What is rocky mountain spotted fever?

Quick Answer
An acute febrile illness caused by Rickettsia rickettsii.
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Causes and Symptoms

In 1906, a young Ohio-born physician, Howard Taylor Ricketts, went to Montana to study the cause of “spotted fever.” Through his efforts, Rocky Mountain spotted fever became the first tickborne infection to be recognized in North America. For the first half of the twentieth century, the disease was found mostly in the western United States, but since then the majority of the cases have been in the south Atlantic and south-central states. Dermacentor andersoni (Rocky Mountain wood tick) and D. variabilis (American dog tick) have been the two dominant tick species serving as vectors in the west and south, respectively.

Following the bite of an infected tick, rickettsia are transmitted through the skin into the lymphatic system and small blood vessels. Once in the bloodstream, they invade endothelial cells. The rickettsia multiply intracellularly by binary fission and spread to adjacent endothelial cells, injuring and killing their cellular hosts, which results in widespread vascular damage.

The incubation period or time from tick bite to illness is usually about seven days. The illness begins with fever, often greater than 102 degrees Fahrenheit, accompanied by headache and myalgias (muscle pain). The classic rash usually begins on the third day, starting on the wrists and ankles and spreading to the trunk. The palms and soles are often involved. The rash begins as small blanching macules and progresses to petechiae. Shock, abdominal pain, and neurological problems may mimic other diseases, making the correct diagnosis more difficult. Specific laboratory confirmation can be obtained by measuring antibodies two to three weeks later.

Treatment and Therapy

Doxycycline, a tetracycline antibiotic, is the treatment of choice for both children and adults. Therapy with doxycycline should be started immediately if Rocky Mountain spotted fever is suspected, as delays in therapy are associated with poor outcomes. Prevention can be accomplished by measures to prevent tick bites, such as wearing light-colored clothing and using insecticides and repellants on clothing and skin. Careful removal of ticks from individuals and their dogs before injection of rickettsia can occur will also prevent the disease.

Perspective and Prospects

Rocky Mountain spotted fever is spreading into new geographic locations with the acquisition of new tick vectors. It is also no longer a rural disease and has been found in urban parks. Prompt treatment with doxycycline has reduced the mortality rate from more than 80 percent in the pre-antibiotic era to less than 1 percent. Early diagnostic tests are lacking but are needed, as the majority of Rocky Mountain spotted fever cases are incorrectly diagnosed upon the first visit to a doctor. Thus far, researchers have been unable to develop a protective vaccine.


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