. Ticks transmit the bacteria primarily by their bites. Less commonly, infections may occur following exposure to crushed tick tissues, fluids, or tick feces. A female tick can transmit R. rickettsii to her eggs in a process called transovarial transmission. Ticks can also become infected with R. rickettsii while feeding on blood from the host in either the larval or nymphal stage. After the tick develops into the next stage, the R. rickettsii may be transmitted to the second host during the feeding process. Furthermore, male ticks may transfer R. rickettsii to female ticks through body fluids or spermatozoa during the mating process. These types of transmission represent how generations or life stages of infected ticks are maintained. Once infected, the tick can carry the pathogen for life. Rickettsiae are transmitted through saliva injected while a tick is feeding. Unlike Lyme disease and other tick-borne pathogens that require a prolonged attachment period to establish infection, a person can become infected with R. rickettsii in a feeding time as short as 2 hours. In general, about 1-3% of the tick population carries R. rickettsii, even in areas where the majority of human cases are reported. Therefore, the risk of exposure to a tick carrying R. rickettsii is low. The disease is spread by the American dog tick (Dermacentor variabilis),<unk><unk>  wood tick (D. andersoni), brown dog tick (Rhipicephalus sanguineus), and Amblyomma sculptum. Not all of these are of equal importance, and most are restricted to certain geographic areas. The two major vectors of R. rickettsii in the United States are the American dog tick and the<unk><unk>  wood tick. American dog ticks are widely distributed east of the<unk><unk> s and they also occur in limited areas along the Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of an adult American dog tick, although it feeds readily on other large mammals, including human beings. This tick is the most commonly identified species responsible for transmitting R. rickettsii to humans.<unk><unk>  wood ticks (D. andersoni) are found in the<unk><unk>  states and in southwestern Canada. The lifecycle of this tick may require up to three years for its completion. The adult ticks feed primarily on large mammals. The larvae and nymphs feed on small rodents. Other tick species have been shown to be naturally infected with R. rickettsii or serve as experimental vectors in the laboratory. These species are likely to play only a minor role in the ecology of R. rickettsii.
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