Rocky Mountain Spotted Fever Facts and Symptoms

American Dog Tick, Dermacentor variabilis, on skin. Found after walking through tall grass in spring, Saskatchewan, Canada
Grambo Photography / Getty Images

Rocky Mountain Spotted Fever (RMSF) is a potentially fatal disease that occurs most often in Oklahoma and the South Atlantic states, although it has been found in most states. It is caused by an infection with the bacteria Rickettsia rickettsii. The bacteria is transmitted to humans by three different types of ticks:

  • Eastern United States - Dermacentor variabilis (American dog tick)
  • West South Central States - Amblyomma americanum (Lone Star tick)
  • Western States - Dermacentor andersoni (wood tick)

Basic Facts

After an infected tick bites a human, the bacteria are released into the bloodstream. There they attack cells that line the blood vessels and smooth muscles that control the constriction of the blood vessel. They set off an immune reaction in the blood vessel causing the vessel to swell and become leaky. This process can occur in any organ system in the body causing a wide variety of symptoms.


The incubation period is 2 to 14 days after the tick bite. The average incubation period is 7 days. The most common symptoms of Rocky Mountain spotted fever are the abrupt onset of fever, severe headache, muscle aches, and vomiting. Other symptoms that are less common are abdominal pain, swollen lymph nodes, cough, stiff neck, confusion, and coma.

What the Rash Looks Like

The rash associated with Rocky Mountain spotted fever usually starts around 4 days into the illness. It looks like small, red, flat spots starting most often on the ankles and wrists, and then moving to the palms, soles, and trunk. As the rash progresses, it becomes bumpier. Approximately 10% of those infected never get a rash.

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Rocky mountain spotted fever
Rocky mountain spotted fever rash. Smith Collection/Gado / Contributor / Getty Images

How Serious Is It?

Overall, Rocky Mountain spotted fever is fatal in 3% to 7% of cases. However, it is fatal in over 30% of those who are not treated. The mortality is higher in people over 40 years of age. Death usually results from shock and kidney failure.

How It's Diagnosed

Rocky Mountain spotted fever is diagnosed mainly based on symptoms. There are no reliable laboratory tests to diagnose Rocky Mountain spotted fever while the patient has the disease. Most laboratory tests that are specific for the bacteria involve obtaining one blood test while the patient is sick and another in 4 weeks to see if the immune system has built up antibodies to the bacteria. Obviously, waiting for this second test to return before making a diagnosis is fruitless and only useful in retrospect. Other lab tests that may indicate Rocky Mountain spotted fever are a low white blood cell count, low platelet count, or elevated liver function tests. The rash is usually the key to diagnosis since not many rashes affect the palms and soles.


Rocky Mountain spotted fever is treated with antibiotics such as doxycycline, tetracycline, or chloramphenicol. Most providers will prescribe one of these antibiotics on the assumption that the disease is Rocky Mountain spotted fever and confirm the diagnosis with another blood test in 4 weeks. Pregnant women should not take doxycycline or tetracycline. Since chloramphenicol is available in the US only in IV form, pregnant women should be admitted to the hospital.


Preventing Rocky Mountain spotted fever involves preventing tick bites. Children and adults who are outside in tick-infested areas should wear long clothing and tuck the end of the pants into the socks. Insect repellant should be applied to shoes and socks. Permethrin products are more effective against ticks than DEET products. Check for ticks attached to the skin every 2-3 hours while outside, then check thoroughly once a day. Favorite hiding places for ticks are in the hair so check the scalp, neck, armpits, and groin.

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