Rocky Mountain Spotted Fever (RMSF), the most virulent of the rickettsial spotted fevers, is caused by the Gram-negative coccobacillus, Rickettsia rickettsii. R. rickettsii is an obligate intracellular pathogen which, after the bite of an infected tick, invades and multiplies within the endothelial cells of veins and arteries. RMSF is not transmitted person-to-person.
Prior to the 1930s, RMSF was predominantly seen throughout the Rocky Mountains states. However, since the 1940s RMSF has become much more common in southeastern and south central states than in western states. The Centers for Disease Control have reported that from 1981 - 1992 there were 9,223 cases of RMSF in the United States. Of these cases, 48% were reported from four states: North and South Carolina, Oklahoma, and Tennessee. RMSF can occur in any month, but 90% of cases occur from April to September. The peak incidence of RMSF is in children ages 5 to 9 years with a second, smaller peak in men older than 60 years of age. RMSF is most common in males (55%) and in whites (85%). A history of a recent tick bite is present in 65% to 85% of patients with RMSF.
The incubation period for RMSF is 2 to 14 days. The onset of symptoms is usually sudden and include: fever (94%), headache (86%), and myalgias (82%). Chills, nausea, and vomiting may also be reported. On examination, 80% of patients have a rash, but only 56% have a rash involving the palms or soles, a distribution often considered to be indicative of RMSF. Two "classic triads" are described in RMSF: (1) fever + rash + report of a tick bite present in only 3% of patients presenting within the first 3 days of illness; and (2) fever + headache + rash present in 55% of confirmed cases. The rash of RMSF usually occurs between the third and forth day of fever, but may occur later. The rash often begins on the wrists and ankles, and then spreads centripetally to the legs, arms, face, and abdomen usually sparing the mucous membranes. Early lesions blanch with pressure, but later become petechial. Biopsy of the rash shows a perivascular infiltration with round cells. Classically the rash involves the palms and soles, ankles and wrists (acral areas), and skin folds of the axillae.
The diagnosis of RMSF requires a high level of suspicion. Most patients have a history of exposure to ticks or tick infested areas. Sudden onset of fever, myalgias, and severe headache (worst headache in patient’s life) with or without a rash suggest RMSF. Because survival is linked to early diagnosis, and rash may not be present in the early stages, it is important to consider RMSF before a rash is evident. The presence of meningismus should suggest other diagnoses (ie. meningitis). The spinal fluid is usually normal, but an elevated spinal fluid protein and a few mononuclear cells may be present in comatose patients. The white cell count is normal or low, and thrombocytopenia and hyponatremia are common. Increased mortality is associated with G6PD deficiency (may result in severe hemolysis), advanced age, and late treatment with effective antibiotics. Immunofluorescence staining of skin or rash biopsies can provide a rapid diagnosis. The indirect immunofluorescent antibody test is superior to the old Weil-Felix reactions, but serologies do not provide a rapid diagnosis.
Early therapy is crucial. The mortality rate for patients treated within the first five days is 6.5% vs. 22.9% after 5 days. Doxycycline (100 mg BID) or other tetracyclines are the drugs of choice. Chloramphenicol therapy results in a higher mortality rate in children and adults than does doxycycline therapy, 8.2% vs 1.6%, respectively.