Porcine epidemic diarrhea virus: an update overview of virus epidemiology, vaccines, and control strategies in South Korea

PEDV structure and genome

PEDV belongs to the subgenus Pedecovirus of the genus Alphacoronavirus within the family Coronaviridae of the order Nidovirales [28]. PEDV is enveloped and roughly spherical or pleomorphic with a diameter of 95–190 nm, including the club-shaped, trimerized projections that measure 18–23 nm in length (Fig. 2A) [29]. A detailed description of the PEDV genome and virion structure can be found elsewhere [5, 6]. The virus has a single-stranded, positive-sense RNA genome of approximately 28 kb, and the ends of the genome contain 5′-cap and 3′-polyadenylated tail structures. The PEDV genome comprises seven canonical coronaviral genes, including open reading frame (ORF) 3, arranged in the order 5′ untranslated region (UTR)-ORF1a-ORF1b-S-ORF3-E-M-N-3′ UTR (Fig. 2B) [5, 6, 30]. ORF1a and 1b encompass the 5′-proximal two-thirds of the genome and encode 16 nonstructural proteins (nsps). The remaining ORFs in the 3′-proximal region of the genome encode four canonical coronaviral structural proteins and a single accessory gene, ORF3. The four structural proteins include three envelope-associated 150–220 kDa glycosylated spike (S), 20–30 kDa membrane (M), and 7 kDa envelope (E) proteins and the 58 kDa nucleocapsid (N) protein that encapsulates the genome to form a long and helical coil structure [5, 6, 30, 31, 32].

Fig. 2
Schematic representation of PEDV structure and genome organization. (A) Model of PEDV structure. The PEDV virion structure is illustrated on the left. The RNA genome inside the virion interacts with the nucleocapsid (N) protein to form a long, helical ribonucleoprotein (RNP) complex that is surrounded by a lipid bilayer envelope where the spike (S), envelope (E), and membrane (M) proteins are embedded; the predicted molecular sizes of each structural protein are indicated in parentheses. A set of corresponding subgenomic mRNAs (sg mRNAs; 2–6), through which canonical structural proteins or nonstructural ORF3 protein are exclusively expressed via a coterminal discontinuous transcription strategy, are also depicted on the right. (B) The structure of PEDV genomic RNA. The 5′-capped and 3′-polyadenylated genome of approximately 28 kb is shown at the top. The viral genome is flanked by UTRs and is polycistronic, harboring replicase ORFs 1a and 1b followed by the genes encoding the envelope (S, E, and M), N, and accessory ORF3 proteins. ORF1a and 1b expression yields two known polyproteins (pp1a and pp1ab) by a −1 programmed RFS; these polyproteins are cotranslationally or posttranslationally processed into at least 16 distinct nonstructural proteins designated nsp1–16 (bottom).
UTR, untranslated region; ORF, open reading frame; RFS, ribosomal frameshift; A(n), polyadenylated tail; PLP, papain-like cysteine protease; 3CL^pro, the main 3C-like cysteine protease; RdRp; RNA-dependent RNA polymerase; HEL, helicase; ExoN, 3′→5′ exonuclease; NendoU, nidovirus uridylate-specific endoribonuclease; 2′OMT, ribose-2′-O-methyltransferase; PEDV, porcine epidemic diarrhea virus.

Adapted from Lee [5, 6].

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PEDV genotypes

As with other CoV S proteins, the PEDV S glycoprotein (consisting of S1 and S2 subunits) plays pivotal roles in infection by interacting with the cellular receptor to mediate viral entry and by inducing neutralizing antibodies [5, 6]. Genetic mutations, including insertions (IN) and deletions (DEL), and recombination in the S gene can drive alterations in viral pathogenicity and tissue or species tropism [33, 34, 35, 36, 37, 38, 39, 40]. Considering the phenotypic and genotypic traits, the S gene is a suitable locus for sequencing to investigate the genetic relatedness (i.e., genotyping) and molecular epidemiology of PEDV [23, 27, 41, 42, 43, 44, 45, 46, 47]. Thus, using S gene-based phylogeny, PEDV can be genetically separated into two main genotypes with two sub-genotypes: low pathogenic (LP)-G1 (classical G1a and recombinant G1b) and HP-G2 (local epidemic G2a and global epidemic or pandemic G2b) [5, 6]. G1a strains include the prototype CV777 and several tissue culture-adapted viruses, whereas G1b represents novel recombinant variants that were first reported in China [11] and then identified in the US, South Korea, and multiple European countries [46, 48, 49, 50, 51]. G1b strains arise from the natural homologous recombination of a minor G1a virus and a major parental G2b virus. G2 includes recent field isolates that are clustered into two subgroups: G2a, which was responsible for the past and current regional epidemics in Asia and G2b, which includes the contemporary dominant strains causing the 2013–2014 pandemic and current outbreaks in the American and Asian continents.

