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Rocky Mountain Spotted Fever

Rocky Mountain spotted fever (RMSF) is caused by infection with Rickettsia rickettsii. In the United States, Rocky Mountain Spotted Fever (RMSF) is the rickettsiosis that causes the most severe disease. The highest incidence occurs in persons between 60 and 69 years of age, but the highest case-fatality rate is among children less than 10 years if age. Rickettsiosis cases occur throughout the United States, but more than 60% of cases originated in five states: Arkansas, Missouri, North Carolina, Oklahoma, and Tennessee.

In the United States, the tick species that is most frequently associated with transmission of Rickettsia rickettsii is the American dog tick, Dermacentor variabilis. This tick is found primarily in the eastern, central, and Pacific coastal United States. D. variabilis ticks often are encountered in wooded, shrubby, and grassy areas and tend to congregate along walkways and trails. These ticks also can be found in residential areas and city parks. Adult Dermacentor ticks are active from spring through autumn, with maximum activity during late spring through early summer.

A history of a tick bite within 14 days of illness onset is reported in only half of RMSF cases. Therefore, absence of a recognized tick bite should never dissuade physicians from considering tickborne rickettsial disease. Symptoms of RMSF typically appear 3 to 12 days after the bite of an infected tick or between the fourth and eighth day after discovery of an attached tick. Initial symptoms include sudden onset of fever, headache, chills, malaise, and myalgia. A rash typically appears 2–4 days after the onset of fever. The classic triad of fever, rash, and reported tick bite is present in only a minority of patients during initial presentation. RMSF rash usually begins as small pink macules on the ankles, wrists, or forearms that subsequently spread to the palms, soles, arms, legs, and trunk, usually sparing the face. As the disease progresses, the rash becomes maculopapular, sometimes with central petechiae. Without treatment, RMSF progresses rapidly. Severe, late-stage manifestations of RMSF include meningoencephalitis, acute renal failure, ARDS, cutaneous necrosis, shock, arrhythmia, and seizure. Case-fatality rates are estimated to be  5% to10%. Patients treated after the fifth day of illness are more likely to die than those treated earlier in the course of illness.

Laboratory findings are not very specific early in the course of the disease. White blood cell count is typically normal or slightly increased and may have a left shift in granulocytes. As the illness progresses, thrombocytopenia may develop. Mild elevations in aspartate transaminase and alanine transaminase and bilirbuin may occur. Hyponatremia is a particularly common finding in patients with central nervous system involvement. Cerebrospinal fluid often shows lymphocytosis, moderate elevation of protein and normal glucose.

R. rickettsii cannot be cultured in most clinical laboratories. Clinical diagnosis must be confirmed by serologic testing for IgG and IgM antibodies to Rickettsia rickettsii. IgM and IgG antibodies typically appear 7 to 10 days after the onset of the illness. The optimal time to obtain a convalescent antibody titer is at 14 to 21 days after the onset of symptoms. A fourfold rise in IgG titers between acute and convalescent sera is consistent with seroconversion and recent illness. The minimum diagnostic titer in most laboratories is 1:64. Falsely negative results may occur if a specimen is drawn too early after onset of symptoms or early antibiotic treatment blunts the rise in antibody titer.

Reference value is not detected. Specimen requirement is a red top tube of blood.

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