Rickettsia rickettsii, the causative organism of RMSF, is transmitted by the bite of an infected tick. Fever, headache, rigors, abdominal pain, myalgias, and malaise occur 2 to 14 days after inoculation. Three to 5 days after the onset of symptoms, the rash begins with erythematous, blanching macules on the distal extremities (wrists and ankles). This is followed by centripetal spread to the trunk and the palms and soles. The lesions evolve into papules and petechia. Without treatment, RMSF has a 25% mortality; delayed diagnosis and treatment result in 3% to 4% mortality.
Management and Disposition
Prompt initiation of doxycycline is recommended for adults and children. Consultation with an infectious disease specialist should be initiated to help guide treatment; this is especially critical for pregnant females. Mildly ill patients may be treated with oral antibiotics on an outpatient basis if close follow-up can be confirmed. More severely ill patients require admission.
Typical distribution of extremity lesions should be treated as RMSF until proven otherwise. Treatment should never be delayed for laboratory confirmation.
Most cases occur between April and October, with the highest US incidence occurring in the Southeast and South-Central states (not Rocky Mountain states).
Forty percent of patients do not recall the inciting tick bite.
Up to 15% of cases present without any cutaneous manifestations. Patients with darker skin types may have less obvious cutaneous findings.
In pediatric patients less than 9 years old, the recommended course of doxycycline has a negligible effect on permanent tooth discoloration. Previously recommended chloramphenicol has a higher mortality rate than doxycycline.
Rocky Mountain Spotted Fever. These erythematous macular lesions will evolve into a petechial rash that will spread centrally. (Photo contributor: Daniel Noltkamper, MD.)
Rocky Mountain Spotted Fever. Early erythematous and hemorrhagic macules and papules appeared initially on the ankles of an adolescent. (Used with permission from Wolff K, Johnson RA, Saavedra AP. Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 7th ed. New York, NY: McGraw Hill; 2013: Fig. 25-49.)