PEDV life cycle

PEDV shows restricted tissue tropism, consistent with other CoVs, and replicates mainly in porcine small intestinal villous epithelial cells or enterocytes. Although porcine aminopeptidase N (pAPN) was long considered to be the putative cellular receptor for PEDV [52, 53, 54, 55, 56], this is now known not to be the case, indicating the presence of an authentic cellular receptor involved in viral entry [57, 58, 59]. In addition, the binding to cell surface-exposed sialic acids plays a role in PEDV infection by facilitating the initial virus attachment to cellular receptors [33, 60]. Nevertheless, PEDV replication begins with binding to the hitherto-unidentified surface receptor of villous epithelial cells of the small intestine via the S protein, followed by virus internalization into target cells by either pH-independent fusion (between viral and plasma membranes) or pH-dependent fusion (between viral and endosomal membranes) following endocytosis (Fig. 3) [5, 6, 61]. The uncoated viral genome is released into the cytosol to start viral mRNA biosynthesis and genome replication through discontinuous synthesis [5, 6, 62]. ORF1a and 1b are immediately translated to replicase polyproteins, pp1a and pp1ab: the former is produced through the initial ORF1a translation, while the latter is expressed from ORF1b translation depending on a −1 ribosomal frameshift that C-terminally extends pp1a into pp1ab. These pp1a and pp1ab are proteolytically matured by internal viral proteases to generate 16 processing end products, named nsp1–16, which comprise the replication and transcription complex that first engages in the negative-strand RNA synthesis using the positive-strand genomic RNA. Both full- and subgenomic (sg)-length negative strands are produced and used to synthesize full-length genomic RNA and 3′-coterminal sg mRNAs. Each sg mRNA is translated into only the protein encoded by the 5′-most ORF of the sg mRNA. The envelope S, E, and M proteins insert into the endoplasmic reticulum (ER) and anchor in the Golgi apparatus. The N protein interacts with newly synthesized genomic RNA to form helical ribonucleoprotein (RNP) complexes. Progeny viruses are assembled by budding of the preformed helical RNP at the ER-Golgi intermediate compartment and then released by the exocytosis-like fusion of smooth-walled, virion-containing vesicles with the plasma membrane [5, 6]. Consequently, PEDV infection destroys target enterocytes and damages the intestinal epithelium, causing severe villous atrophy and vacuolation. These clinical outcomes interfere with efficient digestion and absorption of milk or water, resulting in acute maldigestive and malabsorptive watery diarrhea that eventually produces fatal dehydration in nursing neonates (Fig. 3).

Fig. 3
Overview of the PEDV replication cycle and pathogenesis. The left illustration (left panel) shows viral development from initial entry (pH-dependent or -independent fusion) and release of the viral genome to exocytosis of the mature virion (fusion of smooth-walled, virion-containing vesicles with the plasma membrane). The right diagram describes how PEDV causes maldigestive and malabsorptive watery diarrhea, ultimately producing fatal dehydration.
RTC, replication and transcription complex; ER, endoplasmic reticulum; ERGIC, ER-Golgi intermediate compartment; PEDV, porcine epidemic diarrhea virus.

Adapted from Lee [5, 6].

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PEDV transmission

PEDV can infect pigs of any ages; however, disease severity and mortality are inversely related to the age of the infected animals. The virus is highly transmissible and fatally affects neonatal piglets within their first week of life, reaching morbidity and mortality up to 100% [5, 6]. The oral minimum infectious dose (0.056 median tissue culture infectious dose [TCID₅₀]/mL) of HP-G2b PEDV required to infect 5-day-old nursing pigs was much lower than 3-week-old weaned pigs (56 TCID₅₀/mL) [63]. In weaner and finisher pigs, including gilts and sows, clinical signs are self-limiting within the first week after onset of disease and are not as severe as those of nursing piglets under 2 weeks of age [5, 6]. Onset and duration of diarrhea and fecal virus shedding in pigs of different ages experimentally infected with PEDV are summarized in Table 1.

Table 1
Summary of clinical signs and fecal viral RNA shedding in conventional pigs of different ages experimentally infected with HP-G2b PEDV strains via oral inoculation

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The major means of PEDV transmission is the fecal-oral route via direct or indirect contact with clinically or subclinically infected pigs or diarrheal feces and/or vomitus (Fig. 4). PEDV can invade farms through contact with contaminated equipment, vehicles (transporting pigs or carcasses, delivering feed, or hauling manure), humans (farm employees or visitors, including swine practitioners or trailer drivers, wearing contaminated work clothing and footwear), or wild animals, including birds, stray cats, and mice [5, 6, 64]. Other contaminated fomites, such as feed, feed items, or feed additives or ingredients (e.g., spray-dried porcine plasma), can be potential transmission sources of the virus [20, 65, 66]. PEDV can also be transmitted vertically through milk from the sow to its offspring [67, 68]. PEDV shedding in semen also occurs, suggesting virus spread within the pig population via the use of contaminated semen [69, 70]. In addition, airborne transmission can occur by the fecal-nasal route via aerosolized PEDV particles that are infectious in nursing piglets under certain conditions [71].

