Rocky Mountain spotted fever is the most severe and most frequently reported rickettsial illness in the United States.  The disease is caused by Rickettsia rickettsii, a species of bacteria that is spread to humans by ixodid (hard) ticks.  Initial signs and symptoms of the disease include sudden onset of fever, headache, and muscle pain, followed by development of rash.  The disease can be difficult to diagnose in the early stages, and without prompt and appropriate treatment it can be fatal.  This web page presents a general overview of Rocky Mountain spotted fever; more detailed information can be found in the resources listed in Suggested Reading.

Figure 1.  Characteristic spotted rash of late-stage Rocky Mountain spotted fever on legs of a patient, ca. 1946

Rocky Mountain spotted fever was first recognized in 1896 in the Snake River Valley of Idaho and was originally called "black measles" because of the characteristic rash. It was a dreaded and frequently fatal disease that affected hundreds of people in this area. By the early 1900s, the recognized geographic distribution of this disease grew to encompass parts of the United States as far north as Washington and Montana and as far south as California, Arizona, and New Mexico.  

In response to this severe problem, the Rocky Mountain Laboratory was established in Hamilton, Montana.  This facility is now run by the National Institute of Allergy and Infectious Diseases, National Institutes of Health.  Researchers there continue to study Rocky Mountain spotted fever and other diseases.  Laboratory and epidemiologic studies were also carried out by the Communicable Disease Center (now the Centers for Disease Control and Prevention, or CDC) and are currently conducted by scientists in the Viral and Rickettsial Zoonoses Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Figure 2.  Rocky Mountain Laboratory, ca. 1928  Hamilton, Montana

Howard T. Ricketts was the first to establish the identity of the infectious organism that causes this disease. He and others characterized the basic epidemiologic features of the disease, including the role of tick vectors. Their studies found that Rocky Mountain spotted fever is caused by Rickettsia rickettsii.  This species is maintained in nature by a complex cycle involving ticks and mammals; humans are considered to be accidental hosts and are not involved in the natural transmission cycle of this pathogen.  Tragically, Dr. Ricketts died of typhus (another rickettsial disease) in Mexico in 1910, shortly after completing his remarkable studies on Rocky Mountain spotted fever. 

Figure 3.  Dr. Howard Taylor Ricketts

The name Rocky Mountain spotted fever is somewhat of a misnomer. Beginning in the 1930s, it became clear that this disease occurred in many areas of the United States other than the Rocky Mountain region. It is now recognized that this disease is broadly distributed throughout the continental United States, as well as southern Canada, Central America, Mexico, and parts of South America.  Between 1981 and 1996, this disease was reported from every U.S. state except Hawaii, Vermont, Maine, and Alaska.

Rocky Mountain spotted fever remains a serious and potentially life-threatening infectious disease today. Despite the availability of effective treatment and advances in medical care, approximately 3% to 5% of individuals who become ill with Rocky Mountain spotted fever still die from the infection. However, effective antibiotic therapy has dramatically reduced the number of deaths caused by Rocky Mountain spotted fever; before the discovery of tetracycline and chloramphenicol in the late 1940s, as many as 30% of persons infected with R. rickettsii died.

Figure 4.  Discovery of chloramphenicol and tetracycline antibiotics in the 1940s led to a sharp decline in RMSF-related mortality