Rocky Mountain Spotted Fever

by CDC

Rocky Mountain spotted fever is the most severe and most frequently reported rickettsial illness in the United States. It also occurs in Mexico and in Central and South America. 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.

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 a part of the National Institute of Allergy and Infectious Diseases, National Institutes of Health. 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 still conducted by scientists in the Viral and Rickettsial Zoonoses Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

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 the bacterium, Rickettsia rickettsii. This species is maintained in nature in a complex life 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.

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.

The Organism

Rocky Mountain spotted fever is caused by Rickettsia rickettsii, a small bacterium that grows inside the cells of its hosts. These bacteria range in size from 0.2 x 0.5 micrometers to 0.3 x 2.0 micrometers. They are difficult to see in tissues by using routine histologic stains and generally require the use of special staining methods.

In the human body, rickettsiae live and multiply primarily within cells that line small- to medium-sized blood vessels. Spotted fever group rickettsiae can grow in the cytoplasm or in the nucleus of the host cell. Once inside the host the rickettsiae multiply, resulting in damage and death to these cells. This causes blood to leak through tiny holes in vessel walls into adjacent tissues. This process causes the rash that is traditionally associated with Rocky Mountain spotted fever and also causes damage to organs and tissues.

Taxonomy

The genus Rickettsia is included in the bacterial tribe Rickettsieae, family Rickettsiaceae, and order Rickettsiales. This genus includes many other species of bacteria associated with human disease, including those in the spotted fever group and in the typhus group. More than 20 species are currently recognized in genus Rickettsia but not all are known to cause disease in humans. Other genotypes are also known but they have not been classified yet as valid species and new agents are being discovered in many areas of the world.

Natural History

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 biological vector (e.g., a mosquito, tick, flea, 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.

Rickettsia rickettsii usually infects members of the tick family Ixodidae (hard ticks). Although a closely related rickettsia has been found in the soft bat tick, Carios kelleyi. 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.

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.

Ticks can also become infected with R. rickettsii while feeding on blood from the host in either the larval, nymphal, or adult stage. After an immature tick develops into the next stage, R. rickettsii may be transmitted to a second host during the feeding process. Furthermore, male ticks may transfer R. rickettsii to female ticks through body fluids or spermatozoa during the mating process. Once infected, a tick can carry the pathogen for life. A female tick can also transmit R. rickettsii to her eggs in a process called transovarial transmission. These types of transmission represent how generations or life stages of infected ticks are maintained.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.