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Journal of Virology, May 2002, p. 4750-4763, Vol. 76, No. 10
0022-538X/02/$04.00+0     DOI: 10.1128/JVI.76.10.4750-4763.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Evolution of Porcine Reproductive and Respiratory Syndrome Virus during Sequential Passages in Pigs

C.-C. Chang,1 K.-J. Yoon,1* J. J. Zimmerman,1 K. M. Harmon,1 P. M. Dixon,2 C. M. T. Dvorak,3 and M. P. Murtaugh3

Department of Veterinary Diagnostic and Production Animal Medicine,1 Department of Statistics, Iowa State University, Ames, Iowa 50010,2 Department of Veterinary PathoBiology, University of Minnesota, St. Paul, Minnesota 551083

Received 6 November 2001/ Accepted 12 February 2002

Porcine reproductive and respiratory syndrome (PRRS) viruses are recognized as possessing a high degree of genetic and antigenic variability. Viral diversity has led to questions regarding the association of virus mutation and persistent infection in the host and has raised concerns vis-à-vis protective immunity, the ability of diagnostic assays to detect novel variants, and the possible emergence of virulent strains. The purpose of this study was to describe ongoing changes in PRRS virus during replication in pigs under experimental conditions. Animals were inoculated with a plaque-cloned virus derived from VR-2332, the North American PRRS virus prototype. Three independent lines of in vivo replication were maintained for 367 days by pig-to-pig passage of virus at 60-day intervals. A total of 315 plaque-cloned viruses were recovered from 21 pigs over the 367-day observation period and compared to the original plaque-cloned virus by virus neutralization assay, monoclonal antibody analysis, and sequencing of open reading frames (ORFs) 1b (replicase), 5 (major envelope protein), and 7 (nucleocapsid) of the genome. Variants were detected by day 7 postinoculation, and multiple variants were present concurrently in every pig sampled over the observation period. Sequence analysis showed ORFs 1b and 7 to be highly conserved. In contrast, sequencing of ORF 5 disclosed 48 nucleotide variants which corresponded to 22 amino acid variants. Although no epitopic changes were detected under the conditions of this experiment, PRRS virus was shown to evolve continuously in infected pigs, with different genes of the viral genome undergoing various degrees of change.

* Corresponding author. Mailing address: Department of Veterinary Diagnostic and Production Animal Medicine, College of Veterinary Medicine, Iowa State University, Ames, IA 50011. Phone: (515) 294-1083. Fax: (515) 294-6619. E-mail: kyoon{at}iastate.edu.

Journal of Virology, May 2002, p. 4750-4763, Vol. 76, No. 10
0022-538X/02/$04.00+0     DOI: 10.1128/JVI.76.10.4750-4763.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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