Rocky Mountain Spotted Fever


During the last several decades, between 500 and 1,200 cases of Rocky Mountain spotted fever (RMSF) were reported annually to the Centers for Disease Control and Prevention. The two most endemic regions in the United States are the Southeast and the West South Central regions. The highest incidence rates have been reported from Georgia, Montana, North Carolina, Oklahoma, South Carolina, and Virginia.

RMSF is caused by Rickettsia rickettsii, a bacteria, and is transmitted through the bite of an infected tick. It usually takes a several hours of attachment before the rickettsiae become reactivated and are able to infect humans. In the eastern and southern United States, the common vector is the American dog tick, Dermacentor variabilis. In the southwestern states, the lone star tick, Amblyomma americanum, is the vector responsible for transmission. The wood tick, Dermacentor andersoni has been known to transmit the infection in the western United States. Most cases of RMSF occur between the months of April and October.

Following an incubation period of 3 to 14 days, infected individuals experience a sudden onset of fever, malaise, headache, chills, and conjunctival injection. Many patients, however, may have atypical symptoms such as nausea, vomiting, abdominal pain, diarrhea, joint pains, or stiff neck. Two to three days later, a maculopapular rash appears first on the wrists and ankles and extends to cover the extremities and the rest of the body. One characteristic feature of the rash is that it often covers the palms and soles. In severe cases the rash may become petechial, confluent, or largely hemorrhagic. In some patients, the rash may not progress beyond the maculopapular stage and in others, it may not appear until late in the disease process.

Complications of the disease include pneumonitis, myocarditis, renal failure, gangrene of digits and scrotum, and encephalitis. Thrombocytopenia and disseminated intravascular coagulation are often noted in severe cases.

Rickettsia rickettsii, the etiologic agent of RMSF, is an obligate intracellular parasite in the family Rickettsiaceae. The organism, having antigenically indistinguishable pathogenic variants, belongs to a large, antigenically related group known as the spotted fever rickettsiae (including R. conorii, R. australis, R. akari and R. siberica), which cause similar febrile illnesses in various parts of the world. Spotted fever-group rickettsiae are more closely related to the typhus-group rickettsiae that infect people (including R. typhi and R. prowazekii) than they are to other rickettsiae that infect lower animals, (eg, Neorickettsia helminthoeca and Ehrlichia canis. It is suspected that R. rickettsia or antigenically related organisms infect a wide variety of mammals and birds because many vertebrates have detectable antibodies to spotted fever-group rickettsiae.


The early diagnosis of Rocky Mountain spotted fever is of considerable importance, since the disease is responses well to antibiotic therapy. Unfortunately, the salient features in the first few days may simulate other bacterial or viral infections, especially if no history of a tick bite is evident. Even in the presence of the rash, the disease can be mistaken for other illnesses such as meningococcemia, atypical measles, rubella, and murine typhus. A definitive diagnosis can only be achieved through laboratory testing.

The nonspecific Weil- Felix test (agglutination of bacillus Proteus OX-19 and OX-2 by the serum of Rocky Mountain spotted fever patients) has been used for the past 50 years in the diagnosis of many rickettsial infections. Agglutinins appear on about the fourth day of sickness and increase to a maximum by the beginning of the third week. Care should be taken in interpreting the results of the Weil-Felix reaction, since the test can be positive in many non-rickettsial infections such as urinary tract infections caused by Proteus, lcptospirosis, brucellosis, borrelia infections, typhoid fever, serious liver disease, and occasionally in pregnancy. A single Weil-Felix agglutination titer of at least >1:160, preferably >1:320, or a fourfold rise in paired sera are considered indicative of infection.

The last decade witnessed the introduction of several sensitive and highly specific serologic tests. These include the indirect fluorescent antibody (IFA), latex agglutination (LA), indirect hemagglutination (IHA), and microagglutination (MA) tests. A single titer of >1:64 for IFA or of >1:128 for the LA, IHA, or MA are often considered indicative of infection.

In 1981, the CDC established the following criteria for a laboratory-confirmed case of Rocky Mountain spotted fever: "a fourfold increase in antibody titer between acute and convalescent serum specimens by CF, IFA, IHA, LA, or MA; a single convalescent titer > 1:16 (CF) or >1:64 (IFA) in a clinically compatible case; isolation of spotted fever group rickettsiae; or fluorescent antibody (FA) staining of biopsy or autopsy specimens.

Because of the initially vague symptoms, the disease may simulate upper respiratory and other nonspecific infections. With the appearance of the rash, additional conditions should be considered in the differential diagnosis, namely, atypical measles, meningococcemia, enteroviral infections, typhus, and typhoid fevers. Individuals with fever, a maculopapular rash, a history of tick bite, and a high titer in the Weil-Felix test are often diagnosed as having Rocky Mountain spotted fever.

On the other hand, an atypical rash, lack or questionable history of a tick bite, and low titers in the Weil-Felix reaction should not exclude a diagnosis of Rocky Mountain spotted fever. In many cases the prescription of antibiotics during the early stages of a febrile condition often aborts the full development of the rash and delays the emergence of agglutinins. A presumptive diagnosis of Rocky Mountain spotted fever, based on the clinical symptoms with or without a history of contact with ticks, can often justify the administration of an effective antibiotic such as tetracycline.


Patients suspected of having Rocky Mountain spotted fever should be treated immediately with general antibiotics. Two drugs that have been found effective against Rickettsia rickettsia are chloramphenicol and tetracycline. Chloramphenicol is typically administered orally in a dose of 50 mg/kg/day for adults and 50-100 mg/kg/day for children in four divided doses. It is the drug of choice especially for cases in which meningococcemia could not be ruled out, in severe cases, and in pregnant women. Because of the drug's hematopoictic toxicity, the usual precautionary measures regarding chloramphenicol administration should be taken as directed by a physician.

Tetracycline hydrochloride is administered orally in a dose of 25-40 mg/kg/day for adults and children to a maximum of 2.0 g/day in four divided doses between meals. In order to avoid dental discoloration of the permanent teeth of children under age eight, the dose should not exceed 25 mg/kg/day. Patients should avoid milk and antacids during tetracycline administration. Doxycycline, 50-100 mg orally or intravenously twice/day, is better absorbed than tetracycline and is sometimes recommended when appropriate.

Supportive care is as important as antibiotic therapy. Attention should be given to correcting dehydration, electrolyte imbalance, oliguria, inadequate iiitravascular fluids, etc. Antibiotic therapy can be terminated two to four days after the patient's temperature returns to normal. The duration of therapy is usually seven to ten days.

Page Last Updated: 4/5/2016 3:01:00 PM

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