Porcine reproductive and respiratory syndrome (PRRSV) is a viral disease characterized by two overlapping clinical presentations, reproductive impairment or failure in breeding animals, and respiratory disease in pigs of any age. Porcine reproductive and respiratory syndrome (PRRS) is the most economically significant swine disease in North America. The etiologic agent is an RNA virus named porcine reproductive and respiratory syndrome virus (PRRSV). Large genetic variability exists between PRRSV strains. The whole-PRRSV genome is composed of 15.1 base pairs (bp). The open reading frame-5 (ORF5) genome region represents 4% of the genome (603 bp) and has been primarily used to investigate PRRSV genetic characteristics and diversity.

For more background about the clinical disease, please see the PRRSV entry in the Swine Manual.

PRRSV 1-4-4 variant 

Reports of a very aggressive PRRSV strain affecting swine primarily in Iowa and Minnesota was identified in 2020-2021. This new aggressive PRRSV strain is associated primarily with increased reproductive failure, pre-weaning mortality, downstream mortality, and a higher level of viral concentration in tested samples, i.e., low cycle threshold (Ct) values. Analysis conducted at the ISU VDL found that the PRRSV strain associated with these breaks was a new variant strain classified as PRRSV RFLP 1-4-4 Lineage 1 C variant (PRRSV 1-4-4 L1C variant).

The first detection of this strain at the ISU VDL occurred in June 2020. Until May 2021, > 98% nucleotide homology within their phylogenetic group was observed. Since May 2021, some genetic variability has been observed with some strains having < 98% nucleotide homology within their phylogenetic group.

Between September 1, 2020 and August 31, 2021, more than 5,000 cases have been processed by the ISU VDL requesting PRRSV ORF5 sequencing, and 284 submissions had PRRSV sequencing strain classified as PRRS 1-4-4 L1C variant. From those, 73.6% (209) ORF5 sequences were associated with 180 distinct premises from: Iowa (152), Minnesota (22), Nebraska (2), Illinois (1), Indiana (1), Missouri (1), and South Dakota (1). Premises were identified as grow-finish (81), nursery (18), breeding herd/isolation (18), finisher (5), and other (64). The remaining 75 submissions without a premise identification number come mostly from Iowa (51) and Minnesota (15).

The ISU VDL will continue to work with collaborators and clients to track the spread of the PRRS 1-4-4 L1C variant.