Rickettsia rickettsii is a gram-negative bacterium native to the New World and causes the malady known as Rocky Mountain spotted fever (RMSF). It is transmitted through the bit of an infected tick when it feeds on animals and humans. Humans are not necessary hosts in the rickettsia-tick life cycle but they can be.
S. Burt Wolbach created the first detailed description of the etiologic agent in 1919. He recognized it as an intracellular bacterium seen most frequently in endothelial cells. The nucleus is often filled with minute particles and often was distended. Wolbach didn’t believe that the designation “˜rickettsia’ as appropriate even thought he recognized the similarity to the agent of typhus and tsutsugamushi. He proposed the name Dermacentroxenus rickettsi. It generally affects canines and humans.