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Rocky Mountain Spotted Fever (RMSF)

(Spotted Fever; Tick Fever; Tick Typhus)

By William A. Petri, Jr, MD, PhD

Rocky Mountain spotted fever is a potentially fatal rickettsial infection that is transmitted by dog ticks and wood ticks. It causes a rash, headache, and high fever.

  • People become infected when a tick carrying the infection bites them.

  • A severe headache, chills, extreme exhaustion, and muscle pains develop, usually followed a few days later by a rash.

  • Avoiding tick bites is the best way to prevent the infection.

  • People are given antibiotics immediately if they have been bitten by a tick and have typical symptoms.

Rocky Mountain spotted fever (RMSF) is caused by the bacteria Rickettsia rickettsii. Rickettsiae are a type of bacteria that can live only inside the cells of other organisms (see also Overview of Rickettsial Infections).

Rocky Mountain spotted fever is probably the most common rickettsial infection in the United States. It was first recognized in the Rocky Mountain states but occurs throughout most of the continental United States. It is most common in the southeastern and south central United States (North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri). It also occurs in Central and South America.

The infection occurs mainly from March to September, when adult ticks are active and people are likely to be in tick-infested areas. In the southern states, the disease may occur throughout the year. People who spend a lot of time in tick-infested areas—such as children younger than 15—have an increased risk of infection.

Ticks acquire these rickettsiae by feeding on infected mammals, typically rodents. Infected female ticks can also transmit rickettsiae to their offspring. Infection is spread to people through bites by wood ticks or dog ticks. Rickettsial infection is probably not transmitted directly from person to person.

Rickettsiae live and multiply in the cells lining blood vessels. Blood vessels in and under the skin and in the brain, lungs, heart, kidneys, liver, and spleen are commonly infected. Small infected blood vessels may become blocked by blood clots. If the infection is severe, blood clots may form throughout the body, causing disseminated intravascular coagulation.

Did You Know...

  • Almost three fourths of people with symptoms remember having a tick bite.


Typically, symptoms of Rocky Mountain spotted fever include a severe headache, chills, extreme exhaustion (prostration), and muscle pains. Symptoms begin suddenly 3 to 12 days after a tick bite. The more quickly symptoms begin, the more severe the infection. A high fever develops within several days and, in severe infections, persists for 1 to 3 weeks. A hacking, dry cough may also develop. Nausea and vomiting are common.

Between the first and the sixth day of the fever, a rash appears on the wrists and ankles and rapidly extends to the palms, soles, forearms, neck, face, armpits, buttocks, and trunk. At first, the rash is flat and pink but later darkens and becomes slightly raised. It does not itch. Warm water—for example, in a bath—makes the rash more evident. In about 4 days, small purplish areas (petechiae) develop because of bleeding in the skin. If the infection is severe, areas of skin may die and turn black, indicating gangrene.

About 10% of people with Rocky Mountain spotted fever do not develop a rash.

As this infection progresses, it may cause other symptoms:

  • Restlessness, insomnia, delirium, or sometimes coma if the blood vessels in the brain are affected

  • Abdominal pain

  • Inflammation of the airways (pneumonitis) and pneumonia

  • Heart damage

  • Anemia

  • Severe low blood pressure and death (uncommonly, when the infection is severe)


  • A doctor's evaluation

  • Blood tests and biopsy of the rash

Doctors suspect Rocky Mountain spotted fever if people

  • Live in or near a wooded area anywhere in the Western Hemisphere

  • Have a fever, headache, and muscle aches in the spring, summer, or fall, regardless of whether they have a rash or a tick bite

About 70% of people remember being bitten by a tick.

Doctors may do blood tests that detect antibodies to the bacteria. However, these tests can first detect these antibodies only about 7 to 10 days after symptoms begin. Antibody tests done before this time may be negative. Thus, these tests do not help doctors diagnose the infection immediately after someone becomes ill but can help confirm the diagnosis later.

To confirm the diagnosis, doctors usually do an immunofluorescence assay, which uses a sample from the rash. For immunofluorescence assays, foreign substances produced by the bacteria (antigens) are labeled with a fluorescent dye making the bacteria easier to detect and identify.


There is no vaccine against Rocky Mountain spotted fever, so avoiding tick bites and immediately removing attached ticks are the best prevention. The following measures can help:

  • Tucking trousers into boots or socks and applying insecticide that contains permethrin to clothing limits tick access to skin.

  • Tick repellents such as DEET (diethyltoluamide) may be applied to the skin. These repellents are effective but, rarely, cause toxic reactions, such as seizures, in small children.

  • Searching frequently for ticks helps prevent infection because the tick must be attached for 24 hours on average to transmit infection.

  • Attached ticks should be removed carefully with tweezers. The head of the tick should be grasped as close to the skin as possible. Care must be taken when removing a tick because rickettsiae may be transmitted if an infected tick that is engorged with blood is crushed while being removed.

Tick Bite Prevention

Preventing tick access to skin includes

  • Staying on paths and trails

  • Tucking trousers into boots or socks

  • Wearing long-sleeved shirts

  • Applying repellents with diethyltoluamide (DEET) to skin surfaces

DEET should be used cautiously in very young children because toxic reactions have been reported. Permethrin on clothing effectively kills ticks. Frequent searches for ticks, particularly in hairy areas and on children, are essential in endemic areas.

Engorged ticks should be removed with care and not crushed between the fingers because crushing the tick may result in disease transmission. The tick’s body should not be grasped or squeezed. Gradual traction on the head with a small forceps dislodges the tick. The point of attachment should be swabbed with alcohol. Petroleum jelly, lit matches, and other irritants are not effective ways to remove ticks and should not be used.

No practical means are available to rid entire areas of ticks, but tick populations may be reduced in endemic areas by controlling small-animal populations.


  • Antibiotics

Doctors immediately prescribe antibiotics if they suspect Rocky Mountain fever based on symptoms and the potential for exposure to infected ticks—even if laboratory test results are not yet available. Early treatment with antibiotics has reduced the death rate from about 20% to 5%.

Doxycycline is usually used. It is given by mouth when the infection is mild and intravenously when it is more severe. People take the antibiotic until they improve and have had no fever for 24 to 48 hours, but they must take it for at least 7 days. Chloramphenicol is also effective but can have serious side effects and is not available in the United States.

However, a doctor usually does not prescribe antibiotics for people who have had a tick bite but have no symptoms. Instead, the doctor may ask them to immediately report any symptoms.

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