Sung J. Yoo

Prevalence of novel porcine circovirus 3 in Korean pig populations

Porcine circovirus 3 (PCV3) is a novel porcine circovirus that was identified in pigs with porcine dermatitis and nephropathy syndrome, reproductive failure, and multi-systemic inflammation. However, the distribution and genetic characteristics of emerging PCV3 in Korea remains unclear. In this study, we determined the nationwide prevalence and genetic characteristics of PCV3 using pen-based oral fluid samples. The total prevalence of PCV3 in individual samples and at the farm level was 44.2% (159/360) and 72.6% (53/73), respectively. Korean PCV3 shared 99.2±0.2% (98.9-99.8%) and 98.6±0.5% (97.9-99.8%) nucleotide identity in the complete genome and ORF2, respectively, when compared to those of US strains. These data suggested that PCV3 is widely distributed throughout Korean pig populations.

Genotypic diversity of porcine circovirus type 2 (PCV2) and genotype shift to PCV2d in Korean pig population

Porcine circovirus type 2 (PCV2) is a causative agent of PCV2-associated disease (PCVAD), which leads to enormous economic losses in the swine industry worldwide. A high nucleotide substitution rate allows for the continuous evolution of PCV2 and the emergence of novel PCV2 strains. However, the distribution of emerging PCV2 genotypes and the co-existence of multiple genotypes in Korea have not been elucidated. The objective of this study was to determine the genetic diversity of PCV2 in Korean pig herds between 2009 and 2016. The overall prevalence of PCV2, from various samples originating from commercial pigs, was 53.8% (325/604). Two cases of a genotype shift to PCV2d at the farm level showed that the genotype shift started before 2012. In addition, genotype-specific PCR, on pen-based oral fluid samples for nationwide PCV2 surveillance in 2016, indicated that the infection pattern of PCV2 genotypes at the farm level was as follows; none (6/69), PCV2a (6/69), PCV2b (2/69), PCV2d (33/69), PCV2a/b (2/69), PCV2a/d (4/69), PCV2b/d (11/69), and PCV2a/b/d (2/69), respectively. This suggests that the genotype shift to PCV2d occurred on a nationwide scale and that the co-existence of different genotypes is common in Korean pig herds. In addition, seven sites on the capsid protein of Korean PCV2 were identified as being under positive selection pressure, all of which are related to the epitope region and neutralization activity. These data provide evidence of increased genetic diversity and shifts among Korean PCV2 isolates.

Challenges of influenza A viruses in humans and animals and current animal vaccines as an effective control measure

Influenza A viruses (IAVs) are genetically diverse and variable pathogens that share various hosts including human, swine, and domestic poultry. Interspecies and intercontinental viral spreads make the ecology of IAV more complex. Beside endemic IAV infections, human has been exposed to pandemic and zoonotic threats from avian and swine influenza viruses. Animal health also has been threatened by high pathogenic avian influenza viruses (in domestic poultry) and reverse zoonosis (in swine). Considering its dynamic interplay between species, prevention and control against IAV should be conducted effectively in both humans and animal sectors. Vaccination is one of the most efficient tools against IAV. Numerous vaccines against animal IAVs have been developed by a variety of vaccine technologies and some of them are currently commercially available. We summarize several challenges in control of IAVs faced by human and animals and discuss IAV vaccines for animal use with those application in susceptible populations.

Efficacy of commercial genotype 1 porcine reproductive and respiratory syndrome virus (PRRSV) vaccine against field isolate of genotype 2 PRRSV

Although several recent studies have found that type 1 porcine reproductive and respiratory syndrome virus (PRRSV) modified live virus (MLV) vaccine showed appreciable levels of cross-protection against type 2 PRRSV infection, the possibility of cross-protection between two genotype of PRRSV is still controversial. To determine potential protective efficacy against hetero-genotype field strain of PRRSV and to improve understandings of the mechanisms underlying performance improvement after infection in vaccinated animals, piglets were vaccinated with type 1 PRRSV MLV vaccine and challenged with type 2 field strain of PRRSV. As a result, vaccinated animals gained on average 8.45 kg in comparison to 4.77 kg measured in non-vaccinated animals during a 3-week period after viral challenge, which shows using a certain PRRSV vaccine could be clinically effective against heterologous genotypic virus challenge. In vaccinated animals, viremia was reduced and cleared rapidly, whilst viral load was much higher and reduced more slowly, indicating rebound viremia in non-vaccinated animals. The titers of neutralizing antibody against the type 2 PRRSV did not exceed the protective level in any animal from both vaccinated and control groups. Instead, antibody avidity of vaccinated animals was much higher than in the control group clearly. Furthermore, a strong negative correlation between antibody avidity and viremia was noted in 80% of vaccinated animals. Through those results from tests evaluating degree of antibody maturation and its relevance with clearing viremia, it could be suggested that non-neutralizing antibodies induced by vaccination prior to challenge might play a key role in protection against PRRSV infection, especially in early time course.

Wild boars harboring porcine epidemic diarrhea virus (PEDV) may play an important role as a PEDV reservoir.

Abstract Porcine epidemic diarrhea virus (PEDV) is a burdensome pathogen in the swine industry. Wild boar population poses a high risk for reservoir of viral pathogen. Two hundred eighty seven samples from wild boar (Sus scrofa) collected in South Korea during 2010/11 were analyzed using RT-PCR, revealing a PEDV infection rate of 9.75% (28/287). PEDV positive samples were distributed throughout the mainland of South Korea, clustering at the northern border adjacent to the Demilitarized Zone (DMZ) and in mountainous regions. PEDV in wild boar was genetically similar to Chinese PEDV strains in phylogenetic investigations. Our results indicated that PEDV is circulating in the wild boar and provided a novel knowledge into epidemiology of PEDV infection.

