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Rocky Mountain spotted fever

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Diagnosis

A diagnosis of Rocky Mountain spotted fever is based on a combination of clinical signs, epidemiological data and specialised confirmatory laboratory tests.

Laboratory tests

There is no widely available laboratory assay that provides rapid confirmation of early Rocky Mountain spotted fever. Routine clinical laboratory findings suggestive of Rocky Mountain spotted fever may include normal white blood cell count, thrombocytopenia, hyponatremia, or elevated liver enzyme levels.

 

Serological 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.

Figure 1: IFA reaction of a positive human serum on Rickettsia rickettsii grown in chicken yolk sacs, 400X enlargement

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 human 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 time it takes for antibodies to appear. 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.

These factors also have to be considered for diagnosis of Rocky Mountain spotted fever in dogs. Infected dogs may not have increased titers, so a convalescent sample should be obtained. Furthermore, 5% to 15% of dogs in endemic areas may have positive serology, so a positive diagnosis should never be based upon a single sample unless it is a quite high titer.

Another approach to Rocky Mountain spotted fever diagnostics mainly for human patients is immunostaining by taking a skin biopsy of the rash prior to therapy or within the first 48 hours after antibiotic therapy has been started. 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 2: Red structures indicate immunohistological staining of Rickettsia rickettsii in endothelial cells of a blood vessel from a human patient with fatal RMSF.

   

Further information

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