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Rocky Mountain Spotted Fever (RMSF)

(Spotted Fever; Tick Fever; Tick Typhus)

By William A. Petri, Jr, MD, PhD, Wade Hampton Frost Professor of Medicine and Chief, Division of Infectious Diseases and International Health, University of Virginia School of Medicine

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Rocky Mountain spotted fever (RMSF) is caused by Rickettsia rickettsii and transmitted by ixodid ticks. Symptoms are high fever, severe headache, and rash.

Rocky Mountain spotted fever (RMSF) is a rickettsial disease.


RMSF is limited to the Western Hemisphere. Initially recognized in the Rocky Mountain states, it occurs in practically all of the US and throughout Central and South America. In humans, infection occurs mainly from March to September, when adult ticks are active and people are most likely to be in tick-infested areas. In southern states, sporadic cases occur throughout the year. The incidence is highest in children < 15 yr and in people who frequent tick-infested areas for work or recreation.

Hard-shelled ticks (family Ixodidae) harbor R. rickettsii, and infected females transmit the agent to their progeny. These ticks are the natural reservoirs. Dermacentor andersoni (wood tick) is the principal vector in the western US. D. variabilis (dog tick) is the vector in the eastern and southern US.

RMSF is probably not transmitted directly from person to person.


Small blood vessels are the sites of the characteristic pathologic lesions. Rickettsiae propagate within damaged endothelial cells, and vessels may become blocked by thrombi, producing vasculitis in the skin, subcutaneous tissues, CNS, lungs, heart, kidneys, liver, and spleen. Disseminated intravascular coagulation often occurs in severely ill patients.

Symptoms and Signs

The incubation period for Rocky Mountain spotted fever averages 7 days but varies from 3 to 12 days; the shorter the incubation period, the more severe the infection.

Onset is abrupt, with severe headache, chills, prostration, and muscular pains. Fever reaches 39.5 to 40° C within several days and remains high (for 15 to 20 days in severe cases), although morning remissions may occur.

Between the 1st and 6th day of fever, most patients with RMSF develop a rash on the wrists, ankles, palms, soles, and forearms that rapidly extends to the neck, face, axillae, buttocks, and trunk. Initially macular and pink, it becomes maculopapular and darker. In about 4 days, the lesions become petechial and may coalesce to form large hemorrhagic areas that later ulcerate.

Neurologic symptoms include headache, restlessness, insomnia, delirium, and coma, all indicative of encephalitis.

Hypotension develops in severe cases. Hepatomegaly may be present, but jaundice is infrequent. Nausea and vomiting are common. Localized pneumonitis may occur. Untreated patients may develop pneumonia, tissue necrosis, and circulatory failure, sometimes with brain and heart damage. Cardiac arrest with sudden death occasionally occurs in fulminant cases.


  • Clinical features

  • Biopsy of rash with fluorescent antibody staining to detect organisms

  • Acute and convalescent serologic testing (serologic testing not useful acutely)

  • PCR

Clinicians should suspect Rocky Mountain spotted fever in any seriously ill patient who lives in or near a wooded area anywhere in the Western Hemisphere and has unexplained fever, headache, and prostration, with or without a history of tick contact. A history of tick bite is elicited in about 70% of patients.

Testing is usually required to confirm RMSF but because of the limitations of currently available tests, clinicians typically must make treatment decisions before receiving results of confirmatory testing.

If patients have a rash, a skin biopsy should be taken from the rash site. PCR or immunohistochemical staining, which can provide fairly rapid results, is used. Sensitivity of these tests is about 70% when tissue specimens are collected during the acute illness and before antibiotic treatment is started. However, a negative test result does not justify withholding treatment when clinical manifestations suggest RMSF.

Culture of R. rickettsii is available only at specialized laboratories.

Pearls & Pitfalls

  • Negative test results for RMSF do not justify withholding treatment when clinical manifestations suggest RMSF.

Serologic tests are not useful for acute diagnosis because they usually become positive only during convalescence. Indirect immunofluorescence assay using 2 paired samples is usually done.

For additional specifics of diagnosis, see Overview of Rickettsial and Related Infections : Diagnosis.


  • Doxycycline

Starting antibiotics early significantly reduces mortality, from about 20 to 5%, and prevents most complications. If patients who have been in an endemic area have a tick bite but no clinical symptoms or signs, antibiotics should not be given immediately.

If fever, headache, and malaise occur with or without a rash, antibiotics should be started promptly.

Primary treatment is doxycycline 200 mg po once followed by 100 mg bid until the patient improves, has been afebrile for 24 to 48 h, and has received treatment for at least 7 days.

Chloramphenicol 500 mg po or IV qid for 7 days is 2nd-line treatment.

Severely ill patients with RMSF may have a marked increase in capillary permeability in later stages; thus, IV fluids should be given cautiously to maintain BP while avoiding worsening pulmonary and cerebral edema.

No effective vaccine is available. Measures can be taken to prevent tick bites (see Tick Bite Prevention).

Key Points

  • Despite its name, Rocky Mountain spotted fever (RMSF) occurs in practically all of the US and throughout Central and South America.

  • Small-vessel vasculitis can cause serious illness affecting the CNS, lungs, heart, kidneys, liver, and spleen; untreated mortality is about 20%.

  • Symptoms (severe headache, chills, prostration, muscle pain) begin abruptly, followed by fever and usually a rash.

  • Neurologic symptoms (headache, restlessness, insomnia, delirium, coma) may develop, indicating encephalitis.

  • Suspect RMSF in any seriously ill patient who lives in or near a wooded area anywhere in the Western Hemisphere and has unexplained fever, headache, and prostration, with or without a history of tick contact.

  • Test during acute illness with PCR or immunohistology of a skin biopsy specimen, but because sensitivity is only about 70%, a negative result should not affect the decision to begin antibiotics.

  • Treat with doxycycline and provide supportive care as needed for hypovolemia and/or organ involvement.

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