Fig. 4
Porcine epidemic diarrhea virus transmission sources and routes in epidemic and endemic PED cases. L/K/K, heterologous prime-boost immunization regime with one prime dose of a G2b oral live (L) vaccine and two booster doses of a G2b killed (K) vaccine.
PED, porcine epidemic diarrhea.

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Following epidemic PED, the virus may fade, remain in the farrowing unit, or persist endemically in wean-to-finish (WTF) sites because of inadequate farm management (e.g., improper disinfection and lax biosecurity) (Fig. 4). In farrow-to-finish (FTF) farms, PEDV can spread to and contaminate WTF facilities if infected but surviving piglets move to the nursery and growing barns; this can generate endemic PED where the virus continues to circulate in the affected herd through a contamination-transmission-infection cycle. Upon endemic PED, if newborn piglets cannot obtain sufficient levels of maternal protective immunity from their dams for several reasons, such as inappropriate vaccination, sow replacement, or defective lactation (e.g., mastitis or agalactia), the resident virus circulating within the farm will infect these immunodeficient susceptible piglets and multiply with shedding of large amounts of the virus, which serves as the source of recurrence and ultimately produces a marked increase in neonatal piglet mortality [5, 6, 72, 73]. PEDV infection is asymptomatic in weaned and growing-finishing pigs; however, the virus can be shed in their feces, contaminate their barns, and replicate in a subclinical manner. Therefore, PEDV can continue to subclinically infect growing and finishing pigs in endemically affected farms through the contamination-transmission-infection cycle, which increases the opportunity for gilts to encounter the virus during their acclimation period in contaminated barns [74, 75]. Once infected, asymptomatic infected gilts can serve as “Trojan Pigs” that present no clinical signs but carry the virus in their feces, which will become invisible but invincible carriers that transmit the virus to the farrowing house. Thus, PEDV transferred by “Trojan Pigs” can spread to and infect vulnerable newborn piglets, thereby acting as the source of a recurrent outbreak that affects nursing pigs with low or no passive immunity in a clinical manner [74, 75]. Therefore, gilt management program focusing on infection monitoring and vaccination is a critical practice in the control of PEDV in endemically infected herds.

Genotype shift from classical G1a to G2a PEDV

A retrospective study suggested the existence of PEDV in South Korea as early as 1987 [76], albeit the domestic emergence of G1a PEDV was first reported in 1992 [8]. Since then, the virus has become endemic in the domestic swine population as PEDV circulation (i.e., seropositive weaned and growing-finishing pigs) was detected in > 90% of farms tested in 2007 [77]. However, a genetic diversity study using domestic isolates conducted in 2010 revealed that the dominant PEDV isolates identified during 2007–2009 were classified as G2a strains that were phylogenetically distantly related to the G1a subgroup [42]. G2a PEDV strains contain a genetic signature, namely S insertions-deletions (S INDEL) comprising tripartite discontinuous 4-1-2 IN-IN-DEL at positions 55/56, 135/136, and 160–161, respectively, compared with the G1a prototype CV777 strain [42]. The lack of genetic and molecular epidemiology data before 2010 in South Korea means that it is unknown when or how G2a strains emerged. Nevertheless, G1a PEDV appears to have been the initial dominant genotype during the 1990s or until the early 2000s, and subsequently, a genotype shift from G1a to G2a may have occurred in the mid and late 2000s. This genotype shift event may mirror the domestic circumstances whereby repeated PEDV outbreaks in vaccinated farms occurred, raising questions about the protective efficacy of PED vaccines based on G1a domestic isolates marketed since the early 2000s [5, 6].

Emergence and evolution of HP-G2b PEDV

The status of PEDV outbreaks in South Korea dramatically altered in 2010–2011 when catastrophic foot-and-mouth disease (FMD) emerged nationwide, resulting in the mass culling of > 3 million pigs (one-third of the entire domestic pig population) over two years [23]. The PEDV prevalence was infrequent, with only sporadic outbreaks in South Korea during the 2010–2011 FMD outbreaks, and a lull in PEDV epidemics continued until late 2013 [5, 6]. However, South Korea did not escape from the 2013–2014 pandemic of HP-G2b PEDV that began in the US and disastrously underwent explosive severe outbreaks nationwide starting from November 2013 [23]. HP-G2b PEDV swept through almost 50% of the pig farms across mainland South Korea, and 4 months thereafter (March 2014), the virus hit the Jeju Province, also known as Jeju Island, that had maintained a PEDV-free status since 2004 [23, 27]. As a result of the calamitous impact of HP-G2b PEDV incursion, the South Korean pig industry was assumed to have lost > 10% of its domestic pig population during the 2013–2014 epidemic period, amounting to an estimated one million piglets. Although the source of HP-G2b PEDV invasion into the Korean Peninsula has not been determined, the import of pig breeding stock or feed during or after the unparalleled emergence of PEDV in the US in April 2013 could be a possibility. However, retrospective studies independently identified two South Korean HP-G2b isolates with the S INDEL genetic signature from diarrheic samples collected in November 2012 [78] and May 2013 [23], suggesting the emergence of the second genotype shift from G2a to G2b in the early 2010s. Hence, HP-G2b PEDV may have been present in South Korea as a minor lineage even before the PEDV introduction into the US, and subsequent favorable circumstances might have contributed to the dominance of G2b strains [5, 6].

