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Rocky Mountain spotted fever

What is Rocky Mountain spotted fever?
Rocky Mountain spotted fever (RMSF) is a tickborne disease caused by the bacterium Rickettsia rickettsii. This organism is a cause of potentially fatal human illness in North and South America, and is transmitted to humans by the bite of infected tick species. In the United States, these include the American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (Dermacentor andersoni), and brown dog tick (Rhipicephalus sanguineus). Typical symptoms include: fever, headache, abdominal pain, vomiting, and muscle pain. A rash may also develop, but is often absent in the first few days, and in some patients, never develops. Rocky Mountain spotted fever can be a severe or even fatal illness if not treated in the first few days of symptoms. Doxycycline is the first-line treatment for adults and children of all ages, and is most effective if started before the fifth day of symptoms. The initial diagnosis is made based on clinical signs and symptoms, and medical history, and can later be confirmed by using specialized laboratory tests. Rocky Mountain spotted fever and other tickborne diseases can be prevented.

The first symptoms of Rocky Mountain spotted fever typically begin 2-14 days after the bite of an infected tick. A tick bite is usually painless and about half of the people who develop Rocky Mountain spotted fever do not remember being bitten. The disease frequently begins as a sudden onset of fever and headache and most people visit a healthcare provider during the first few days of symptoms. Because early symptoms may be non-specific, several visits may occur before the diagnosis of Rocky Mountain spotted fever is made and correct treatment begins. The following is a list of symptoms commonly seen with this disease, however, it is important to note that few people with the disease will develop all symptoms, and the number and combination of symptoms varies greatly from person to person.

• Fever
• Rash (occurs 2-5 days after fever, may be absent in some cases; see below)
• Headache
• Nausea
• Vomiting
• Abdominal pain (may mimic appendicitis or other causes of acute abdominal pain)
• Muscle pain
• Lack of appetite
• Conjunctival injection (red eyes)

Rocky Mountain spotted fever is a serious illness that can be fatal in the first eight days of symptoms if not treated correctly, even in previously healthy people. The progression of the disease varies greatly. Patients who are treated early may recover quickly on outpatient medication, while those who experience a more severe course may require intravenous antibiotics, prolonged hospitalization or intensive care.

Rash. While most people with Rocky Mountain spotted fever (90%) have some type of rash during the course of illness, some people do not develop the rash until late in the disease process, after treatment should have already begun. Approximately 10% of patients with Rocky Mountain spotted fever never develop a rash. It is important for physicians to consider Rocky Mountain spotted fever if other signs and symptoms support a diagnosis, even if a rash is not present.

A classic case of Rocky Mountain spotted fever involves a rash that first appears 2-5 days after the onset of fever as small, flat, pink, non-itchy spots (macules) on the wrists, forearms, and ankles and spreads to include the trunk and sometimes the palms and soles. Often the rash varies from this description and people who fail to develop a rash, or develop an atypical rash, are at increased risk of being misdiagnosed.

The red to purple, spotted (petechial) rash of Rocky Mountain spotted fever is usually not seen until the sixth day or later after onset of symptoms and occurs in 35-60% of patients with the infection. This is a sign of progression to severe disease, and every attempt should be made to begin treatment before petechiae develop.

Long-term health problems. Patients who had a particularly severe infection requiring prolonged hospitalization may have long-term health problems caused by this disease. Rickettsia rickettsii infects the endothelial cells that line the blood vessels. The damage that occurs in the blood vessels results in a disease process called a "vasculitis", and bleeding or clotting in the brain or other vital organs may occur. Loss of fluid from damaged vessels can result in loss of circulation to the extremities and damaged fingers, toes or even limbs may ultimately need to be amputated. Patients who suffer this kind of severe vasculitis in the first two weeks of illness may also be left with permanent long-term health problems such as profound neurological deficits, or damage to internal organs. Those who do not have this kind of vascular damage in the initial stages of the disease typically recover fully within several days to months.

