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Background:In China,PRRS is first recognized in 1996 and has become one of the most severe diseases affect swine production.Moreover,an outbreak of highly pathogenic PRRS (HPPRRS),a highly virulent form of PRRS whose principal clinical sign is high fever and high mortality in pigs of all ages,which occurs in 2006,inflicts severe damage on the Chinese pork industry.Highly pathogenic PRRSV (HP-PRRSV) is identified as the main pathogen of HPPRRS.