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Rocky Mountain Spotted Fever

Rocky Mountain Spotted Fever Overview

Rocky Mountain spotted fever (RMSF) is a disease caused by Rickettsia rickettsii (transmitted by tick bites to humans) that has nonspecific symptoms of fever, chills, headache, and muscle aches with progression to a rash about five to 10 days after an initial bite by an infected tick. RMSF is the most common fatal tick-transmitted disease in the U.S. Worldwide, there are about 18 other closely related Rickettsia species belonging to a group (spotted fever group) that cause a similar disease process known by many different names (for example, Boutonneuse fever, African tick bite fever, Japanese spotted fever). RMSF was first diagnosed in 1896 in Idaho and was first named "black measles" but then was termed Rocky Mountain spotted fever. In 1906, Howard Ricketts showed that ticks were vectors (carriers) of the pathogenic bacteria and in 1909 showed evidence that bacteria (later named after him) caused the disease. Ticks can act as vectors for many diseases (for example, Lyme disease, tularemia, Q fever) in addition to RMSF.

Rocky Mountain Spotted Fever Causes

RMSF is caused by R. rickettsii, which are bacteria that are Gram stain negative, aerobic coccobacilli (round and rod shaped) that must live within eukaryotic cells (cells, such as human cells, that have a nucleus bound by a membrane). The organisms only proliferate if they are inside of a eukaryotic cell. In most cases of human infections, the endothelial cells that line blood vessels are the cells that harbor the bacteria. These cells can be disrupted by the organisms, thus producing vascular permeability (leaky blood vessels) that results in macules (flat areas of skin color change) and eventually a petechial rash (red or purple spots about 1 mm-2 mm in diameter usually caused by broken blood vessels). This vascular permeability can occur throughout the body and eventually cause death.

Although RMSF is caused by R. rickettsii, the bacteria need to be transmitted to humans by a tick vector (carrier) infected with R. rickettsii. RMSF will not occur in people bitten by uninfected ticks. The following ticks have been shown to be vectors for RMSF: the mountain wood tick (Dermacentor andersoni), the American dog tick (Dermacentor variabilis), and the brown dog tick (Rhipicephalus sanguineus), with the first two as the primary vectors. Other ticks can be responsible as vectors in various worldwide regions.

Picture of female and male wood ticks
Picture of female and male wood ticks; Source: CDC
Medically Reviewed by a Doctor on 9/9/2015

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