EPIDEMIOLOGY

AGE Any age

GENDER M > F.

RACE More common in Native Americans > whites, blacks

INCIDENCE Uncommon, though increasing. Highest incidence = 60 cases/million, southeastern U.S.

ETIOLOGY Caused by Rickettsia rickettsii, a gram-negative obligate intracellular bacterium.

SEASON April to September in the United States (95% of patients).

GEOGRAPHY Endemic in Virginia, North Carolina, South Carolina, Georgia, Kansas, Oklahoma, Texas, New York, northern Mexico, central, and South America.

PATHOPHYSIOLOGY

Occurs through bite of an infected tick or inoculation through abrasions contaminated with tick feces or tissue juices. The reservoirs and vectors are the wood tick (Dermacentor andersoni) in the western United States, the dog tick (D. variabilis) in the eastern and southern United States, and rarely the lone star tick (Amblyomma americanum) in the southern United States. Patients either live in or have recently visited an endemic area; however, only ∼62% may have knowledge of a recent tick bite. Following inoculation, there is an initial local replication of the organism in endothelial cells followed by hematogenous dissemination. Focal infection of vascular smooth muscle causes a generalized vasculitis. Hypotension, local necrosis, gangrene, and DIC may follow. The classic petechial rash results from extravasated blood after vascular necrosis.

HISTORY

The typical incubation period for RMSF after tick exposure ranges from 5 to 7 days, but can be as short as 2 days or as long as 2 weeks. The rash, present in 90% of cases, begins on day 3 or 4 on the extremities and spreads proximally. By day 6 or 7, the rash is generalized. Systemic symptoms include abrupt onset of fever (94%); severe headache (94%); generalized myalgia, especially the back and leg muscles (87%); a sudden shaking rigor; photophobia; prostration; and nausea with occasional vomiting.