Infection in children. Children with Rocky Mountain spotted fever infection may experience nausea, vomiting, and loss of appetite. Children are less likely to report a headache, but more likely to develop an early rash than adults. Other frequently observed signs and symptoms in children with Rocky Mountain spotted fever are abdominal pain, altered mental status, and conjunctival injection. Occasionally, symptoms like cough, sore throat, and diarrhea may be seen and can lead to misdiagnosis.

For more in-depth information about signs and symptoms of Rocky Mountain spotted fever, please visit

Physician diagnosis. There are several aspects of Rocky Mountain spotted fever that make it challenging for healthcare providers to diagnose and treat. The symptoms of Rocky Mountain spotted fever vary from patient to patient and can easily resemble other, more common diseases. Treatment for this disease is most effective at preventing death if started in the first five days of symptoms. Diagnostic tests for this disease, especially tests based on the detection of antibodies, will frequently appear negative in the first 7-10 days of illness. Due to the complexities of this disease and the limitations of currently available diagnostic tests, there is no test available at this time that can provide a conclusive result in time to make important decisions about treatment.

For this reason, healthcare providers must use their judgment to treat patients based on clinical suspicion alone. Healthcare providers may find important information in the patient’s history and physical examination that may aid clinical suspicion. Information such as recent tick bites, exposure to high grass and tick-infested areas, contact with dogs, similar illnesses in family members or pets, or history of recent travel to areas of high incidence can be helpful in making the diagnosis. Also, information about the presence of symptoms such as fever and rash may be helpful. The healthcare provider may also look at routine blood tests, such as a complete blood cell count or a chemistry panel. Clues such as a low platelet count (thrombocytopenia), low sodium levels (hyponatremia), or elevated liver enzyme levels are often helpful predictors of Rocky Mountain spotted fever but may not be present in all patients. After a suspect diagnosis is made on clinical suspicion and treatment has begun, specialized laboratory testing should be used to confirm the diagnosis of Rocky Mountain spotted fever.

The diagnosis of Rocky Mountain spotted fever must be made based on clinical signs and symptoms, and can later be confirmed using specialized confirmatory laboratory tests. Treatment should never be delayed pending the receipt of laboratory test results, or be withheld on the basis of an initial negative finding for R. rickettsii.

Laboratory confirmation. R. rickettsii infects the endothelial cells that line blood vessels, and does not circulate in large numbers in the blood unless the patient has progressed to a very severe phase of infection. For this reason, blood specimens (whole blood, serum) are not always useful for detection of the organism through polymerase chain reaction (PCR) or culture. If the patient has a rash, PCR or immunohistochemical (IHC) staining can be performed on a skin biopsy taken from the rash site. This test can often deliver a rapid result. These tests have good sensitivity (70%) when applied to tissue specimens collected during the acute phase of illness and before antibiotic treatment has been started, but a negative result should not be used to guide treatment decisions. PCR, culture, and IHC can also be applied to autopsy specimens (liver, spleen, kidney, etc) collected after a patient dies. Culture of R. rickettsii is only available at specialized laboratories; routine hospital blood cultures cannot detect R. rickettsii.

During Rocky Mountain spotted fever infection, a patient’s immune system develops antibodies to R. rickettsii, with detectable antibody titers usually observed by 7-10 days after illness onset. It is important to note that antibodies are not detectable in the first week of illness in 85% of patients, and a negative test during this time does not ruleout Rocky Mountain spotted fever as a cause of illness.

The gold standard serologic test for diagnosis of Rocky Mountain spotted fever is the indirect immunofluorescence assay (IFA) with R. rickettsii antigen, performed on two paired serum samples to demonstrate a significant (four-fold) rise in antibody titers. The first sample should be taken as early in the disease as possible, preferably in the first week of symptoms, and the second sample should be taken 2 to 4 weeks later. In most Rocky Mountain spotted fever cases, the first IgG IFA titer is typically low or negative, and the second typically shows a significant (fourfold) increase in IgG antibody levels. IgM antibodies usually rise at the same time as IgG near the end of the first week of illness and remain elevated for months or even years. Also, IgM antibodies are less specific than IgG antibodies and more likely to result in a false positive. For these reasons, physicians requesting IgM serologic titers should also request a concurrent IgG titer.

