Study flags novel recombinant PRRSV-2 linked to MLV-like strains

A newly published study in Animals describes a novel PRRSV-2 strain in China, SCMS2025, that the authors say emerged through recombination involving two modified live virus-like strains. The report stands out because the virus looked NADC30-like by ORF5 genotyping, yet whole-genome analysis placed it between lineage 1 and lineage 5, illustrating how recombinant PRRSV can blur familiar classification lines. In experimentally infected piglets, SCMS2025 caused fever, poorer weight gain, viremia, and tissue lesions, but the disease course was transient and the strain was categorized as low pathogenic. (pubmed.ncbi.nlm.nih.gov)

That finding lands in a field already shaped by PRRSV’s well-known genetic instability. Reviews and prior studies have documented that recombination is a major driver of PRRSV evolution, including events involving wild-type strains, vaccine-like strains, and, more rarely, recombination between two modified live vaccine strains. A 2022 U.S. report described what researchers called the first field detection of a PRRSV-2 recombinant consisting entirely of two modified live vaccine strains, and a more recent herd-level report tracked an MLV-derived recombinant along with its production impact and eventual elimination from a breeding herd. (pmc.ncbi.nlm.nih.gov)

In the new paper, the farm history matters. According to the MDPI full-text summary, the herd used the HP-PRRSV-derived MLV vaccine TJM-F92 from 2020 through 2022, then switched to RespPRRS MLV in May 2023. The authors isolated SCMS2025 from a PRRSV-positive serum sample and reported a complex recombination pattern involving NADC30-like virus and two MLV-like strains, RespPRRS MLV and TJbd14-1 MLV-like virus. The isolate carried a characteristic discontinuous NSP2 deletion pattern, shared its highest nucleotide identity, 87.6%, with lineage 5 strains, and showed discordant placement depending on whether ORF5 or the full genome was analyzed. (pubmed.ncbi.nlm.nih.gov)

The pathogenicity data suggest the virus is not benign, even if it is less severe than some contemporary PRRSV variants. In the 14-day piglet study, infected animals developed transient overt clinical signs and significantly reduced weekly weight gain. The highest tissue viral loads were found in the tonsils, and pathology included hemorrhagic pneumonia, lymph node swelling and hemorrhage, and tonsillar damage. Notably, the piglets remained seronegative for PRRSV-specific antibodies throughout the observation period, which the authors interpreted as evidence of delayed humoral response. (pubmed.ncbi.nlm.nih.gov)

Industry and expert-oriented sources reinforce two practical takeaways. First, suspected recombinants identified by next-generation sequencing should be confirmed with additional methods, because mixed infections can complicate interpretation. Second, several reports suggest that keeping vaccine strategy consistent and avoiding situations where pigs are simultaneously exposed to multiple PRRSV strains may reduce opportunities for recombination. Those points don’t negate the value of modified live vaccination in control programs, but they do sharpen the conversation around how those tools are used in complex field settings. (nationalhogfarmer.com)

Why it matters: For veterinary professionals working in swine health, this study is less about one Chinese isolate and more about surveillance blind spots. ORF5 sequencing remains useful, but this case shows it can misclassify recombinant viruses that only become clear through whole-genome work. That matters for outbreak interpretation, vaccine decisions, communication with producers, and biosecurity planning. If clinical performance, herd history, and sequence data don’t line up, this paper supports escalating to whole-genome sequencing and taking a hard look at recent vaccine use, strain circulation, and co-infection risk. (pubmed.ncbi.nlm.nih.gov)

What to watch: The next questions are whether SCMS2025-like viruses are found beyond the index setting, whether they remain low pathogenic in broader field conditions, and whether regulators, diagnosticians, or industry groups push harder for whole-genome sequencing in PRRS investigations where vaccine-like or recombinant strains are suspected. (nationalhogfarmer.com)