Comparative Analysis of Infection by Rickettsia rickettsii Sheila Smith and Taiacu Strains in a Murine Model

Rocky Mountain spotted fever (RMSF) is a life-threatening tick-borne disease caused byRickettsia rickettsii, which is widely distributed throughout the Americas. Over 4000 cases of RMSF are recorded annually in the United States, while only around 100 cases are reported in Brazil. Conversely, while case fatality rates in the United States oscillate around 5%, in Brazil they can surpass 70%, suggesting that differences in tick vectoring capacity, population sensitivity, and/or variability in virulence of the rickettsial strains may exist. In this study, we compared the susceptibility of C3H/HeN mice to two highly virulent strains ofR. rickettsii, one from the United States (Sheila Smith) and the other from Brazil (Taiacu). Animals inoculated with the Taiacu strain succumbed to infection earlier and exhibited severe histological lesions in both liver and spleen sooner than mice infected with the Sheila Smith strain. These differences in survival and signs of the disease are not related to a greater proliferation of the Taiacu strain, as there were no significant differences in the rickettsial load in mice tissues inoculated with either strain. The present study is the first step to experimentally assess differences in fatality rates of RMSF in two different regions of the American continent.