Genetic evolution of classical swine fever virus under immune environments conditioned by genotype 1-based modified live virus vaccine

Modified live vaccines (MLVs) based on genotype 1 strains, particularly C-strain, have been used to prevent and control classical swine fever virus (CSFV) worldwide. Nevertheless, a shift in the predominant CSFV strains circulating in the field from genotype 1 or 3 to genotype 2 is seen. Genotype 2 is genetically distant from the vaccine strains and was recently reported during outbreaks after vaccine failure; this has raised concerns that vaccination has influenced viral evolution. In Korea in 2016, there was an unexpected CSF outbreak in a MLV-vaccinated commercial pig herd. The causative CSFV strain was genetically distinct from previously isolated Korean strains but similar to recent Chinese strains exhibiting enhanced capacity to escape neutralization; this suggests the need for global cooperative research on the evolution of CSFV. We analysed global E2 sequences, using bioinformatics tools, revealing the evolutionary pathways of CSFV. Classical swine fever virus genotypes 1 and 2 experienced different degrees and patterns of evolutionary growth. Whereas genotype 1 stayed relatively conserved over time, the genetic diversity of genotype 2 has progressively expanded, with few fluctuations. It was determined that genotype 2 evolved under lower immune pressures and at a higher evolutionary rate than genotype 1. Further, several selected codons, under diversifying selection in genotype 1 but under purifying selection in genotype 2, correspond to antigenic determinants, which could lead to evasion of vaccine-induced immunity. Our findings provide evidence that evolutionary changes in CSFV are the result of the disproportionate usage of the CSF MLVs in endemic areas; this underscores the need to develop mitigation strategies to minimize the substantial risk associated with the emergence of vaccine-escaping mutants.

Genetic diversity of ORF 4–6 of type 1 porcine reproductive and respiratory syndrome virus in naturally infected pigs

Genotype 1 porcine reproductive and respiratory syndrome virus (PRRSV) has been highly prevalent throughout Korea since the virus was first detected in 2005. However, genetic analyses of genotype 1 PRRSV in Korea have been limited to ORF5 and/or ORF7. In the present study, we determined 10 representative sequence covering ORF4 to ORF6 and each individual ORFs of genotype 1 PRRSV in Korea, and performed molecular analyses. The most variable gene among the individual ORFs of field strains was ORF4, and this gene exhibited only 74.5-87.3% sequence homology compared with strains reported elsewhere. However, the strains showed analogous sequence arrangements with each other. In the phylogenetic analysis, the sequences of Korean field strains formed a distinct cluster with some Austrian and German strains compared to genotype 1 PRRSV strains available in GenBank. In the amino acid analysis, the putative antigenic region of GP4 was highly variable, whereas the predicted epitope regions of ORF5 and ORF6 were relatively conserved. The hydropathy plots of GP4 showed a highly variable pattern in the antigenic region. The non-synonymous and synonymous substitution analysis suggested that ORF4 presumably had more immunogenic pressure compare with the other ORFs. According to these findings, genotype 1 PRRSV in Korea has been diversified and indigenized in Korea, and these strains might have multifarious immunological and genetic properties. This study provides novel insights into genotype 1 PRRSV in a geographically remote area and contributes to the information for further research on the evolution of type 1 PRRSV in the Korean peninsula.

Cross-Reactivity of Porcine Immunoglobulin A Antibodies with Fecal Immunoglobulins of Wild Boar (Sus scrofa) and Other Animal Species.

Fecal samples obtained from wild boar habitats are useful for the surveillance of diseases in wild boar populations; however, it is difficult to determine the species of origin of feces collected in natural habitats. In this study, a fecal IgA ELISA was evaluated as a method for identifying the porcine species from fecal samples. Both domestic pigs (Sus scrofa domestica) and wild boars (Sus scrofa coreanus) showed significantly higher levels of fecal IgA than other animal species. Additionally, age dependent changes in the level of Ig A in wild boars and domestic pigs were identified; Titers of Ig A were highest in suckling period and lowest in weanling period.

Classical Swine Fever Outbreak after Modified Live LOM Strain Vaccination in Naive Pigs, South Korea

We report classical swine fever outbreaks occurring in naive pig herds on Jeju Island, South Korea, after the introduction of the LOM vaccine strain. Two isolates from sick pigs had >99% identity with the vaccine stain. LOM strain does not appear safe; its use in the vaccine should be reconsidered.

Genetic identification and serological evaluation of commercial inactivated foot-and-mouth disease virus vaccine in pigs

Vaccination is considered a frequently used tool to prevent and control foot-and-mouth disease (FMD). However, the effectiveness of conventional FMD virus (FMDV) vaccines in pigs has been controversial because the massive prophylactic vaccination could not elicit proper immune response nor prevent the broad spread of FMD outbreak, mainly in pig farms, in South Korea during outbreaks of 2014. In addition, there has been little information on the efficacy of inactivated, high potency, multivalent, oil-based FMDV vaccine in pigs, because an evaluation of FMDV vaccines had been mainly carried out using cattle. In this study, we evaluated the genetic identification of commercial inactivated FMDV vaccine and monitored the immune responses in pigs under the field condition. Results implied that it contained three different serotypes with a high level of antigen payload. However, serological results showed low mean percentage of inhibition, and positive rate reached its peak at 6-week post-vaccination, indicating current FMDV vaccine need to improve for a prophylactic vaccination policy in pigs. Therefore, there is an imperative need to develop FMDV vaccine that can provide rapid and long-lasting protective immunity in pigs.