Afterward, HP-G2b PEDV infections have remained uncontrolled until now, leading to year-round small- to large-scale outbreaks nationwide with considerable economic costs in the domestic pig industry (Fig. 5). Furthermore, a large number of PED-affected pig farms have experienced recurrent outbreaks within one or two years. This situation indicates that endemic persistence of resident PEDV has become established on farms, thereby making disease control difficult and aggravating financial losses [74, 75]. G2b has continued to independently evolve and undergo genetic diversity with specific geographic clustering, ranging from 96.3% to 99.4% amino acid (aa) identity with an HP-G2b South Korean prototype strain (KNU-141112). In particular, some domestic HP-G2b strains (except for one isolate with a 200-aa DEL in the S1/S2 junction region [79]) possessed small IN or/and DEL in the N-terminal domain (NTD) of S1 [13, 44, 45, 47], which differed from G2b variants with large 194–216-aa DELs within the S1 NTD reported in Japan, Taiwan, and the US [25, 37, 80, 81, 82, 83, 84]. The domestic isolates are phylogenetically clustered into six disjunct clades according to the geographic origin, that includes nationwide (NW clade with 1.3%–1.9% aa variation), Kyeongnam and Jeonnam Provinces (KJ clade with 2.2%–2.9% aa variation), Chungcheong and Kyeongbuk Provinces (CK clade with 0.6%–1.7% aa variation), Jeju Hallim (JH clade with 1.8%–2.6% aa variation), Jeju Daejeong (JD clade with 1.6%–2.7% aa variation), and not geographically classified (NC clade with 0.9%–3.7% aa variation) (Fig. 6A). Two Jeju clades, JH and JD, have been commonly circulating in the corresponding geographic origins, the Hallim and Daejeong areas of Jeju Island (Province), respectively, but have not been found in mainland South Korea. Similarly, the mainland clades, NW, KJ, and CK, did not cross into Jeju Island until early 2022, even though they have moved across different geographic origins in the mainland. During late February to early March 2022, large-scale outbreaks of PEDV simultaneously occurred in southern South Korea and Jeju Island. Genetic and phylogenetic analysis confirmed the emergence of two CK variants, namely CK.1 (0.9%–1.1% aa variation) and CK.2 1.1%–1.4% aa variation) clades (Fig. 6B). Further spatiotemporal investigations suggested that the CK.1 and CK.2 clades that first emerged in the Kyeongnam area (the southeast region of the Korean peninsula) around early 2021 were almost simultaneously introduced into the northeast areas of Jeju Island that have a low pig population density via an unidentified source(s) and subsequently spread to the Hallim district (northwest Jeju Island) that has a high pig density. Since then, these CK.1 and CK.2 clades that includes the epidemic strains with the lowest variation compared with KNU-141112 has now become dominant on Jeju Island and further expanded its distribution throughout the mainland.

Fig. 5
The number of PED cases in South Korea from 2013 to 2023 (until June). (A) Number of PED occurrences per year since 2013. Heat maps (upper panel) of nine provinces (including Jeju Island located southwest of the mainland) in South Korea illustrate the nationwide distribution of PED cases per province by year (2013–June 2023) with the legend from red to white, representing most to least. A thick (azure) arrow (lower panel) represents the timeline indicating release years of G2b porcine epidemic diarrhea virus vaccines in the domestic market. (B) Number of PED cumulative cases per month since 2013. The line graph indicates the cumulative number of cases by month, demonstrating a phenomenon where seasonal PED (usually occurred from November to March annually) has changed to the year-round incidence of PED.
KV, killed vaccine; LAV, live attenuated vaccine; PED, porcine epidemic diarrhea.

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Fig. 6
Phylogenetic analysis based on the full S gene of the PEDV strains globally identified before (A) and after (B) 2022. Four genotypes, G1a (red), G1b (blue), G2a (green), and G2b (purple), are indicated. Different colored triangles on the blue (G1b) branches indicate the recombinant LP-G1b PEDV strains identified in South Korea. Dark orange triangles indicate 2018–2019 G1b strains, indigo triangles indicate the 2017 G1b strains, and a green triangle indicates the emergent Korean G1b prototype strain identified in 2014. Different colored dots on the purple (G2b) branches indicate the HP-G2b PEDV strains identified nationwide in South Korea that were clustered into six geographic disjunct clades (before 2022) and contemporarily into eight clades with two subclades (after 2022): NW clade (orange), KJ (dark red), CK (light green), JH (sky blue), JD (neon), and not geographically classified NC (purple) clades. Two CK variants, namely CK.1 and CK.2 clades, responsible for large-scale outbreaks in 2022 are indicated by red and blue dots, respectively. A black dot indicates the HP-G2b Korean prototype strain (KNU-141112).
PEDV, porcine epidemic diarrhea virus.

