Evaluation of infection status in Chinese swine with porcine reproductive and respiratory syndrome virus by nested RT-PCR targeting nsp2 gene.

One of the biggest obstacles in containing porcine reproductive and respiratory syndrome virus (PRRSV) results from its genetic diversity due to the high mutation rate. The nsp2 gene of PRRSV is the most hypervariable region of the genome. Since the emergence of highly pathogenic (HP)-PRRSV, many of PRRSV strains with a mutated nsp2 gene have been reported. To decipher the epidemiology of the PRRSV and identify the epidemic strains, a nested RT-PCR able to differentiate the nsp2 gene from different PRRSV strains was developed and used to test 550 clinical samples. The amplified products of 301-bp, 211-bp and 154-bp were corresponding to low pathogenic PRRSV (LP-PRRSV) infection without deletion in nsp2 gene, HP-PRRSV infection with 90-bp deletion in nsp2 gene and a variant PRRSV strain with 147-bp deletion in nsp2 gene, respectively. Of the 550 clinical samples, 192 including 108 serum samples and 84 tissue samples were tested PRRSV RNA positive. Of the 192 positive samples, 107 were infected with a single strain and 85 were infected with two strains. 84 out of 85 samples harboring two PRRSV strains of HP-PRRSV and PRRSV variant strain were detected. 97 out of 107 samples with single strain were detected with HP-PRRSV infection. The data indicated that HP-PRRSV containing a 90-bp deletion in the nsp2 gene remained the predominant strain in China.