Objective: The protein capsule of Yersinia pestis, known as Fraction 1 or F1, is a protective immunogen and is an assumed, but not proven, virulence factor. Our objectives were to determine if inhaled F1-negative and/or F1-positive strains of Y pestis were virulent in the African green monkey and, if so, to differentiate F1-negative from F1-positive monkeys. Because F1-negative strains have been isolated from natural sources and have caused experimental fatal disease, we felt that this information was crucial to the development of future vaccines and diagnostic tests.
Materials and methods: Adult African green monkeys were exposed by aerosol to F1-positive (CO92, n=15) or F1-negative (CO92-C12, n=6; Java-9, n=2) Y pestis strains.
Results: All monkeys died 4 to 10 days postexposure and had lesions consistent with primary pneumonic plague. Antibodies to F1 antigen and other Y pestis antigens allowed us to differentiate F1-positive from F1-negative Y pestis strains in fixed tissues.
Conclusions: In this study, F1 antigen was not a required virulence factor. Therefore, there may be a need for vaccines and diagnostic assays that are not solely based on the F1 antigen.