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Rocky Mountain Spotted Fever (RMSF) (Rickettsia rickettsii)
2008 Case Definition

Clinical Description

Rocky Mountain spotted fever (RMSF) is an illness caused by Rickettsia rickettsii, a bacterial pathogen transmitted to humans through contact with ticks. Dermacentor species of ticks are most commonly associated with infection, including Dermacentor variabilis (the American dog tick), Dermacentor andersoni (the Rocky Mountain wood tick), and more recently Rhiphicephalus sanguineus (the brown dog tick). Disease onset averages one week following a tick bite. Age-specific illness is highest for children and older adults. Illness is characterized by acute onset of fever, and may be accompanied by headache, malaise, myalgia, nausea/vomiting, or neurologic signs; a macular or maculopapular rash appears 4-7 days following onset in many (~80%) patients, often present on the palms and soles. RMSF may be fatal in as many as 20% of untreated cases, and severe, fulminant disease can occur.

Acute illness is best detected by polymerase chain reaction (PCR) and immunohistochemical methods (IHC) in skin biopsy specimens, and occasionally by PCR in appropriate whole blood specimens taken during the first week of illness, prior to antibiotic treatment. Serology can also be employed for detection, however an antibody response may not be detectable in initial samples, and paired acute and convalescent samples are essential for confirmation.

Clinical Criteria

Any reported fever and one or more of the following: rash, headache, myalgia, anemia, thrombocytopenia, or any hepatic transaminase elevation.

Laboratory Criteria for Diagnosis

For the purposes of surveillance,
Laboratory confirmed:

  • Serological evidence of a fourfold change in immunoglobulin G (IgG)-specific antibody titer reactive with Rickettsia rickettsii antigen by indirect immunofluorescence assay (IFA) between paired serum specimens (one taken in the first week of illness and a second 2-4 weeks later), OR
  • Detection of R. rickettsii DNA in a clinical specimen via amplification of a specific target by PCR assay, OR
  • Demonstration of spotted fever group antigen in a biopsy or autopsy specimen by IHC, OR
  • Isolation of R. rickettsii from a clinical specimen in cell culture.
Laboratory supportive:
  • Has serologic evidence of elevated IgG or immunoglobulin M (IgM) antibody reactive with R. rickettsii antigen by IFA, enzyme-linked immunosorbent assay (ELISA), dot-ELISA, or latex agglutination.
Note: Current commercially available ELISA tests are not quantitative, cannot be used to evaluate changes in antibody titer, and hence are not useful for serological confirmation. IgM tests are not strongly supported for use in serodiagnosis of acute disease, as the response may not be specific for the agent (resulting in false positives) and the IgM response may be persistent. Complement fixation (CF) tests and other older test methods are neither readily available nor commonly used. CDC uses in-house IFA IgG testing (cutoff of ≥1:64), preferring simultaneous testing of paired specimens, and does not use IgM results for routine diagnostic testing.

Exposure

Exposure is defined as having been in potential tick habitats within the past 14 days before onset of symptoms. A history of a tick bite is not required.

Case Classification

Suspected

A case with laboratory evidence of past or present infection but no clinical information available (e.g., a laboratory report).

Probable

A clinically compatible case (meets clinical evidence criteria) that has supportive laboratory results.

Confirmed

A clinically compatible case (meets clinical evidence criteria) that is laboratory confirmed.



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