Porcine epidemic diarrhea (PED) first appeared in the United Kingdom and Belgium in the early 1970s and then gradually spread to other European countries. After the 1990s, the disease showed sporadic local outbreaks in Europe. Porcine epidemic diarrhea virus (PEDV) has gradually become popular in Asian countries since it was first reported in Asia in 1982. In October 2010, the highly virulent variant PEDV broke out on a large scale and then spread across the country in China. Survey data showed that the detection rates of PEDV-positive samples ranged from 61.10–78.49%, while the rates of PEDV-positive pig farms ranged from 71.43–83.47%[5, 6]. Pigs of all ages are affected by PEDV, among which the mortality rate of suckling piglets is as high as 100% [4, 7, 8]. The high morbidity and mortality have brought huge economic losses to the pig industry.
PEDV is a member of the alpha coronavirus family with a particle approximately 95–190 nm in diameter and has an envelope, a genome length of approximately 28 kb containing the 5' noncoding region (5'-UTR), a single-stranded positive-stranded RNA and a cap structure and a poly A tail in the 3'-end noncoding region (3'-UTR) . The viral genome consists of a 5'-UTR, 3'-UTR and seven open reading frames (ORF1a, ORF1b, ORF2, ORF3, ORF4, ORF5 and ORF6). The replicase polyprotein 1ab (pp1ab) encoded by ORF1a and ORF1b is cleaved by papain (PL pro) and 3C-like protease (3CL pro) into 16 nonstructural proteins (Nsp1-16) [9, 10]; ORF2-6 encode ORF3 nonstructural proteins and four structural proteins, including spike protein (S protein), small membrane protein (E protein), membrane glycoprotein (M protein) and nucleocapsid protein (N protein) .
Sequence analysis of the whole PEDV genome showed that its hypervariable regions were mainly concentrated in the nsp2 gene, nsp3 gene, S gene, and ORF3 gene. Among them, the S gene was the most highly mutagenic. The PEDV S protein is composed of 1383 amino acids with a size of approximately 180–200 kDa. It is located on the surface of the coronavirus and contains a signal peptide, a neutralizing epitope, a transmembrane domain and a short intracellular region. The S gene of some coronaviruses contains a furin cleavage site, which can be cleaved into two subunits, S1 and S2, but the PEDV S protein cannot be naturally recognized due to the lack of this endoprotease recognition site. The PEDV S protein is divided into two domains according to its function: the N-terminal S1 subunit (aa 1-789) is responsible for recognizing cell surface receptors, and the C-terminal S2 subunit (aa 790–1383) is responsible for mediating virus and cell membrane fusion. When the virus invades the cell, the S1 protein binds to the surface of the host cell and induces the production of neutralizing antibodies; the S2 protein mediates the fusion of the virus with the host cell membrane, allowing the virus genome to enter the host cell. As the most mutagenic protein in PEDV, the S protein can change the pathogenicity and tissue tropism of the virus by gene insertion, deletion and mutation, so it is often used in epidemiological investigations.
Antibody detection tests can be used to evaluate the immune effect of vaccines and monitor the disease status. The current methods for detecting PEDV antibodies mainly include indirect immunofluorescence (IFA), serum neutralization tests (SVNs), enzyme-linked immunosorbent assays (ELISAs) and fluorescent microsphere immunoassays (FMIAs). Indirect immunofluorescence (IFA) is based on the antigen-antibody reaction and evaluates the immune status of the population by specifically detecting PEDV antibodies in the serum of suspicious populations . Zhu Weizheng and others collected pig serum samples at different time points after recovery, and the positive rate was approximately 89.25% by IFA. Song et al.  verified the reliability of IFA as the gold standard for serological testing by analyzing the correlation between IFA and ELISA. ELISA detection methods for PEDV antibodies mainly include indirect ELISA and blocking ELISA [19–21]. Wager et al. first used indirect ELISA to detect IgG antibodies in human serum albumin in 1978. Yorde et al. established a blocking ELISA detection method in 1976. The established ELISA detection methods for PEDV mainly focus on PEDV S protein, N protein and M protein. M. Knuchel et al. extracted viral proteins from Vero cells infected with PEDV, adjusted the solubility of the protein in the buffer by changing the pH value, enriched PEDV N protein and S protein, and established an ELISA detection system as a coating antigen . Studies have found that S-ELISA is more sensitive than N-ELISA, and antibodies against S protein last longer in the body than N protein .
Secretory immunoglobulin A (IgA), which can neutralize viruses and prevent viruses from invading the intestinal epithelium, plays an important role in intestinal mucosal immunity. A few ELISA methods could detect IgA antibodies against PEDV and be applied to clinical diagnosis or vaccine effect evaluation. However, there is no more information on the correlation between neutralization activity and ELISA detection based on recombinant S1 and its truncated proteins for clinical serum samples. In this study, we compared the sensitivity and specificity of ELISA detection of PEDV IgA with full-length S1 and its truncated proteins as coating antigens via ROC curves and analyzed the correlation between neutralization activity in vitro and PEDV IgA antibody levels measured by ELISA. The anti-PEDV IgA antibody levels detected by S1-ELISA or S1T2-ELISA can reflect the neutralization capability for pig serum.