Concurrent porcine circovirus type 2a (PCV2a) or PCV2b infection increases the rate of amino acid mutations of porcine reproductive and respiratory syndrome virus (PRRSV) during serial passages in pigs.

•The genetic variability of PRRSV during serial passage in PCV2-infected pigs was investigated.•PRRSV structural genes ORF6 and ORF7 mutated at various degrees over time in vivo.•A significantly higher mutation rate was observed when pigs were co-infected with PCV2.