text: Ecology In addition to having vectors in the United States, R. rickettsii commonly infect certain ticks in South America. The cayenne tick (Amblyomma cajennense) and the brown dog tick (Rhipicephalus sanguineus) are both common vectors found in South America. The yellow dog tick (Amblyomma aureolatum) is another vector that infects specific parts of Brazil. Infecting horses and capybaras, the cayenne tick turns vertebrate hosts into amplifier hosts of the bacteria. Other small rodent species serve as amplifier hosts in the United States and South America.
  Specific requirements must be fulfilled when looking at vertebrate hosts to become amplifiers for R. rickettsii. First, there should be a large population of said host in the endemic area. Then, the vertebrate must be a primary host for the vector, so it can carry out its life cycle. Furthermore, it must be prone to infection from R. rickettsii, and once inside, the bacteria must infect the host to a great enough degree that its blood will infect other vectors that attach to the host. Finally, the host should be part of a prolific species, so that the species has a continuous supply of nonimmune amplifiers.
  Transmission in mammals
  Due to its confinement in the midgut and small intestine, Rickettsia rickettsii can be transmitted to mammals, including humans.