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

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

Answer: (A)

Rickettsia rickettsii is the bacterium that causes Rocky Mountain spotted fever, and it is transmitted to people by the bite of an infected tick. The ticks most often involved are Dermacentor species, such as the American dog tick, which pass the organism to humans during feeding. This link between the specific bacterium and a tick vector distinguishes RMSF from other rickettsial diseases caused by different species and different vectors—for example, Rickettsia akari is spread by mites and causes rickettsialpox; Rickettsia conorii causes Mediterranean spotted fever transmitted by a different tick in other regions; and Rickettsia prowazekii causes epidemic typhus spread by the human body louse. The key idea is that Rocky Mountain spotted fever is driven by infection with Rickettsia rickettsii transmitted by tick bites.