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Rocky Mountain Spotted Fever (Rickettsia
rickettsii) (RMSF)
2004 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) and Dermacentor andersoni (the Rocky Mountain
wood tick). Disease onset averages one week following a tick bite.
Age specific illness is highest for children. 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 is reported in most patients, and a rash is often present
on the palms and soles. RMSF is fatal in approximately 20% of untreated
cases, and severe fulminant disease is possible.
Laboratory criteria for diagnosis
- Serological evidence of a significant change in serum
antibody titer reactive with Rickettsia rickettsii antigens between
paired serum specimens, as measured by a standardized assay conducted
in a commercial, state, or reference laboratory.
- Demonstration of R. rickettsii antigen in a clinical specimen
by immunohistochemical methods.
- Detection of R. rickettsii DNA in a clinical specimen by
the polymerase chain reaction (PCR assay).
- Isolation of
R. rickettsii from a clinical specimen in cell culture.
Note: For confirmed cases, a significant change in titer must
be determined by the testing
laboratory; examples of commonly used measures of significant change
include, but are not limited to, a four-fold or greater change
in antibody titer as determined by indirect
immunoflourescent antibody (IFA) assay or an equivalent change
in optical density measured by enzyme-linked immunosorbent assay
(EIA or ELISA).
Case classification
Confirmed: A person with a clinically compatible illness that
is laboratory confirmed.
Probable: A person with a clinically compatible illness and serologic
evidence of antibody reactive with R. rickettsii in a single serum
sample at a titer considered indicative of current or past infection
(cutoff titers are determined by individual laboratories).
See also:
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