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Emergence and evolution of LP-G1b PEDV

Novel recombinant LP-G1b PEDV isolates, which had been previously reported in China [11] and the US [16, 51], were first reported in South Korea in March 2014 [46] and later, in several European countries [48, 49, 50, 51, 85, 86, 87]. The G1b PEDV strains originated from a recombination event between G1a as the minor parent and G2b as the major parent around the early 2010s [11, 46, 51]. Thus, the S genes possess typical genetic and phylogenetic features: identical size and no S INDELs compared with the classical G1a CV777 strain and a different phylogenetic classification (G1b or G2) based on the S gene or whole genome [5, 6]. Although we cannot exclude the probability of the existence of the G1b virus in South Korea before its first identification, the close genetic relatedness between the US and South Korean G1b strains indicates that each G1b and G2b ancestor could have been simultaneously introduced into South Korea from the US [5, 6, 46]. Unlike the dominant HP-G2b strains, the LP-G1b virus infrequently causes small-scale local, economically insignificant outbreaks across the mainland in recent years [45]. Interestingly, the inter-subgroup G1b variants emerged in 2017 by recombining the NTD of S1 from existing G1b (minor) into the backbone of G2b (major) circulating in the domestic field, followed by genetic drift (Fig. 6) [41]. This study further indicated that the NTD of the S1 gene is a common target for natural recombination between different PEDV genotypes, thereby affording several advantages that may allow the virus to evade host immune defenses [41].

Future direction (genotype shift or variant emergence)

In South Korea, the G1b and G2b strains have been circulating for the past decade: the former has evolved through recombination in the host environment, whereas the latter, responsible for most domestic outbreaks, continues to evolve through genetic mutations, such as IN or/and DEL in the S1 NTD [13, 44, 45, 47]. Since the virus is assumed to undergo an evolutionary process to accumulate mutations or/and recombination to ensure viral fitness in the field, the advent of new genotype shift or the emergence of variants likely arising from domestic clades will be inevitable. Moreover, these circumstances may arrive earlier than expected, creating local or global outbreaks. Therefore, performing active monitoring and surveillance (MoS) is critical to hunt hitherto-unidentified PEDV variants (or another genotype) with distinct antigenic and pathogenic properties that may emerge locally or globally through genetic drift (e.g., nonsilent mutations, including IN or/and DEL) or genetic shift (e.g., recombination events) to predict and prepare for the next epidemics or pandemics [5, 6, 45].

G1a (1st generation) PEDV vaccines

As the large-scale occurrence of PEDV was repeated yearly during the 1990s, the South Korean government concentrated intensive efforts on developing PEDV vaccines. The first PEDV inactivated vaccine was developed using the South Korean G1a PEDV strain SM98-1 and was launched in the domestic market in 2004. Subsequently, two South Korean G1a PEDV strains, SM98-1 and DR-13, were independently attenuated through serial passages in Vero cell culture: the former was commercialized as a parenteral live attenuated vaccine (LAV), whereas the latter was available as an oral LAV [101, 102]. In addition, a parenteral LAV (P-5V) using the cell culture-attenuated Japanese G1a 83P-5 was imported and marketed in South Korea [103]. These domestic and imported G1a vaccines provided protection to piglets in independent experimental trials, and their nationwide implementation led to a decline in the prevalence of PEDV compared with that in previous years [5]. However, despite the nationwide vaccination campaign, PEDV persistence in vaccinated herds instigated a debate on issues of their effectiveness and the pros and cons of their use in the field. Finally, the advent of HP-G2b PEDV outbreaks in South Korea ended the debate concerning the efficacy of the aforementioned G1a PEDV vaccines; the first generation G1a vaccines are now barely employed in South Korea because they offer only partial protection against the heterologous G2b PEDV strains.

