Rocky Mountain Spotted Fever - Natural History

Natural History

Rocky Mountain spotted fever, like all rickettsial infections, is classified as a zoonosis. Zoonoses are diseases of animals that can be transmitted to humans. Some zoonotic diseases require a vector (e.g., a mosquito, tick, or mite) to be transmitted from the animal host to the human host. In the case of Rocky Mountain spotted fever, ticks are the natural hosts, serving as both reservoirs and vectors of R. rickettsii. Ticks transmit the organism to vertebrates 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 to a vertebrate host through saliva while a tick is feeding. It usually takes about 24 hours of attachment and feeding before the rickettsiae are transmitted to the host. The risk of exposure to a tick carrying R. rickettsii is low. In general, about one to three percent of the tick population carries R. rickettsii, even in areas where the majority of human cases are reported.

Vectors include Dermacentor variabilis, Dermacentor andersoni, Rhipicephalus sanguineus, and Amblyomma cajennense. However, not all of these are of equal importance, and most are restricted to certain geographic areas.

There are two major vectors of R. rickettsii in the United States: the American dog tick and the Rocky Mountain wood tick. American dog ticks (Dermacentor variabilis) are widely distributed east of the Rocky Mountains and they also occur in limited areas along the Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of adult D. variabilis, 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. Rocky Mountain wood ticks (Dermacentor andersoni) are found in the Rocky Mountain states and in southwestern Canada. The life cycle 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. However, these species are likely to play only a minor role in the ecology of R. rickettsii.

There are only 800 cases of Rocky Mountain Spotted Fever reported in the United States per year, and in only about 20 percent can the tick be found.