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Rocky Mountain Spotted Fever Home > 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.
Many zoonotic diseases require a biological vector (e.g.,
a mosquito, tick, flea, 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.
Rickettsia rickettsii usually infects members
of the tick family Ixodidae (hard ticks). Although a
closely related rickettsia has been found in the soft
bat tick, Carios kelleyi. 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.
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.
Ticks can also become infected with R. rickettsii
while feeding on blood from the host in either the larval,
nymphal, or adult stage. After an immature tick develops
into the next stage, R. rickettsii may be transmitted
to a 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. Once infected, a tick can carry
the pathogen for life. A female tick can also transmit
R. rickettsii to her eggs in a process called
transovarial transmission. These types of transmission
represent how generations or life stages of infected
ticks are maintained.
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. However, other ticks can acquire
R. rickettsii in nature and some may serve as
experimental vectors of R. rickettsii in the
laboratory.
American Dog 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.
Rocky Mountain Wood Tick
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.
Other Tick Species
Other tick species like Rhipicephalus sanguineus
and Amblyomma cajennense have been shown to be
naturally infected with R. rickettsii or serve
as experimental vectors in the laboratory. While these
species appear to play only a minor role in the ecology
and transmission of R. rickettsii in the United
States, they can cause infections in visitors to Central
and South America.
Date last reviewed: 05/20/2005 |
