Virologic, serologic and pathogenic characteristics of porcine reproductive and respiratory syndrome virus

Porcine Reproductive and Respiratory Syndrome (PRRS) has been recognized since 1987 in the U.S. swine industry. Little was known about the disease except that it caused severe economic losses due to reproductive failure in breeding swine and respiratory problems in piglets. The objectives were to define the cause of the syndrome, to characterize the isolates during the early stage, to develop serological methods and to understand pathogenic characteristics of PRRS virus infection in swine. Isolation of the causative virus was possible using porcine alveolar macrophage cultures. The syndrome was reproduced in young pigs and pregnant sows by experimental infection with an isolate, and major properties of the isolates were investigated. An indirect fluorescent antibody (IFA) test was developed for the detection of antibody to PRRS virus in swine sera. The assay was found to be simple to perform, and the results were highly sensitive and specific. Using the IFA, high prevalence of PRRS virus infection in the U.S. swine farms was confirmed. The growth of different PRRS virus isolates was demonstrated on a permissive clone of MA-104 cell line. A plaque method using the cell clone was standardized and proven useful for virus purification and differentiation of virus variants. Serum neutralization (SN) test was modified to improve its sensitivity. The modified SN method detected antibody earlier, and the method could be used to differentiate the serogroups of PRRS virus isolates. Virus infection and shedding from pigs infected or placed in contact at different intervals with infected pigs was investigated. The results indicated that different management practices could reduce the opportunity for pig-to-pig transmission of PRRS virus on endemically infected farms. Pathogenic differences between PRRS virus isolates in pregnant sows were described. The ability of the 3 PRRS virus isolates to cause transplacental infection was successfully demonstrated but fetal pathogenicity varied among the viruses. These results support the prevalence of different pathogenic PRRS virus strains under field conditions. From these studies, a better understanding of virologic, serologic and pathogenic characteristics of PRRS virus is achieved.