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Causes of Rocky Mountain spotted fever

Primary Cause of Rocky Mountain spotted fever

The primary cause of Rocky Mountain spotted fever is the result:

  • of an infectious agent.

Rocky Mountain spotted fever Causes: Risk Factors

The following conditions have been cited in various sources as potentially causal risk factors related to Rocky Mountain spotted fever:

Rocky Mountain spotted fever: Related Medical Conditions

To research the causes of Rocky Mountain spotted fever, consider researching the causes of these these diseases that may be similar, or associated with Rocky Mountain spotted fever:

Rocky Mountain spotted fever: Causes and Types

Causes of Broader Categories of Rocky Mountain spotted fever: Review the causal information about the various more general categories of medical conditions:

Rocky Mountain spotted fever as a complication of other conditions:

Other conditions that might have Rocky Mountain spotted fever as a complication may, potentially, be an underlying cause of Rocky Mountain spotted fever. Our database lists the following as having Rocky Mountain spotted fever as a complication of that condition:

What causes Rocky Mountain spotted fever?

Article excerpts about the causes of Rocky Mountain spotted fever:

Rocky Mountain Spotted Fever: Epidemiology: DVRD (Excerpt)

A history of tick bite or exposure to tick-infested habitats is reported in approximately 60% of all cases of Rocky Mountain spotted fever. (Source: excerpt from Rocky Mountain Spotted Fever: Epidemiology: DVRD)

Rocky Mountain spotted fever: Natural History: DVRD (Excerpt)

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

American dog tick (Dermacentor variabilis)

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

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. (Source: excerpt from Rocky Mountain spotted fever: Natural History: DVRD)

Medical news summaries relating to Rocky Mountain spotted fever:

The following medical news items are relevant to causes of Rocky Mountain spotted fever:

Related information on causes of Rocky Mountain spotted fever:

As with all medical conditions, there may be many causal factors. Further relevant information on causes of Rocky Mountain spotted fever may be found in:

 

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