Rocky Mountain Spotted Fever (cont.)

Medical Author:
Medical Editor:

What causes Rocky Mountain spotted fever?

RMSF is caused by a small bacterium known as Rickettsia rickettsii. R. rickettsii lives inside the cells that line the blood vessels of infected animals and humans.

The tick is the primary home or reservoir for R. rickettsii. Because ticks can also spread the organism to humans and other animals, they are sometimes referred to as vectors for transmission of RMSF. Several different types of ticks can carry R. rickettsii. For example, the American dog tick (Dermacentor variabilis) is the most common vector in the eastern, central, and Pacific U.S. In the West, the Rocky Mountain wood tick (Dermacentor andersoni) is the primary vector for RMSF. Ticks can pass the organism to their offspring, creating a new generation of infected ticks. Ticks can also be infected by feeding on an infected person or animal. Even in the woodlands and fields of high-risk areas, only a small proportion of ticks will carry R. rickettsii.

Picture of a Rocky Mountain wood tick
Picture of a Rocky Mountain wood tick. SOURCE: CDC/Dr. Christopher Paddock.
Picture of American dog ticks.
Picture of American dog ticks.

What are risk factors for Rocky Mountain spotted fever?

Cases of RMSF occur when the appropriate tick vector comes in contact with human populations. Risk factors include traveling to an area with a high rate of RMSF, especially in seasons when ticks are plentiful, such as summer. People at highest risk are those who frequent forested areas or fields, such as those who are hiking or camping. Even urban outdoor areas pose a risk, however. Dog ownership is a risk factor in areas where the American dog tick exists. Dogs acquire ticks while roaming outside, and these ticks can be transferred to humans during petting or other contact.

The risk of RMSF is thought to increase with the duration of tick attachment. A feeding tick can remain attached to a human for up to two weeks. Crushing an attached tick improperly may cause secretions from the tick to be injected into the skin and increase the risk of infection.

Severe disease is more common in the elderly, alcoholics, and in African Americans. The latter is at least partly due to delays in diagnosis of the typical rash in dark-skinned people. In addition, blacks are more likely to have a genetic enzyme deficiency (G6PD) that can make the disease more severe.

Medically Reviewed by a Doctor on 4/30/2014

Patient Comments

Viewers share their comments

Rocky Mountain Spotted Fever - Location Question: Please share your experience with RMSF, including the region in which you contracted it.
Rocky Mountain Spotted Fever - Symptoms Question: What were your signs and symptoms associated with RMSF?
Rocky Mountain Spotted Fever - Diagnosis Question: What tests or methods were used to detect and diagnose your case of RMSF?
Rocky Mountain Spotted Fever - Treatment Question: What kinds of treatment or medication did you receive for RMSF?
Rocky Mountain Spotted Fever - Prevention Question: If you live in a region with ticks or previous outbreaks of RMSF, how do you try to prevent it?