Rocky Mountain spotted fever (RMSF) is a tick-borne disease caused by the intracellular bacterium Rickettsia rickettsii.
Antibody reactivity to Rickettsia rickettsii antigen should be considered Spotted Fever group reactive. Other organisms within the group include R. akari, R. conorrii, R. australis, and R. sibirica.
The standard serologic test for diagnosis of RMSF is the indirect immunofluorescence antibody (IFA) assay for immunoglobulin G (IgG) using R. rickettsii antigen.
R. rickettsii is closely related to other pathogenic spotted fever group Rickettsia (SFGR) species, including R. akari, R. parkeri, and Rickettsia 364D. Closely related species of SFGR share similar antigens such that antibodies directed to one of these antigens can cross-react with other heterologous spotted fever group antigens.
Early recognition and treatment with doxycycline is critical to survival.
Rocky Mountain spotted fever (RMSF) can be difficult to diagnose due to the non-specific signs and symptoms in early stages of illness. Signs and symptoms can vary from patient to patient and can resemble other, more common diseases. Nonetheless, early consideration of rickettsial disease in the differential diagnosis and empiric treatment is crucial to prevent severe illness and death.
Testing for RMSF should be considered for any person with a compatible illness and known risk factors, such as those who remember a tick bite or have been exposed to areas with ticks.
- The standard serologic test for diagnosis of RMSF is the indirect immunofluorescence antibody (IFA) assay for immunoglobulin G (IgG) using R. rickettsii antigen.
- IgG IFA assays should be performed on paired acute and convalescent serum samples collected 2–4 weeks apart to demonstrate evidence of a fourfold seroconversion.
- Antibody titers are frequently negative in the first week of illness. RMSF cannot be confirmed using single acute antibody results.
- Immunoglobulin M (IgM) IFA assays are available through some reference laboratories, however results might be less specific than IgG IFA assays for diagnosing a recent infection.
- R. rickettsii is closely related to other pathogenic spotted fever group Rickettsia (SFGR) species, including R. akari, R. parkeri, and Rickettsia 364D. Closely related species of SFGR share similar antigens such that antibodies directed to one of these antigens can cross-react with other heterologous spotted fever group antigens.
- Most commercial labs are unable to differentiate one spotted fever infection from another using these serologic methods.
- Antibodies to R. rickettsii might remain elevated for many months after the disease has resolved.
- In certain people, high titers of antibodies against R. rickettsii have been observed up to four years after the acute illness.
- Ten percent or more of healthy people in some areas might have elevated antibody titers due to past exposure to R. rickettsii or other SFGR.
- Comparison of paired, and appropriately timed, serologic assays provides the best evidence of recent infection.
- Single or inappropriately timed serologic tests, in relation to clinical illness, can lead to misinterpretation of results.
Signs & Symptoms:
- Rocky Mountain spotted fever, (RMSF) is the most severe rickettsiosis in the United States.
- RMSF is a rapidly progressive disease and without early administration of doxycycline can be fatal within days.
- Signs and symptoms of RMSF begin 3-12 days after the bite of an infected tick. However, because tick bites are not painful, many people do not remember being bitten.
- Illness generally begins with sudden onset of fever and headache and most people visit a healthcare provider during the first few days of symptoms.
Early illness (days 1-4)
- Gastrointestinal symptoms (nausea, vomiting, anorexia)
- Abdominal pain (may mimic appendicitis or other causes of acute abdominal pain)
- Rash (typically occurs 2-4 days after the onset of fever)
- Edema around the eyes and on the back of hands
Late illness (day 5 or later)
- Altered mental status, coma, cerebral edema
- Respiratory compromise (pulmonary edema, acute respiratory distress syndrome)
- Necrosis, often requiring amputation
- Multiorgan system damage (CNS, renal failure)
Risk factors for severe illness
- Delayed treatment
- Children < 10 years
- Persons with glucose-6-phosphate dehydrogenase (G6PD) deficiency
The CDC does not use IgM results for routine diagnostic testing of Rocky Mountain Spotted Fever, as the response may not be specific for the agent (resulting in false positives) and the IgM response may be persistent from past infection.
ABS; Aby; Antby; Anti; Antibodies; Antibody; Arbitrary concentration; Autoantibodies; Autoantibody; Immune globulin G; Immunoglobulin G; Microbiology; Point in time; QNT; Quan; Quant; Quantitative; R rickettsi; R rickettsii; Random; Ricket; Rickettsial; RMSF; Rocky mountain spotted fever; Serum; Spotted fever; SR
Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you.
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