G2b (2nd generation) PEDV inactivated killed vaccines

The inadequate effectiveness of the G1a PEDV vaccines was anticipated to some extent since field strains dominant in South Korea after the 2013–2014 pandemic showed genetic and antigenic variations compared with the existing G1a vaccine [5, 6, 23, 42, 78, 104]. Therefore, G2b PEDV strains were considered as the seeds for developing next-generation vaccines. Although G2b PEDV vaccine development was hindered because of the difficulty in obtaining a field isolate in cell culture, three types of the 2nd generation inactivated killed vaccine using different G2b PEDV isolates (one imported US strain and two South Korean strains) were independently marketed by domestic vaccine manufacturers in late 2015 and 2017. Despite the supply of new G2b inactivated vaccines, local PEDV outbreaks proved unstoppable, and the number of PEDV occurrences grew rather than declined in 2018, with the highest number of PED cases officially reported nationwide since 2014 (https://www.kahis.go.kr/home/lkntscrinfo/selectLkntsOccrrnc.do) (Fig. 5). This conflicting status may mirror the limitation of a homologous prefarrow prime-boost immunization regime with two or three doses of the parenteral G2b killed vaccine alone (i.e., K/K or K/K/K) in fulfilling the aforementioned core strategies (i.e., the induction and maintenance of protective mucosal immunity) and in reducing disease morbidity (i.e., PEDV fecal shedding) [98]. During this period, to complement the drawbacks of this homologous K/K/(K) regime, G1a (1st generation) oral or parenteral LAV or oral intentional exposure (feedback) was used to domestically implement the traditional heterologous L/K/K strategy. However, because of a deep mistrust of G1a vaccines, a modified heterologous prime-boost regime with feedback and the G2b inactivated vaccine (i.e., feedback/K/K) or feedback alone was mostly adopted as the alternative to control PED. Although oral exposure to autogenous PEDV (i.e., feces or minced gut tissues collected from PEDV-infected neonatal piglets) may offer benefits, this may fail to induce complete maternal lactogenic immunity and to prevent PEDV shedding due to the inaccurate amount and heterogeneity of infectious PEDV in autogenous viral materials [75]. Furthermore, before accepting this approach, we should contemplate that if uncontrolled, this can serve as a potential source for virus transmission to nearby pig farms or endemic PED in the affected farm, for the extensive dissemination of other viral and bacterial pathogens that exist in the fecal or intestinal contents, for the accelerated evolution and diversification of PEDV, and for the advent of PEDV variants or recombinant novel CoVs that can cross the species barrier to infect other animals or humans [5, 6, 105, 106].

G2b (2nd generation) PEDV oral live vaccine

Current knowledge remains limited in understanding the factors that influence the induction of lactogenic immunity, such as administration dose, vaccine strain, and the age or parity of the gilt/sow, as well as other variables [6]. However, multiple scientific evidence highlights the oral route of prime immunization for the effectual priming of the gut with a live vaccine or through controlled feedback to enhance and sustain the sow mucosal immunity to confer local passive protection to the offspring [18, 107, 108]. The development of a new oral LAV using the HP-G2b strain was therefore required to overcome the defect in G1a live vaccines and to replace the indiscriminate feedback practice. With coordinated research and development efforts, the world’s first G2b oral LAV was launched in late 2020 and was demonstrated to be safe and immunogenic under experimental and field conditions [74, 108]. Moreover, a heterologous prefarrow G2b L/K/K regime combining G2b oral live and parenteral killed vaccines provided full passive lactogenic protection against HP-G2b PEDV in suckling neonates by accomplishing the aforementioned core strategies for PEDV vaccination [108]. However, to maximize vaccine effectiveness, the prefarrow oral L/K/K scheme will be the most favorable practical tool for the prevention and/or control of PEDV if the vaccination is combined with other control measures that include stringent biosecurity/disinfection practices, regular molecular (virus) and serological (host) monitoring, and optimal farm husbandry and sanitation management.

Future direction (rapid-responsive and applicable vaccine platform)

Despite the genetic diversity of HP-G2b strains in South Korea (eight possible clades including two subclades), their antigenicity has remained unchanged, implying that G2b vaccines remain effective against homologous dominant strains responsible for recent outbreaks [45]. However, new genotypes or variants of PEDV that can escape from G2b vaccine efficacy will inevitably emerge [109], and this may require the replacement of the current G2b vaccines. Considering the timeline for the domestic market launch of new G2b-based inactivated vaccines (late 2015 and 2017) and LAV (late 2020), challenges in the fastidious and laborious processes of virus isolation and attenuation should be surmounted for the rapid development of new vaccines against prospective variants. These hindrances can now be overcome by using a reverse genetics-based platform for PEDV. This vaccine platform proved to be a rational backbone to carry the heterologous S gene of any future epidemic strains and can serve as a template for the prompt generation of new vaccines and pave the way for future PEDV vaccine research [109, 110].

