Rocky Mountain Spotted Fever Facts and Symptoms

Rocky Mountain Spotted Fever (RMSF) is a potentially fatal disease 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: Dermacentor variabilis (American dog tick); Amblyomma americanum (Lone Star tick); Dermacentor andersoni (wood tick).

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

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.

According to the CDC, Rocky Mountain Spotted Fever occurs most often in Arkansas, Missouri, North Carolina, Tennessee, and Virginia.


The incubation period is two to 14 days after the tick bite. The average incubation period is seven 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 four days into the illness, usually after the severe symptoms have started. 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.

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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 can be 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 easily accessible 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 four 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. A rash is not present in most patients when they present for medical care, and the rash can present without palm and sole involvement, or may not appear at all.


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 four weeks.

Most pregnant women with RMSF are treated with doxycycline; in special circumstances, tetracycline or chloramphenicol may be used.


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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