Emergence of Two different recombinant PRRSV strains with low neutralizing antibody susceptibility in China

PRRSV causes major economic loss in global swine industry. 41 of 131 (31.29%) tissue samples collected from pig farms in central and east China from 2016 to 2017 were confirmed as PRRSV positive in RT-PCR. Base on phylogenetic analysis for ORF5 and ORF6, 3 isolates closely related to QYYZ strain form a new subgroup IV, while 3 other ones were clustered into subgroup III, represented by NADC30. Numerous amino acid substitutions involved in viral neutralization susceptibility were identified in GP5 among these isolates. Two emerging PRRSV strains (ZJnb16-2, SDbz16-2) were successfully isolated and sequenced. ZJnb16-2 was identified as a recombinant virus between strain QYYZ and JXA1 while SDbz16-2 was an inter-subgenotype recombinant virus of strains NADC30 and JXA1. As shown in the pathogenicity evaluation in piglets, ZJnb16-2 is highly pathogenic while SDbz16-2 is mild. Hyper-immune sera against major vaccine strains HUN4-F112 and JK-100 failed to neutralize either ZJnb16-2 or SDbz16-2. Only 0.8-2.0% of pig serum samples which were confirmed as PRRSV-positive with commercial ELISA kits presented neutralization reactivity against either ZJnb16-2 or SDbz16-2. The study confirmed that the viral genomic recombination contributes to the emergence of new pathogenic PRRSVs in China, which may escape from the protective immunity elicited by the conventional vaccines, highlighting the necessity in updates of vaccine strains and the need for a universal vaccine against PRRSV.