Genomic Motifs as a Novel Indicator of the Relationship between Strains Isolated from the Epidemic of Porcine Epidemic Diarrhea in 2013-2014

Porcine epidemic diarrhea virus (PEDV) is a positive-sense RNA virus that causes infectious gastroenteritis in pigs. Following a PED outbreak that occurred in China in 2010, the disease was identified for the first time in the United States in April 2013, and was reported in many other countries worldwide from 2013 to 2014. As a novel approach to elucidate the epidemiological relationship between PEDV strains, we explored their genome sequences to identify the motifs that were shared within related strains. Of PED outbreaks reported in many countries during 2013–2014, 119 PEDV strains in Japan, USA, Canada, Mexico, Germany, and Korea were selected and used in this study. We developed a motif mining program, which aimed to identify a specific region of the genome that was exclusively shared by a group of PEDV strains. Eight motifs were identified (M1–M8) and they were observed in 41, 9, 18, 6, 10, 14, 2, and 2 strains, respectively. Motifs M1–M6 were shared by strains from more than two countries, and seemed to originate from one PEDV strain, Indiana12.83/USA/2013, among the 119 strains studied. BLAST search for motifs M1–M6 revealed that M3–M5 were almost identical to the strain ZMDZY identified in 2011 in China, while M1 and M2 were similar to other Chinese strains isolated in 2011–2012. Consequently, the PED outbreaks in these six countries may be closely related, and multiple transmissions of PEDV strains between these countries may have occurred during 2013–2014. Although tools such as phylogenetic tree analysis with whole genome sequences are increasingly applied to reveal the connection between isolates, its interpretation is sometimes inconclusive. Application of motifs as a tool to examine the whole genome sequences of causative agents will be more objective and will be an explicit indicator of their relationship.