Rickettsia rickettsii whole cell antigens offer protection against Rocky Mountain spotted fever in the canine host

Rocky Mountain Spotted Fever (RMSF) is a potentially fatal tick-borne disease in people and dogs. RMSF is reported in the USA and several countries in north, central and south Americas. The causing agent of this disease, Rickettsia rickettsii , is transmitted by several species of ticks, including Dermacentor andersoni, Rhipicephalus sanguineus , and Amblyomma americanum . RMSF clinical signs generally include fever, headache, nausea, vomiting, muscle pain, lack of appetite, and rash. If untreated, it can quickly progress into a life-threatening illness in people and dogs with high fatality rates ranging from 30-80%. While RMSF has been known for over a century, recent epidemiological data suggest that the documented cases and the fatality rates remain high in people, particularly during the last two decades in parts of North America. Currently, there are no vaccines available to prevent RMSF in either dogs or people. In this study, we investigated the efficacy of two experimental vaccines, a subunit vaccine containing two recombinant outer membrane proteins (RCA) and a whole cell inactivated antigen vaccine (WCA), in conferring protection against virulent R. rickettsii infection challenge in a newly established canine model for RMSF. Dogs vaccinated with WCA were protected from RMSF, whereas those receiving RCA developed the disease similar to non-vaccinated R. rickettsii -infected dogs. WCA also reduced the pathogen loads to nearly undetected levels in the blood, lungs, liver, spleen and brain, and induced bacterial antigen-specific immune response. This study provides the first evidence of the protective ability of WCA against RMSF in dogs.