Surveillance of classical swine fever in wild boar in South Korea from 2010-2014

Haplotype of Wild Korean Boars Infected by Classical Swine Fever Virus Subgenotype 2.1d

Simple Summary

Classical swine fever is a highly contagious disease that infects both domestic pigs and wild boars. Classical swine fever virus (CSFV) has not been detected in domestic pigs in South Korea since 2016, but has been increasing in wild boars since 2017. Two cases of CSFV subgenotype 2.1d were detected in wild Korean boars in 2011, but then no cases were detected until 2016; however, 16 cases of CSFV were detected between 2017 and 2019. In this study, we report seven CSFV-positive samples obtained from wild boars in 2020. In addition, although 13 mtDNA haplotypes were detected in wild boars in South Korea, all 25 cases of CSFV that occurred in wild boars between 2011 and 2020 were detected in animals with haplotype 01.

Abstract

Classical swine fever virus (CSFV) is one of the major pathogens that causes severe economic damage to the swine industry. Circulation of CSFV in wild boars carries the potential risk of reintroducing the virus into CSFV-free pig farms. This study carried out a genetic analysis of CSFV isolates from wild boars and analyzed the mtDNA haplotypes of the wild boars. Blood samples (n = 2140) from wild Korean boars captured in 2020 were subjected to qRT-PCR to detect CSFV, which was classified as subgenotype 2.1d based on phylogenetic analysis. CSFV had been detected in wild boars only in northern regions (Gangwon and Gyeonggi) of South Korea between 2011 and 2019. However, CSFV was identified in wild boars in the more southern regions (Chungbuk and Gyeongbuk) in 2020. Based on mitochondrial DNA analysis, all wild boars with CSFV were haplotype 01 (H01). Thus, we presume that the H01 haplotype is more susceptible to CSFV. In the future, infection of wild boars by CSFV is expected to occur intermittently every year, and we predict that most wild boars infected with CSFV will be haplotype H01.

Rapid Spread of Classical Swine Fever Virus among South Korean Wild Boars in Areas near the Border with North Korea

There has been a rapid increase in the number of classical swine fever (CSF) sero-positive wild boars captured near the demilitarized zone (DMZ), located the border with North Korea. In 2015-2016, few CSFV-positive antibody boars were detected; however, the number has increased steeply since 2017. Most occurred in the northern region of Gyeonggi before spreading slowly to Gangwon (west to east) in 2018-2019. Multi-distance spatial cluster analysis provided an indirect estimate of the time taken for CSFV to spread among wild boars: 46.7, 2.6, and 2.49 days/km. The average CSF serum neutralization antibody titer was 4-10 (log 2), and CSFV Ab B-ELISA PI values ranged from 65.5 to 111.5, regardless of the age and sex of wild boars. Full genome analysis revealed that 16 CSFV strains isolated from wild boars between 2017 and 2019 were identical to the YC16CS strain (sub-genotype 2.1d) isolated from an outbreak in breeding pigs near the border with North Korea in 2016. The rapid increase in CSF in wild boars may be due to a continuously circulating infection within hub area and increased population density. The distribution pattern of CSFV in Korean wild boars moves from west to southeast, affected by external factors, including small-scale hunting, geographical features and highways.

Role of Wild Boar in the Spread of Classical Swine Fever in Japan

Since September 2018, nearly 900 notifications of classical swine fever (CSF) have been reported in Gifu Prefecture (Japan) affecting domestic pig and wild boar by the end of August 2019. To determine the epidemiological characteristics of its spread, a spatio-temporal analysis was performed using actual field data on the current epidemic. The spatial study, based on standard deviational ellipses of official CSF notifications, showed that the disease likely spread to the northeast part of the prefecture. A maximum significant spatial association estimated between CSF notifications was 23 km by the multi-distance spatial cluster analysis. A space-time permutation analysis identified two significant clusters with an approximate radius of 12 and 20 km and 124 and 98 days of duration, respectively. When the area of the identified clusters was overlaid on a map of habitat quality, approximately 82% and 75% of CSF notifications, respectively, were found in areas with potential contact between pigs and wild boar. The obtained results provide information on the current CSF epidemic, which is mainly driven by wild boar cases with sporadic outbreaks on domestic pig farms. These findings will help implement control measures in Gifu Prefecture.

Evolutionary dynamics of classical swine fever virus in South Korea: 1987-2017

The 5' UTR (n=102) and full-length E2 (n=37) genes of classical swine fever viruses (CSFVs) circulating in South Korea over the past 30 years (1987-2017) were examined to determine the evolutionary rate and estimated time of the most recent common ancestor (tMRCA). From 2000, the Korean classical swine fever (CSF) antigen changed from genotype 3 to 2, which comprises subgenotypes 2.1b (2002-2013) and 2.1d (2011-2017). There are genotypic variations in the full-length E2 gene of Korean CSFV genotypes 2.1b and 2.1d (seven separate amino acid substitutions); these are useful distinguishing markers. The mean substitution rate (×10³ substitutions/site/year) for Korean CSFV was estimated to be 2.2088 (95% highest posterior density (HPD): lower, 1.7045; upper, 2.7574) and the mean tMRCA was estimated to be 1901 (95% HPD: lower, 1865; upper, 1933). The effective population size of Korean CSFV genotype 2 increased rapidly from 2002 to 2003, after which it remained constant. The occurrence of CSF in Korea is expected to decline in the future; however, it will likely be more prevalent in wild boar than in domestic pigs. Thus, there is a risk of transmission from wild boar to breeding pigs.

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 and virulence characterization of classical swine fever viruses isolated in Mongolia from 2007 to 2015

Classical swine fever (CSF), a highly contagious viral disease affecting domestic and wild pigs in many developing countries, is now considered endemic in Mongolia, with 14 recent outbreaks in 2007, 2008, 2011, 2012, 2014, and 2015. For the first time, CSF viruses isolated from these 14 outbreaks were analyzed to assess their molecular epidemiology and pathogenicity in pigs. Based on the nucleotide sequences of their 5'-untranslated region, isolates were phylogenetically classified as either sub-genotypes 2.1b or 2.2, and the 2014 and 2015 isolates, which were classified as 2.1b, were closely related to isolates from China and Korea. In addition, at least three different viruses classified as 2.1b circulated in Mongolia. Experimental infection of the representative isolate in 2014 demonstrated moderate pathogenicity in 4-week-old pigs, with relatively mild clinical signs. Understanding the diversity of circulating CSF viruses gleans insight into disease dynamics and evolution, and may inform the design of effective CSF control strategies in Mongolia.