Characterization of Porcine Reproductive and Respiratory Syndrome (PRRS) Virus

Porcine reproductive and respiratory syndrome is a newly described disease of swine that is now widespread in the U.S. and Europe. It affects pigs of all ages and causes significant morbidity and mortality. At the onset of this study, little information was available regarding the etiologic agent of PRRS. Therefore, the purpose of this study was to determine the basic characteristics of the PRRS virus, characterize the viral proteins, evaluate antigenic variation among PRRS virus isolates, and evaluate the humeral immune response of experimentally infected pigs to PRRS viral proteins. Porcine reproductive and respiratory syndrome virus was determined to be a small, enveloped RNA virus with morphological and physicochemical properties similar to the arteriviruses. Five putative virus structural proteins were identified. Proteins of 15-, 16-, 19-, 22- and 26-kDa relative molecular mass were associated with the U.S. isolate of PRRS virus, VR-2332. Proteins of 15-, 15.5-, 18-, 22- and 26-kDa were associated with the European prototype isolate, the Lelystad isolate. The 15- and 19-kDa proteins and the 15- and 18-kDa proteins of the VR-2332 and Lelystad isolates, respectively, were non-glycosylated while the 26-kDa protein was identified as a glycoprotein. The 15-kDa protein was detemined to be the nucleocapsid protein of PRRS virus. European isolates of PRRS virus were determined to be antigenically distinct from U.S. isolates of the virus. However, immunoblotting experiments using polyclonal antisera demonstrated that European and U.S. isolates share some common antigenic epitopes on the nucleocapsid protein. Some monoclonal antibodies prepared to the nucleocapsid protein of PRRS virus recognized epitopes common to all isolates of PRRS virus, while other monoclonal antibodies recognized epitopes on U.S. isolates but not European isolates of PRRS virus. Circulating antibodies to PRRS virus were detected in experimentally infected pigs within 7 to 21 days post-inoculation. Neutralizing antibodies were not detected until 6 to 8 weeks post-inoculation. The antibody response of experimentally infected pigs was primarily directed toward the 15-, 19- and 26-kDa PRRS virus proteins