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There is no widely available laboratory assay that provides rapid
confirmation of early Rocky Mountain spotted fever. Therefore,
treatment decisions should be based on epidemiologic and clinical
clues, and should never be delayed while waiting for confirmation by
laboratory results. Fundamental understanding of the signs,
symptoms, and epidemiology of the disease is crucial in
guiding requests for tests for Rocky Mountain spotted fever and
interpretation of results. Routine clinical laboratory
findings suggestive of Rocky Mountain spotted fever may include
normal white blood cell count, thrombocytopenia, hyponatremia,
or elevated liver enzyme levels.
Serologic assays are the most widely available and frequently
used methods for confirming cases of Rocky Mountain spotted fever.
The
indirect immunofluorescence assay (IFA) is generally considered
the reference standard in Rocky Mountain spotted fever serology and
is the test currently used by CDC and most state public health
laboratories (Figure 15).
Figure 15. IFA reaction of a positive human serum on
Rickettsia rickettsii grown in chicken yolk sacs, 400X
IFA can be used to detect either IgG or IgM antibodies. Blood
samples taken early (acute) and late (convalescent) in the disease
are the preferred specimens for evaluation. Most patients
demonstrate increased IgM titers by the end of the first week of
illness. Diagnostic levels of IgG antibody generally do not appear
until 7-10 days after the onset of illness. It is important to
consider the amount of time it takes for antibodies to appear when
ordering laboratory tests, especially because most patients visit
their physician relatively early in the course of the illness,
before diagnostic antibody levels may be present. The value of
testing two sequential serum or plasma samples together to show a
rising antibody level is considerably more important in confirming
acute infection with rickettsial agents because antibody titers may
persist in some patients for years after the original exposure.
Another approach to Rocky Mountain spotted fever diagnostics is
immunostaining. This method is used by taking a skin biopsy of the
rash from an infected patient prior to therapy or within the first
48 hours after antibiotic therapy has been started. Because
rickettsiae are focally distributed in lesions of Rocky Mountain
spotted fever, this test may not always detect the agent. Even in
laboratories with expertise in performing this test, the sensitivity
is only about 70% on biopsied tissues. This assay may also be used
to test tissues obtained at autopsy and has been used to confirm
Rocky Mountain spotted fever in otherwise unexplained deaths (Figure
16). Immunostaining for spotted fever group rickettsiae is offered
by the CDC, a few state health departments, and some
university-based hospitals and commercial laboratories in the United
States.
Figure 16. Red structures indicate immunohistological
staining of Rickettsia rickettsii in endothelial cells of a
blood vessel from a patient with fatal RMSF
Other Pages with information on Rocky Mountain Spotted Fever:
Rocky
Mountain Spotted Fever - History
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Rocky Mountain Spotted Fever Overview
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Rocky
Mountain Spotted Fever The Organism
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Rocky Mountain Spotted Fever
Epidemiology
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Rocky Mountain Spotted Fever
Signs and Symptoms
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Rocky Mountain spotted fever
Detection
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Rocky Mountain Spotted Fever
Treatment
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Rocky Mountain Spotted Fever
Prevention and Control
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