Interestingly, the inability of different endosymbiotic Rickettsia species to co-infect the same organ in the same tick, called “interference,” has been demonstrated, although the exact mechanisms are unknown. Early studies seeking to understand the epidemiology of RMSF in the Bitterroot Valley in Montana demonstrated that the non-pathogenic tick endosymbiont Rickettsia peacockii (found on the east side of the valley and originally called the East side agent) colonized the ovaries of D. andersoni ticks and excluded pathogenic Rickettsia rickettsii (more prevalent on the west side of the valley) from infecting the ovaries and being transmitted to eggs (Burgdorfer, Hayes & Mavros, 1981). Similarly, studies of Dermacentor variabilis (Say) infected with R. montanensis or R. rhipicephali demonstrated resistance to transovarial transmission of the reciprocal Rickettsia in challenge experiments (Macaluso et al., 2002). Negative influences between co-infecting species of Rickettsia and other symbionts has been suggested to occur in other vectors such as fleas (Azad & Beard, 1998; Jones et al., 2012). Interference has been postulated to have significant effects in altering the distribution of Rickettsia pathogens in the environment and, consequently, the presence of human disease (Burgdorfer, Hayes & Mavros, 1981).