R. rickettsii is transmitted to a human or small animal by the prolonged bite (several hours) of an adult tick—the wood tick (Dermacentor andersoni) in the western United States and the dog tick (Dermacentor variabilis) in the eastern part of the country. Occasionally, R. rickettsii is acquired through inhalation (such as in laboratory settings in which aerosolization of blood and specimens may occur) or through the contact of abraded skin with tick excreta or tissue juices. (This explains why people shouldn’t crush ticks between their fingers when removing them from other people and animals.) In most tick-infested areas, 1% to 5% of the ticks harbor R. rickettsii.

Endemic throughout the continental United States, RMSF is particularly prevalent in the Southeast and Southwest. Because RMSF is associated with outdoor activities, such as camping and backpacking, the incidence of this illness is usually higher in the spring and summer months. Epidemiologic surveillance reports for RMSF indicate that the incidence is also higher in children ages 5 to 9, in men and boys, and in whites.

Complications of RMSF include lobar pneumonia, pneumonitis, otitis media, parotitis, disseminated intravascular coagulation, shock, renal failure, meningoencephalitis, and hepatic injury.

The incubation period is usually about 7 days but can range from 2 to 14 days. Initial signs and symptoms include fever (102° to 104° F [38.9° to 40° C]), severe headache, nausea, vomiting, neurologic symptoms, and muscle pain. Children may also experience severe abdominal pain, altered mental status, and conjunctival infection. A maculopapular rash commonly develops on the palms of the hands and soles of the feet 4 to 7 days after the onset of symptoms. Blanching, pink maculae develop on the ankles, wrists, or forearms and commonly evolve to maculopapules. In about 50% of cases, the rash becomes petechial over the next several days of illness. The rash may spread to the entire body; however, spread to the face is typically limited. Patients with petechial rash are typically severely ill.