Current Status of Porcine Epidemic Diarrhoea (PED) in European Pigs

Antas, Marta; Wozniakowski, Grzegorz

The coronaviruses are pleomorphic and have one of the largest RNA viral genomes and a unique replication strategy (28, 33) . Virions are spherical with diameter of approximately 125 nm. The most prominent and the defining feature of coronaviruses are the spike structures on the surface of the virion, which give them the appearance of a solar corona (5) . The PEDV genome is single-stranded RNA (28,000 nucleotides in length) of positive-sense polarity containing 3′ and 5′ untranslated regions (UTR) at both ends. Two thirds of the genome from the 5′ end encodes proteins necessary for RNA replication, while the one third on the 3′ side of the genome comprises at least seven open reading frames (ORFs) that encode four structural proteins: N (capsule), S (spike), E (envelope), and M (membrane), and three nonstructural proteins (replicases 1a and 1b and ORF3). In the genome, they are arranged in the order 5′-replicase (1a/1b)-S-ORF3-E-M-N-3′. The RNA genome of the virus is associated with the N protein to form a long, helical ribonucleoprotein (RNP) complex. The virus core is enclosed by a lipoprotein envelope, which contains the S, E, and M proteins ( Fig. 1) (1, 17, 28, 37) . The M protein is the most abundant component in the viral envelope, is necessary for the assembly process, and affects production of protective antibodies with virus-neutralising activity. The small E protein plays an important role during coronavirus budding. The N protein has multiple functions in viral replication, interacts with viral genomic RNA, and associates with other N proteins to protect the viral genome. It disturbs antiviral responses by antagonising interferon production (17, 31) . The S protein is encoded by a gene sequence which determines the virulence of a PEDV strain (35) . This protein is composed of 1,383 amino acids (aa), significant among which are a signal peptide (1-18 aa), neutralizing epitopes (499-638, 748-755, 764-771, and 1,368-1,374 aa), a transmembrane domain (1,334-1,356 aa), and a short cytoplasmic domain. It consists of the S1 (1-789 aa) and S2 (790-1,383 aa) domains, which are responsible for binding and fusion of the virus. The S1 domain is involved in a specific interaction with the cellular receptor and induction of neutralising antibodies (14, 28) . Only one serotype of PEDV has been reported. The phylogenetic analysis of the S gene indicates that PEDV could be genetically divided into two groups, these being the classical genotype 1 (GI) and the field epidemic or pandemic genotype 2 (GII). The early European CV777 strains (GenBank accession number AF353511), vaccine strains, and strains adapted to cell cultures were classified to genotype 1, while genotype 2 includes strains responsible for the epidemics in the USA and Asia. Strains identified in the USA have been (17, 18, 25, 31) . All PEDV pandemic strains circulating in China after 2014 were also clustered to genotype 2, and they were genetically separated from other global PEDV strains and from earlier Chinese strains (29) .