Rocky Mountain Spotted Fever
المؤلف:
Mary Louise Turgeon
المصدر:
Immunology & Serology in Laboratory Medicine
الجزء والصفحة:
5th E, P255-256
2025-08-23
444
Etiology
Rocky Mountain spotted fever (RMSF) is a tickborne disease caused by the bacterium Rickettsia rickettsii. This organism is a cause of potentially fatal human illness in North and South America, and is transmitted to human beings by the bite of infected tick species. In the United States, these include the American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (Dermacentor andersoni), and brown dog tick (Rhipicephalus sanguineus).
Epidemiology
The CDC has noted that the geographic distribution of RMSF correlates with the type of tick found in that area. For example, American dog tick is found in the eastern, central, and Pacific coastal United States; the Rocky Mountain wood tick resides in the western United States. In 2005, the brown dog tick, a vector of RMSF in Mexico was implicated as a vector of this disease in a confined geographic area in Arizona. The cayenne tick (Amblyomma cajennense) is a common vector for RMSF in Central and South America and its range extends into the United States in Texas.
During 1997 to 2002, the estimated average annual incidence of RMSF, based on passive surveillance, was 2.2 cases/ million persons. More than half (56%) of reported cases of RMSF were from only five states—North Carolina, South Carolina, Tennessee, Oklahoma, and Arkansas—but cases have been reported from each of the contiguous 48 states, except Vermont and Maine. RMSF is also endemic throughout several countries in Central and South America, including Argentina, Brazil, Columbia, Costa Rica, Mexico, and Panama.
Signs and Symptoms
The first symptoms of RMSF typically begin 2 to 14 days after the bite of an infected tick. A tick bite is usually painless and about 50% of those who develop RMSF do not remember being bitten. The disease frequently begins as a sudden onset of fever and headache. Most patients with RMSF (90%) have some type of rash during the course of the illness. The number and combination of symptoms vary greatly from person to per son. Symptoms can include fever, rash (occurs 2 to 5 days after fever; may be absent in some cases), headache, nausea, and vomiting.
Diagnostic Evaluation
Blood specimens are not always useful for detection of the organism through PCR assay or culture. If the patient has a rash, PCR testing or immunohistochemical (IHC) staining can be performed on a skin biopsy taken from the rash site or on autopsy specimens. This can yield rapid results with good sensitivity (70%) when applied to tissue specimens collected during the acute phase of illness and before antibiotic treatment has been started, but a negative result should not be used to guide treatment decisions.
During RMSF infection, a patient’s immune system devel ops antibodies to R. rickettsii, with detectable antibody titers usually observed within 7-10 days of illness onset. It is important to note that antibodies are not detectable in the first week of illness in 85% of patients; a negative test during this period does not rule out RMSF as a cause of illness.
The gold standard serologic test for diagnosis of RMSF is the IFA with R. rickettsii antigen, performed on two paired serum samples to demonstrate a significant (fourfold) 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.
Typically, in most RMSF cases, the first IgG IFA titer is low or negative and the second 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 yield a false-positive result. 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 per sons who were previously exposed to antigenically related organ isms. 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. If only one sample is tested, it can be difficult to interpret, whereas two paired samples taken weeks apart that demonstrate a significant (fourfold) rise in antibody titer provide the best evidence for the correct diagnosis of RMSF.
Treatment and Prevention
The progression of the disease varies greatly. Patients who are treated early may recover quickly on outpatient medication, whereas those who experience a more severe course may require IV antibiotics, prolonged hospitalization, or intensive care.
Doxycycline is the first-line treatment for adults and children of all ages and is most effective if started before the fifth day of symptoms. Standard duration of treatment is 7 to 14 days.
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