Rickettsial Diseases, including Typhus and Rocky Mountain Spotted Fever (page 3)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Rocky Mountain spotted fever, like all rickettsial infections, is
as a zoonosis. Zoonoses are diseases of animals that can
transmitted to humans. Many zoonotic diseases require a vector (e.g., a
mosquito, tick, or mite) in order to be transmitted from the animal
to the human host. In the case of Rocky Mountain spotted fever, ticks
the natural hosts, serving as both reservoirs and vectors of R.
Ticks transmit the organism to vertebrates primarily by their bite.
commonly, infections may occur following exposure to crushed tick
fluids, or feces.
Only members of the tick family Ixodidae (hard ticks) are
naturally infected with Rickettsia rickettsii. These ticks have
four stages in their life cycle: egg, larva, nymph, and adult. After
eggs hatch, each stage must feed once to develop into the next stage.
male and female ticks will bite.
A female tick can transmit R. rickettsii to her eggs in a
called transovarial transmission. Ticks can also become infected with R.
rickettsii while feeding on blood from the host in either the
or nymphal stage. After the tick develops into the next stage, R.
may be transmitted to the second host during the feeding process.
Furthermore, male ticks may transfer R. rickettsii to female
through body fluids or spermatazoa during mating. In this
manner, generations or each life stage of infected ticks are
Once infected, the tick can carry the rickettsiae for life.
Rickettsiae are transmitted to a vertebrate host through saliva
a tick is feeding. It usually takes several hours of attachment and
before the rickettsiae are transmitted to the host. The risk of
to a tick carrying R. rickettsii is low. Generally, about 1 -3%
of the tick population carries R. rickettsii, even in areas
the majority of human cases are reported.
Major Tick Vectors in the United States
There are two major vectors of R. rickettsii in the United
the American dog tick and the Rocky Mountain wood tick.
The American dog tick (Dermacentor variabilis) is widely
east of the Rocky Mountains and also occurs in limited areas along the
Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of
adult D. variabilis, although it feeds on other large mammals,
humans. This tick is the most commonly identified species responsible
transmitting R. rickettsii to humans.
Figure 5. American dog tick
Figure 6. Approximate
distribution of the American dog tick. (CDC)
The Rocky Mountain wood tick (Dermacentor andersoni) is
in the Rocky Mountain states and in southwestern Canada. The life cycle
of this tick may require up to 2 to 3 years for completion. Adult ticks
feed primarily on large mammals. Larvae and nymphs feed on small
Figure 7. Rocky Mountain wood
tick (Dermacentor andersoni). (CDC)
Figure 8. Approximate
distribution of the Rocky Mountain wood tick. (CDC)
Other tick species have been shown to be naturally infected with R.
rickettsii, but these species are likely to play only a minor role
the ecology of R. rickettsii.