Rocky Mountain Spotted Fever

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. 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. Despite the name, few cases of Rocky Mountain spotted fever are reported from the Rocky Mountain region. Most cases occur in the southeastern United States.

Rocky Mountain spotted fever is still a serious and potentially life-threatening infectious disease 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, die from the infection. However, effective antibiotic therapy has dramatically reduced the number of deaths caused by this disease. Before the discovery of tetracycline and chloramphenicol in the late 1940s, as many as 30% of persons infected with Rocky Mountain spotted fever died.

Rocky Mountain spotted fever 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 for the causative agent a bacterium called Rickettsia rickettsii. Ticks transmit the rickettsiae to vertebrates primarily by their bite. Less commonly, infections may occur following exposure to crushed tick tissues, fluids, or tick feces. Once inside the human body, rickettsiae live and multiply primarily within the cells that line small-to-medium sized blood vessels. This results 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.

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.

There are two major vectors of R. rickettsii in the United States, the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni). Both ticks belong to the hard tick family Ixodidae. In Colorado, the Rocky Mountain wood tick is the tick that transmits Rocky Mountain spotted fever.

People infected with R. rickettsii generally visit a physician in the first week of their illness, following an incubation period of about 5-10 days after a tick bite. The early clinical presentation of Rocky Mountain spotted fever is nonspecific and may resemble a variety of other infectious and non-infectious diseases. Initial symptoms may include fever, nausea, vomiting, severe headache, muscle pain, and lack of appetite. The rash first appears 2-5 days after the onset of fever and is often not present or may be very subtle when the patient is initially seen by a physician. Most often it begins as small, flat, pink, non-itchy spots on the wrists, forearms, and ankles. These spots turn pale when pressure is applied and eventually become raised on the skin. Later signs and symptoms include rash, abdominal pain, joint pain, and diarrhea. The characteristic red, spotted rash of Rocky Mountain spotted fever is usually not seen until the sixth day or later after onset of symptoms, and this type of rash occurs in only 35% to 60% of patients with Rocky Mountain spotted fever, and as many as 10% to 15% of patients may never develop a rash.

Rocky Mountain spotted fever can be a very severe illness and people often require hospitalization. Because R. rickettsii infects the cells lining blood vessels throughout the body, severe manifestations of this disease may involve the respiratory system, central nervous system, gastrointestinal system, or renal system. This disease can have long-term health problems following acute infection and may include partial paralysis of the lower extremities; gangrene requiring amputation of fingers, toes, or arms or legs; hearing loss; loss of bowel or bladder control; movement disorders; and language disorders. These complications are most frequent in persons recovering from severe, life-threatening disease, often following lengthy hospitalizations.

Currently, no licensed vaccine is available for prevention of Rocky Mountain spotted fever, so limiting exposure to ticks is the most effective way to reduce the likelihood of getting the disease. For people exposed to tick-infested habitats, prompt careful inspection and removal of crawling or attached ticks is an important method of preventing disease. It usually takes extended attachment time before the virus is passed from the tick to the human. Removing a tick before it has been attached for more that 4 hours greatly reduces the risk of infection. Use tweezers, and grab as closely to the skin as possible. Do not handle ticks with bare hands. Do not try to remove ticks by squeezing them, coating them with petroleum jelly, or burning them with a match. After removing the tick, thoroughly disinfect the bite site, and wash your hands. See or call a doctor if you think that tick parts may remain in your skin. If you get a fever, headache, rash or nausea within 2 weeks of a possible tick bite or exposure, see a doctor right away.

When going into possible tick infected areas, wear a light colored long sleeved shirt and long pants tucked into socks. The light color will allow you to see ticks crawling on your clothing. Use insecticides to repel or kill ticks. Repellents containing the compound DEET can be used on exposed skin except for the face, but they do not kill ticks and are not 100% effective in discouraging ticks from biting. Products containing permethrin kill ticks, but they cannot be used on the skin, only on clothing. When using any insecticides or repellents read and follow the label directions carefully. Be especially cautious when using insecticides or repellents on children. After returning from a tick-infested area, conduct a body search using a mirror or the buddy system. Be sure to check children and pets for ticks in hidden areas such as hair.

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