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What is the diagnosis of Rocky Mountain spotted fever?

By  ,  Onlymyhealth editorial team
Jun 14, 2013
4.8 / 5(4 Ratings)

There are several aspects of Rock Mountain Spotted Fever (RMSF) that renders it challenging for healthcare providers to diagnose and subsequently treat. The symptoms of RMSF vary between patients and can easily resemble other, more common diseases.

Leg with spotsDiagnostic tests for this disease are mostly suited to detect antibodies, which usually appear negative in the first 7-10 days of the illness. Due to the complexities of this disease and the limitations of current available diagnostic tests, there is a mounting difficulty in the timely diagnosis of RMSF.

For this very same reason, healthcare providers are generally expected to use their judgment to treat patients based primarily on clinical suspicion. Stressed concentration on a patient’s medical history and physical examination are prioritised. Information such as recent tick bites, exposure to high grass and tick-infested areas, contact with dogs, similar illnesses in family members or pets, or history of recent travel to areas of high incidence helpful in correct diagnosis of this disease. The healthcare provider may also look at routine blood tests, such as a complete blood cell count or a chemistry panel. Indicators such as a low platelet count (thrombocytopenia), low sodium levels (Hyponatremia), or elevated liver enzyme levels are often helpful predictors of RMSF but may not be present in all patients. After a suspect diagnosis is made on clinical suspicion and treatment has begun, specialized laboratory testing is carried out to confirm the diagnosis of RMSF.

Laboratory Confirmation

R. rickettsii infects the endothelial cells that line blood vessels. This bacteria is not known to  circulate in large numbers in the blood unless the patient has progressed to a very severe phase of infection. For this reason, blood specimens (whole blood, serum) are not always useful for detection of the organism through polymerase chain reaction (PCR) or culture. If the patient has had a rash, PCR or immune histo-chemical (IHC), staining can be performed on a skin biopsy taken from the affected area. This test is highly sensitive (70%) when applied to tissue specimens gathered during the acute phase of illness and before antibiotic treatment has been started. However, a negative result should not be used to guide treatment decisions. PCR, culture, and IHC can also be applied to autopsy specimens (liver, spleen, kidney, etc) collected after a patient dies. Culture of R. rickettsii is only available at specialized laboratories; routine hospital blood cultures cannot detect R. rickettsii.

During RMSF infection, a patient’s immune system develops antibodies to fight R. Rickettsii. It is important to note that antibodies are not detectable in the first week of illness in 85% of patients, and a negative test during this time should not be ruled out as a potential cause of the illness.

The gold standard serologic test for diagnosis of RMSF is the indirect immunofluorescence assay (IFA) with R. rickettsii antigen, performed on two paired serum samples to demonstrate a significant (four-fold) rise in antibody titers. The first sample should be taken as early in the disease as possible, preferably in the first week of symptoms, and the second sample should be taken 2 to 4 weeks later. In most RMSF cases, the first IgG IFA titer is typically low or negative, and the second typically shows a significant (fourfold) increase in IgG antibody levels. IgM antibodies usually rise at the same time as IgG near the end of the first week of illness and remain elevated for months or even years. Also, IgM antibodies are less specific than IgG antibodies and more likely to result in a false positive. For these reasons, physicians requesting IgM serologic titers should also request a concurrent IgG titer.

Both IgM and IgG levels may remain elevated for months or longer after the disease has resolved, or may be detected in persons who were previously exposed to antigenically related organisms. Up to 10% of currently healthy people in some areas may have elevated antibody titers due to past exposure to R. rickettsii or similar organisms. Therefore, if only one sample is tested it can be difficult to interpret, whereas two paired samples taken weeks apart demonstrating a significant (four-fold) rise in antibody titer provide the best evidence for the correct diagnosis of RMSF.

Expected Duration of Rocky Mountain Spotted Fever

Symptoms of Rocky Mountain spotted fever begin two to fourteen days post a bite by an infected tick. Most cases of RMSF respond to appropriate antibiotic treatment within a week. Once symptoms develop, a person can die within 2 weeks without proper treatment.
It is therefore essential that good measures are taken to avoid contraction of this illness, since the know cures are not yet full proof.


Read more Rocky Mountain Spotted Fever.

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