PEDV risk assessment

Once introduced into pig populations, PEDV will likely persist endemically in FTF farms, and consequently, the endemic nature of the disease has made PED control more challenging in swine FTF operating systems. Furthermore, eliminating endemic PED from the FTF herds is a complex task because of the continual flow nature of pigs and the level of environmental contamination [74, 75]. Thus, to prevent repetitive PEDV infection in the affected farms, it is necessary to monitor four-pillar factors in PED control strategies, which include: 1) biosecurity; 2) vaccination; 3) rapid diagnostics and active MoS; and 4) herd management (husbandry and sanitation) [5, 6, 72, 74, 75]. When endemic PED is suspected, coordinated four-pillar-based control measures should be implemented to break the chain of disease transmission at the farm level. This involves: 1) tightening biosecurity protocols and disinfection practices; 2) testing herds for virus and serological monitoring to check the immunity level in the farrowing herd and to identify growing or finishing pigs with exposure to resident PEDV; 3) vaccinating sows and gilts if necessary; and 4) improving farm management [5, 6, 74, 75]. To strengthen these strategies, a PED risk assessment was established to determine whether the farm is endemically affected and faces a risk of recurrence (or occurrence in PEDV-naïve farms) and to provide customized intervention measures in accordance with the farm condition [74, 75]. Thus, the goal of conducting this farm assessment is to gather information regarding the virus, herd (host), and farm (environment) by: 1) subjective biosecurity monitoring to understand how well the producer or staff follows biosecurity protocols; 2) rapid virus screening, including genetic diagnostics, upon an outbreak, to obtain information on whether the virus was newly introduced from outside the farm or had persisted inside the farm after the initial outbreak; 3) serological screening in farrowing herds to obtain information concerning herd immunity in sows and their offspring; and 4) virus and serological screening in WTF herds to obtain information on virus circulation within the farm [74, 75]. Individual swine farms are risk assessed to evaluate their own grades of biosecurity performance, herd immunity, and virus circulation as follows:

2) Herd immunity evaluation

Since passive lactogenic immunity is a key protection strategy for newborn piglets against PEDV, determining the presence or absence of PEDV-specific antibodies in sow herds would not be significant [5, 6]. Instead, measuring the stability and quantity (or titers) of PEDV-specific antibodies in sows is necessary to monitor the level of sow herd immunity that confers passive protection to neonatal piglets through lactation. Therefore, S1-based indirect enzyme-linked immunosorbent assay (S1-iELISA) and/or virus neutralization test (VNT) methods are recommended for assessing the protective capacity of IgA and neutralizing antibodies in lactogenic secretions as well as the neutralizing antibody (NAb) titers in sera of sows (both low- and high parities) and suckling piglets (18–20-day-old) [5, 6, 74, 75, 108]. The S1-iELSIA measures the stability of IgA antibodies in colostrum and milk, while the VNT quantifies the stability of NAb in lactogenic secretion and serum samples. The level of herd immunity is estimated based on the stability profile of colostral IgA and NAb in sows and serum NAb in sows and suckling piglets with a 0–5 scoring scale (0 = worst; 5 = best) (Table 2).

Table 2
Sow herd immunity profiles with the 0–5 scoring scale system

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3) Virus circulation (endemic PED) status

The presence of PEDV or antibody or both in the WTF herd could be used to determine whether the virus has been circulating within the farm. Quantitative real-time RT-PCR (rRT-PCR) can detect and quantify PEDV RNA using fecal (individual) and slurry (pen-side) samples [74, 75]. Simultaneously, serological examinations are performed on nursery and growing pigs using VNT to identify those with past or recent exposure to PEDV. The virus circulation status is profiled based on the presence or absence of PEDV in individual fecal and pen-side slurry samples and/or seroconversion against PEDV in weaned and growing-finishing pigs with a 0–5 scoring scale (0 = best; 5 = worst) (Table 3).

Table 3
PEDV circulation (endemic PED) profiles with the 0–5 scoring scale system

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4) Pentagon profile system

Using the assessment scores independently obtained from the biosecurity-related questionnaire in three sections (0 = worst; 5 = best), the herd immunity level (0 = worst; 5 = best), and the endemic (virus circulation) status (5 = worst; 0 = best), the outcome of the risk assessment can be visualized by a pentagon profile system [74, 75]. Individual scores from the five parameters (yard, staff, and barn biosecurity, herd immunity, and virus circulation) are used to construct the five arms of a risk pentagon. The risk pentagon profiles are classified into three categories presenting a low-, medium-, and high-risk of PEDV recurrence or occurrence if the farm has been PEDV-free, as shown in Fig. 7.

Fig. 7
Illustrated examples of the potential risk of porcine epidemic diarrhea virus recurrence in PED-affected farms (or occurrence in PED-free farms) determined using the risk pentagon profile diagram. The risk pentagon system assessed the levels of five parameters: yard biosecurity, staff biosecurity, barn biosecurity, sow immunity, and virus circulation (endemic infection). Each factor, given a score of 0–5, is represented by a shaded area relative to the total area of the pentagon. The first yellow color pentagon diagram depicts a standard model with no risk of the recurrence or occurrence of PED. A green color pentagon diagram represents no or low risk of PED recurrence (second). Blue color pentagon diagrams indicate a medium-risk of PED recurrence due to unstable herd immunity (third) or virus circulation (fourth). Red color pentagon diagrams represent a high-risk of PED outbreak due to poor biosecurity, unstable herd immunity, and virus circulation (fifth and sixth).
PED, porcine epidemic diarrhea.

