Etiology

Caused by infection with the obligately intracellular aerobic bacterium Rickettsia rickettsii. Ticks are both natural reservoirs of this organism and vectors to infect humans.[1]

The predominant vector in the eastern, central, and western US is the dog tick Dermacentor variabilis. In the northern Rocky Mountains most cases are transmitted by the wood tick Dermacentor andersoni. The brown dog tick Rhipicephalus sanguineus was identified as a vector along the US-Mexico border in 2003 and is now the most common vector in southwestern US and Mexico.[1][6]​​[9][12]​​​ Tick species in the genus Amblyommaare the causative vector in Central and South America.[1][8]​​​​​

Centers for Disease Control and Prevention: tick ID Opens in new window

Transmission via blood transfusion is extremely rare.[1][Figure caption and citation for the preceding image starts]: Dorsal view of a female American dog tick Dermacentor variabilisCenters for Disease Control and Prevention [Citation ends].com.bmj.content.model.Caption@68791b85

Pathophysiology

The incubation period is 3 to 12 days. Rickettsiae are introduced via the bite of an infected tick, multiply in the endothelial and occasionally adjacent smooth muscle cells of small blood vessels, and are hematogenously disseminated throughout the body. A multisystem vasculitis ensues, with thrombosis and leakage of red blood cells and protein into surrounding tissues. Vascular lesions account for the principal signs and symptoms of Rocky Mountain spotted fever, such as rash and headache. Hyponatremia develops frequently, in part because water shifts from intracellular to extracellular spaces and secretion of antidiuretic hormone.​[13]​ As the infection progresses, end-organ damage and potentially death may occur.

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