The pathogenesis and spotted-fever-group complement-fixing (CF) antibody responses of black-tailed jack rabbits and cottontails following inoculation with Rickettsia rickettsii were investigated. Cottontails inoculated 1 month after being trapped did not have rickettsemia that persisted for as long as it did in cottontails 9 months after capture. Of nine jack rabbits tested, rickettsemia lasting for 6 days developed in two. There was a marked variation in the persistence of rickettsemia in the cottontails and jack rabbits tested. There did not appear to be any tendency for latent infection in cottontails, as rickettsiae did not persist for longer than 20 days in the tissues of the animals tested. CF antibody was readily found in jack rabbits, but the response in cottontails was variable, and some cottontails known to have been infected did not have CF antibodies injected with a soluble antigen.
It was concluded that the jack rabbit may not be as important in the cycle of R. rickettsii in nature as previously believed and that cottontails are of greater importance in the dissemination of R. rickettsii in nature.
Present address: Department of Microbiology, The Lovelace Foundation for Medical Education and Research, Albuquerque, New Mexico 87108.