Introduction
SADS-CoV, also name PEAV (Gong et al.,
2017) and SeACoV (Y. Pan et al., 2017),
as the newest strain of porcine coronaviruses (CoVs), was first detected
in pig herds with diarrhea-outbreak in Guangdong, China in 2017
(P. Zhou et al., 2018). SADS-CoV belongs
to the genus Alphacoronavirus of the family Coronaviridae
(P. Zhou et al., 2018), which also
includes Transmissible gastroenteritis virus (TGEV)
(Doyle & Hutchings, 1946), Porcine
epidemic diarrhea virus (PEDV) (Pensaert
& de Bouck, 1978) and Porcine deltacoronavirus (PDCoV)
(Woo et al., 2012). SADS-CoV genome is an
enveloped, single-stranded, positive-sense RNA virus, which is
approximately 27 kb in length, and arranged in the order of: 5’
UTR-ORF1a/1b-S-NS3-E-M-N-NS7a-3’ UTR, encoding 16 non-structural
proteins and 4 structural proteins (Gong et
al., 2017; Y. Pan et al., 2017;
P. Zhou et al., 2018). SADS-CoV caused
clinical symptoms similar to other porcine enteric pathogens, such as
TGEV, PEDV and PDCoV, characterized by vomiting, diarrhea, dehydration,
and a mortality rate as high as 90% in piglets
(Xu et al., 2019;
P. Zhou et al., 2018). Since SADS-CoV was
first reported in southern China in 2017
(Gong et al., 2017), the retrospective
investigation of 236 samples from 45 swine farms showed that the
prevalence of PEAV in pigs was reported to be 43.53% in Guangdong,
China in August 2016 to May 2017 (L. Zhou,
Sun, et al., 2019). And yet from May 2017 to January 2019, there were
no new SADS cases reported in pig herds in Guangdong, but the
re-emerging of SADS-CoV infection in pig herds in Guangdong on February
2019 (L. Zhou, Li, et al., 2019),
indicating a continuing threat to pig farms. Apart from Guangdong, a
SADS-CoV-like strain, CN/FJWT/2018, was recently discovered in Fujian,
China (K. Li et al., 2018). Recently, it
was reported that SADS-CoV could infect human cell lines
(Edwards et al., 2020;
Yang et al., 2019) and in view of the
great harm of SARS-CoV2 (Hu, Guo, Zhou, &
Shi, 2020), which same cross-species infection likely originate from
bats as SADS-CoV (Hu et al., 2020;
P. Zhou et al., 2018), reveal that the
monitoring of epidemic situation of SADS-CoV in pig herds might have
important public health significance.
Since first detection in swine herds in 2017,
multiple detection methods
including real time reverse transcription loop-mediated isothermal
amplification method (RT-LAMP) (H. Wang et
al., 2018), SYBR green-based real-time PCR
(Ma et al., 2019), multiplexTaq Man-probe-based real-time PCR
(Z. Pan et al., 2020)),
microfluidic-RT-LAMP chip (L. Zhou et al.,
2020) are developed and widely used to detect SADS-CoV infection.
However, these detection methods only target viral nucleic acid,
serological assays for SADS-CoV to undertake epidemiological studies are
not available to date. Enzyme-linked immunosorbent assay (ELISA), which
mainly including indirect ELISA, double-antibody sandwich ELISA and
blocking ELISA, has strong specificity and high sensitivity and is
widely used in pathogens antibodies detection and vaccines evaluation
(Knuchel, Ackermann, Muller, & Kihm,
1992). The basic principle of ELISA is the specific response between
antigen and antibody. Therefore, for pathogen specific antibody
detection, it is critical to select the appropriate detect antigen. S
protein of CoVs belong to type I membrane fusion protein, locate on the
envelope of the virus particle (Yu, Qiao,
Guo, & Wang, 2020). It is known that the S protein, which can be
divided into domain 1 (S1) and domain 2 (S2), plays an important role in
virus attachment and entry, and induction of neutralizing antibodiesin vivo (Wen et al., 2018). In
addition, the S protein has higher antigenicity than any of the other
CoVs’ proteins, and anti-S antibodies persist longer than anti-
nucleocapsid (N) antibodies in vivo(Knuchel et al., 1992). Therefore, Many
studies have selected the whole or part of S protein as a coating
antigen to develop indirect ELISA methods for the detection of TGEV
(Sestak, Zhou, Shoup, & Saif, 1999),
PEDV (Lin et al., 2018) and PDCoV
(Lu et al., 2020) antibodies. Although
SADS-CoV has the smallest S protein of only 1130 amino acids
(Gong et al., 2017) compared to other
reported CoVs, the structural features of SADS-CoV S protein are
evolutionarily related to other CoVs genera
(Guan et al., 2020), indicating that S
protein of SADS-CoV can also be used as detect antigen.
SADS-CoV is an important enteropathogen in pigs, but currently no
suitable serology assays to detect the specific antibodies for SADS-CoV.
The objective of this study was to develop an anti-SADS-CoV IgG indirect
ELISA based on recombinant S protein and utilize this ELISA to
investigate the prevalence rates of SADS-CoV in the pig farms of China,
which might provide a tool for advancing understanding of the
epidemiology of SADS-CoV.