Viral Fitness and Antigenic Determinants of Porcine Parvovirus at the Amino Acid Level of the Capsid Protein.

Since 2001, strains of porcine parvovirus (PPV), designated 27a-like strains, were observed in Europe, suggesting a predominance of these viruses over older strains. The reasons for the obvious evolutionary advantage are unknown. Here, a series of mutants containing amino acid replacements found in the predominant field strains were generated in a PPV-NADL2 background, and their impact on replication efficiency and antibody binding activity was determined. Some amino acid substitutions observed in the 27a-like strains significantly increased viral fitness and decreased neutralization activity of serum samples raised against commercial vaccines and old virus strains (e.g., NADL2). These mutant viruses and a monoclonal antibody raised against a classical PPV strain defined a 27a-specific neutralizing epitope around amino acid 228 of the capsid protein VP2. Based on the analysis of the mutant viruses, it is hypothesized that the predominant factor for the global spread of the PPV-27a strain substitutions is an increased viral fitness of the 27a-like viruses, possibly supported by partial immune selection. This is reminiscent to the evolution of canine parvovirus and worldwide replacement of the original virus by the so-called new antigenic types. IMPORTANCE Porcine parvovirus is one of the most important causes of reproductive failure in swine. Recently, despite the continuous use of vaccines, "new" strains emerged, leading to the hypothesis that the emergence of new amino acid substitutions could be a viral adaptation to the immune response against the commercial vaccines. Our results indicate the amino acid substitutions observed in the 27a-like strains can modify viral fitness and antigenicity. However, an absolute immune escape was not evident.