Isolation and Characterization of Porcine Deltacoronavirus from Pigs with Diarrhea in the United States

Porcine deltacoronavirus (PDCoV) is a novel coronavirus that causes diarrhea in nursing piglets. Following its first detection in the United States in February 2014, additional PDCoV strains have been identified in the United States and Canada. Currently, no treatments or vaccines for PDCoV are available. In this study, U.S. PDCoV strain OH-FD22 from intestinal contents of a diarrheic pig from Ohio was isolated in swine testicular (ST) and LLC porcine kidney (LLC-PK) cell cultures by using various medium additives. We also isolated PDCoV [OH-FD22(DC44) strain] in LLC-PK cells from intestinal contents of PDCoV OH-FD22 strain-inoculated gnotobiotic (Gn) pigs. Cell culture isolation and propagation were optimized, and the isolates were serially propagated in cell culture for >20 passages. The full-length S and N genes were sequenced to study PDCoV genetic changes after passage in Gn pigs and cell culture (passage 11 [P11] and P20). Genetically, the S and N genes of the PDCoV isolates were relatively stable during the first 20 passages in cell culture, with only 5 nucleotide changes, each corresponding to an amino acid change. The S and N genes of our sequenced strains were genetically closely related to each other and to other U. S. PDCoV strains, with the highest sequence similarity to South Korean strain KNU14-04. This is the first report describing cell culture isolation, serial propagation, and biological and genetic characterization of cell-adapted PDCoV strains. The information presented in this study is important for the development of diagnostic reagents, assays, and potential vaccines against emergent PDCoV strains.