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Rocky Mountain
spotted fever, like all rickettsial infections, is classified as a
zoonosis. Zoonoses are diseases of animals that can be
transmitted to humans. Many zoonotic diseases require a vector
(e.g., a mosquito, tick, or mite) in order 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 bite. Less commonly,
infections may occur following exposure to crushed tick tissues,
fluids, or tick 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 the eggs hatch,
each stage must feed once to develop into the next stage. Both male
and female ticks will bite.
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
spermatazoa 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 several 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 1%-3% of the tick population carries R. rickettsii,
even in areas where 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 States, the
American dog tick and the Rocky Mountain wood tick.
American
dog tick
(Dermacentor variabilis) is widely distributed east of the
Rocky Mountains and also occurs in limited areas on 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 humans. This tick is the most commonly identified
species responsible for transmitting R. rickettsii to humans.
Figure 7. Approximate distribution of the American dog
tick
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American dog tick (Dermacentor variabilis) |
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Rocky
Mountain wood tick
(Dermacentor andersoni) is found 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 rodents.
Figure 8.
Approximate distribution of the Rocky Mountain wood tick
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Rocky Mountain wood tick (Dermacentor andersoni) |
Other Tick
Species
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.
Other Pages with information on Rocky Mountain Spotted Fever:
Rocky
Mountain Spotted Fever - History
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Rocky Mountain Spotted Fever Overview
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Rocky
Mountain Spotted Fever The Organism
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Rocky Mountain Spotted Fever
Epidemiology
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Rocky Mountain Spotted Fever
Signs and Symptoms
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Rocky Mountain spotted fever
Detection
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Rocky Mountain Spotted Fever
Treatment
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Rocky Mountain Spotted Fever
Prevention and Control
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