Genetic interaction between porcine reproductive and respiratory syndrome virus (PRRSV) strains in cell culture and in animals

Objective: The need to obtain protective immunity and full reduction of disease associated with porcine reproductive and respiratory syndrome virus (PRRSV) infections has encouraged the use of multiple vaccine types to possibly obtain broader protection against genotypically variable PRRSV isolates. This strategy introduces potential risks of genetic rearrangements that might accelerate the natural rate of genetic change, facilitating the emergence of new PRRSV strains. Our goal was to determine if recombination was occurring between vaccine strains in porcine cells and in vivo. Methods: Attenuated vaccine viruses were grown together in cultured cells or administered simultaneously to pigs. Cell culture fluids and pig sera were evaluated for recombinant viruses. Results: Genetic recombination occurred between attenuated vaccine strains of PRRSV grown together in porcine alveolar macrophages and in simian MA-104 cell cultures. However, animals inoculated with both strains simultaneously did not provide evidence of viral recombination between vaccine strains in vivo. Implications: Practices which favor coinfection of cells with different virus strains might accelerate the rate of genetic change in PRRSV due to recombination. However, the probability of recombination of vaccine strains in animals appears to be low and recombinants appear to grow poorly. Mixing of vaccine strains for simultaneous administration in swine must be considered with the knowledge that recombination could occur. Vaccinating infected herds also introduces the possibility of recombination between vaccine and field strains. The likelihood of producing viable recombinant vaccine viruses is low, since they did not persist in cell culture and were not observed in vivo.