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Rocky Mountain Spotted Fever (RMSF)

(Spotted Fever; Tick Fever; Tick Typhus)


William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Last full review/revision Jul 2020| Content last modified Jul 2020
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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 is a rickettsial disease.

Epidemiology of RMSF

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 years and in people who frequent tick-infested areas for work or recreation. (See also Centers for Disease Control and Prevention: RMSF–epidemiology and statistics.)

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.

Pathophysiology of RMSF

Small blood vessels are the sites of the characteristic pathologic lesions of Rocky Mountain spotted fever. Rickettsiae propagate within damaged endothelial cells, and vessels may become blocked by thrombi, producing vasculitis in the skin, subcutaneous tissues, central nervous system, lungs, heart, kidneys, liver, and spleen. Disseminated intravascular coagulation Disseminated Intravascular Coagulation (DIC) Disseminated intravascular coagulation (DIC) involves abnormal, excessive generation of thrombin and fibrin in the circulating blood. During the process, increased platelet aggregation and coagulation... read more often occurs in severely ill patients.

Symptoms and Signs of RMSF

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.

Diagnosis of RMSF

  • Clinical features

  • Biopsy of rash with fluorescent antibody staining to detect organisms

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

  • Polymerase chain reaction (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.

Treatment of RMSF

  • Doxycycline

Starting antibiotics early significantly reduces mortality, from about 20 to 5%, and prevents most complications of Rocky Mountain spotted fever . 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 orally once followed by 100 mg twice a day in adults and 2.2 mg/kg body weight given twice a day in children under 45 kg (100 lb) for at least 3 days after the fever has subsided, and there is evidence of clinical improvement. Minimum course of treatment is 5 to 7 days. Doxycycline is given to both adults and children. The Centers for Disease Control and Prevention has found that short courses of doxycycline (5 to 10 days) can be used in children without causing tooth staining or weakening of tooth enamel. Use of antibiotics other than doxycycline increases the risk of severe illness and death.

Chloramphenicol 500 mg orally or IV 4 times a day for 7 days is 2nd-line treatment. Oral chloramphenicol is not available in the US and its use is associated with adverse hematologic effects, which requires monitoring of blood indices.

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 blood pressure while avoiding worsening pulmonary and cerebral edema.

Prevention of RMSF

No effective vaccine is available to prevent Rocky Mountain spotted fever. Measures can be taken to prevent tick bites. (See also Centers for Disease Control and Prevention: Preventing tick bites.)

Deer and dog tick size comparison

Deer ticks

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.

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 central nervous system, 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.

More Information

The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

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