Detection of Porcine Norovirus GII.18 Strains in Pigs Using Broadly Reactive RT-qPCR Assay for Genogroup II Noroviruses

Noroviruses, belonging to the family Caliciviridae, are classified into at least ten genogroups (G) based on their major capsid protein (VP1). The common genogroup to be identified in both humans and pigs is GII, although porcine noroviruses (PoNoVs) belong to genotypes of their own (GII.11, GII.18, and GII.19). So far, PoNoVs have not been studied much in Finland, possibly due to their rather symptomless nature in pigs. In the present study, we enrolled a total of 189 fecal samples collected from pigs from Finnish farms. Samples were taken from 12 farms in 2010, 2019 and 2020. We analyzed feces from growing pigs ranging from 2.1 to 6 months of age. RNA was extracted from fecal suspensions using a commercial viral RNA extraction kit, followed by RT (reverse transcription)-qPCR. The genotypes were determined by Sanger sequencing of the PCR fragments amplified by conventional PCR. Of the 12 farms, 6 (50%) had at least one PoNoV-infected pig. Altogether 18 (9.5%) of the 189 pigs tested positive for PoNoVs. Pigs mostly aged over 4 months were infected with PoNoVs. Eventually, 12 positive samples were determined as genotype GII.18. We could demonstrate the presence of PoNoVs in Finnish pigs. In future, more studies in which longer sequences from PoNoV genome can be obtained, are required.