Both IgM and IgG levels may remain elevated for months or longer after the disease has resolved, or may be detected in persons who were previously exposed to antigenically related organisms. Up to 10% of currently healthy people in some areas may have elevated antibody titers due to past exposure to R. rickettsii or similar organisms. Therefore, if only one sample is tested it can be difficult to interpret, whereas two paired samples taken weeks apart demonstrating a significant (four-fold) rise in antibody titer provide the best evidence for a correct diagnosis of Rocky Mountain spotted fever. For more in-depth information about testing, please visit

Doxycycline is the first line treatment for adults and children of all ages and should be initiated immediately whenever Rocky Mountain spotted fever is suspected.

Use of antibiotics other than doxycycline is associated with a higher risk of fatal outcome. Treatment is most effective at preventing death if doxycycline is started in the first 5 days of symptoms. Therefore, treatment must be based on clinical suspicion alone and should always begin before laboratory results return or symptoms of severe disease, such as petechiae, develop.

If the patient is treated within the first 5 days of the disease, fever generally subsides within 24 to 72 hours. In fact, failure to respond to doxycycline suggests that the patient’s condition might not be Rocky Mountain spotted fever. Severely ill patients may require longer periods before their fever resolves, especially if they have experienced damage to multiple organ systems. Resistance to doxcycline or relapses in symptoms after the completion of the recommended course of treatment have not been documented.

Recommended dosage:
Doxycycline is the first-line treatment for adults and children of all ages:
• Adults: 100 mg every 12 hours
• Children under 45 kg (100 lbs): 2.2 mg/kg body weight given twice a day

Patients should be treated for at least 3 days after the fever subsides and until there is evidence of clinical improvement. Standard duration of treatment is 7-14 days.

Treating children. The use of doxycycline to treat suspected Rocky Mountain spotted fever in children is standard practice recommended by both CDC and the AAP Committee on Infectious Diseases. Use of antibiotics other than doxycycline increases the risk of patient death. Unlike older tetracyclines, the recommended dose and duration of medication needed to treat Rocky Mountain spotted fever has not been shown to cause staining of permanent teeth, even when five courses are given before the age of eight. Healthcare providers should use doxycycline as the first-line treatment for suspected Rocky Mountain spotted fever in patients of all ages.

Other treatments. In cases of life-threatening allergies to doxycycline and in some pregnant patients for whom the clinical course of Rocky Mountain spotted fever appears mild, chloramphenicol may be considered as an alternative antibiotic. Oral forumulations of chloramphenicol are not available in the United States, and use of this drug carries the potential for other adverse risks, such as aplastic anemia and Grey baby syndrome. Furthermore, the risk for fatal outcome is elevated in patients who are treated with chloramphenicol compared to those treated with doxycycline. Other antibiotics, including broad-spectrum antibiotics are not effective against R. rickettsii, and the use of sulfa drugs may worsen infection.

Prophylaxis (preventive treatment). Antibiotic treatment following a tick bite is not recommended as a means to prevent Rocky Mountain spotted fever. There is no evidence this practice is effective, and may simply delay onset of disease. Instead, persons who experience a tick bite should be alert for symptoms suggestive of tickborne illness and consult a physician if fever, rash, or other symptoms of concern develop.

For more in-depth information about treatment, please visit

This information was developed by the Centers for Disease Control and Prevention.

Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Viral and Rickettsial Diseases, Viral and Rickettsial Zoonoses Branch. Rocky Mountain spotted fever. Available at: Accessed January 22, 2014.

The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink Corporation, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.

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