Rocky Mountain spotted fever natural history
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
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 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.
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 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.
There are 2 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 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 ticks (Dermacentor andersoni) are 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 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.
Complications
Possible complications include:
- Brain damage
- Clotting problems
- Heart failure
- Kidney failure
- Lung failure
- Meningitis
- Pneumonitis (lung inflammation)
- Shock
Prognosis
Treatment usually cures the infection. Complications are rare but can include paralysis, hearing loss, nerve damage, and, rarely, death. Rocky Mountain spotted fever can be a severe illness, and the majority of patients are hospitalized. Infection with R. rickettsii is thought to provide long lasting immunity against re-infection. However, prior illness with Rocky Mountain spotted fever should not deter persons from practicing good tick-preventive measures or visiting a physician if signs and symptoms consistent with Rocky Mountain spotted fever occur, especially following a tick bite, as other diseases may also be transmitted by ticks.
Before antibiotics were readily available to fight off Rocky mountain spotted fever, the mortality rate was as high as 30%. Now, with drugs such as doxycycline available, the mortality rate is approximately 1.4%.