Rocky Mountain spotted fever is transmitted by the bite of a tick infected with which Rickettsia species?

• A. Rickettsia akari
• B. Rickettsia conorii
• C. Rickettsia prowazekii
• D. Rickettsia rickettsii

Answer: (B)

Rocky Mountain spotted fever is caused by an intracellular bacterium that is transmitted to humans through the bite of infected ticks. The species responsible is Rickettsia rickettsii. Ticks such as Dermacentor variabilis (the American dog tick) and Dermacentor andersoni are common vectors in North America, and after the bite the bacteria invade endothelial cells, leading to the vasculitis and rash that characterize the illness.

Other Rickettsia species are associated with different vectors and diseases. Rickettsia akari causes rickettsialpox and is transmitted by mites from infected house mice. Rickettsia conorii is linked to Mediterranean spotted fever and is transmitted by ticks in certain regions, not the one classically described for RMSF. Rickettsia prowazekii causes epidemic typhus and is transmitted by human body lice.

So the correct agent for Rocky Mountain spotted fever is Rickettsia rickettsii.