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Customized intervention measures

According to risk assessment results, the evaluated herds can be split into nonendemically (viro- and sero-negative in WTF herds) and endemically (viro-positive and/or sero-positive in WTF herds) affected farms by identifying on-farm risk factors that can influence PEDV recurrence. The main factors can be divided into four different categories that include unstable sow immunity, poor biosecurity, virus contamination in farrowing houses, and virus circulation in WTF barns. Once the weaknesses of the farm are confirmed, respective customized control measures are tailored and implemented based on the endemic status to counteract and ameliorate the corresponding risk factors, thereby establishing herd immunity stabilization and virus elimination for PEDV eradication in FTF farms. These include: 1) strict internal and external biosecurity performance (prerequisite); 2) prime-boost prefarrow oral L/K/K vaccination and longitudinal monitoring of protective immunity in sows and their offspring; 3) secure cleaning/disinfection/drying practices, in parallel with the all-in-all-out management, if needed, in farrowing houses and longitudinal MoS of PEDV in the herd (feces) and the environment (slurry); and 4) disinfection in WTF barns and gilt management covering infection monitoring and vaccination, combined with longitudinal PEDV MoS in the herd (feces) and the environment (slurry) and serological monitoring in weaned and growing-finishing pigs. Low-dose infections and high survivability (> 28 days at −20°C to 4°C) of PEDV in the environment (slurry) are known to be major challenges in virus elimination from contaminated barns [108, 111]. Thus, PEDV should be traced and removed in the environment by regularly collecting and testing slurry samples from WTF barns. Fig. 8 illustrates a decision tree to describe the rationale of customized control measures following PEDV exposure in FTF farms that are nonendemically or endemically infected.

Fig. 8
Diagram of customized PED control strategies and measures for nonendemically and endemically affected farms. Upon epidemic PED, the affected farm is recommended to conduct a risk assessment of recurrence to identify on-farm weaknesses that must be managed to control and prevent re-exposure. The main risk factors that influence PED recurrence in FTF farms include: 1) unstable sow immunity, 2) poor biosecurity, 3) virus contamination in farrowing houses, and 4) virus circulation in WTF barns. The four-pillar-based collaborative control strategies are implemented to enhance the herd immunity stabilization and/or eliminate the virus for PED eradication in FTF herds.
L(oral)/K/K, heterologous prime-boost immunization regime with oral live (one shot) and inactivated killed (two booster shots) vaccines; AIAO, all-in-all-out; WTF, wean-to-finish; PED, porcine epidemic diarrhea; FTF, farrow-to-finish.

Adapted from Jang et al. [74].

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We conducted a PED risk assessment with the pentagon profile system in 10 FTF farms with G2b PEDV outbreak history during 2021–2022. All farms had implemented one of the following immunization programs: a homologous K/K or K/K/K regime (G2b inactivation vaccine alone), a modified heterologous feedback/K/K regime (feedback and G2b inactivated vaccine), or feedback alone. Individual pig samples (feces, sera, and colostrum) and pen-side samples (slurry) were collected at 3–4-month intervals as described previously [75]. Stool and slurry samples were collected from diarrheic or nondiarrheic pigs and slurry pits in the FTF sites, respectively. Blood was taken from first-parity (primiparous) and second-parity or greater (multiparous) sows and pigs of different ages (20-, 40-, 70-, 100-, 130-, and 160-day-old). Colostrum was collected on the day of farrowing from primiparous and multiparous sows. Fecal and slurry specimens were subjective to rRT-PCR to detect PEDV RNA, while serum and colostrum samples were tested using VNT or/and S1-iELISA as described previously [74, 75, 108]. External and internal biosecurity checklists were evaluated by a herd veterinarian.

The risk pentagon profiling for each farm was created based on the assessment scores from the aforementioned factors, indicating that all the farms were a medium or high risk of PEDV recurrence because poor biosecurity, unstable sow immunity with low levels of IgA and/or NAb in colostrum (or serum), and/or virus circulation in WTF barns (Fig. 9). Following the risk assessment, each farm’s vulnerabilities to recurrence were ascertained and ameliorated to control PEDV by implementing customized intervention measures as follows: 1) improving weaknesses in internal and external biosecurity protocols strengthened biosecurity management; 2) prime-boost prefarrow oral L/K/K vaccination followed by longitudinal immunity monitoring stabilized herd protective immunity with high persisting levels of IgA and NAb in colostrum and serum; and 3) continuous disinfection practices, in parallel with longitudinal PEDV MoS in feces and slurries and serological monitoring in weaned and growing-finishing pigs, reduced or eliminated virus circulation (Fig. 9).

Fig. 9
Illustration of PED risk pentagon profiles before and after the implementation of customized control measures in 10 farrow-to-finish farms (Farm A–J) with G2b porcine epidemic diarrhea virus outbreak history during 2021–2022. The initial risk assessment (Before) indicated that all farms were a medium (blue) or high (red) risk of PED recurrence. The second assessment (After) showed that the risks of PED recurrence in all farms were improved and were consequently classified as medium (blue) or low (green).
PED, porcine epidemic diarrhea.

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