Endemic outbreaks due to the re‐emergence of classical swine fever after accidental introduction of modified live LOM vaccine on Jeju Island, South Korea

Abstract

Summary After the unintentional vaccination of the LOM vaccine strain in 2014, classical swine fever virus (CSFV) reemerged in naïve pig herds on Jeju Island, South Korea, which had been a CSF‐free region with a non‐vaccination policy for a decade. Since the re‐emergence, endemic outbreaks of CSFV have occurred in the island, causing enormous damage to provincial pig farms. The present study reports the complete genome sequences and molecular characterization of the LOM‐derived field CSFV strains responsible for the current outbreaks on Jeju Island. The emergent Jeju LOM‐derived isolates shared 98.9%–99.7% and 98.7%–99.0% nucleotide sequence identity at the E‐gene and whole‐genome levels compared to the LOM vaccine strain respectively. Genetic and phylogenetic analyses indicated that the CSFV field isolates were closest to the LOM strains, but appeared to have undergone substantial evolution. The total number of nucleotide and amino acid differences between the LOM vaccine strain and LOM‐derived field isolates ranged from 111 and 28 to 148 and 42. These variations were found to be widely distributed throughout the genome and particularly accumulated in non‐structural proteins, which might be associated with the potential for LOM to revert to its original low pathogenic form and subsequent horizontal transmission in Jeju swine herds. These data improve our knowledge regarding safety of the LOM vaccine and inherent risk of reversion to natural virulence in host animals.

Major swine viral diseases: an Asian perspective after the African swine fever introduction

“…Even strains with considerably low virulence could be formidable, including MLV vaccines. One of the recent and still ongoing cases is the CSF situation in Jeju Island, South Korea [63]. While sporadic outbreaks have occurred in mainland South Korea, Jeju Island successfully eradicated the disease and became CSF-free in 1999.…”

Evaluation of classical swine fever E2 (CSF-E2) subunit vaccine efficacy in the prevention of virus transmission and impact of maternal derived antibody interference in field farm applications

“…In 2015, Japan was officially announced by OIE to be CSF-free and added to the list of countries with CSF-free status after conducting a successful 10-year eradication program; however, several sporadic reemerging incursions transmitted from wild boar to the domestic population have been reported recently [39][40][41]. In addition, CSF reemerged accidentally in the Jeju island of South Korea after the unintentional vaccination of live attenuated CSF vaccine (LOM strain) on naïve pigs in 2014 [42][43][44]. The alignment of whole-genome sequences of field virulent CSFV strains in the Jeju island (Jeju LOM strain) and the LOM vaccine strain revealed high identity on nucleotide (98.7-99.0%) and amino acid sequence (98.9-99.2%) [44].…”

Evaluation of classical swine fever E2 (CSF-E2) subunit vaccine efficacy in the prevention of virus transmission and its potential application for virus eradication in field farms

“…In 2015, Japan was o cially announced by OIE to be CSF-free and added to the list of countries with CSF-free status after conducting a successful 10-year eradication program; however, several sporadic reemerging incursions transmitted from wild boar to the domestic population have been reported recently [39][40][41]. In addition, CSF reemerged accidentally in the Jeju island of South Korea after the unintentional vaccination of live attenuated CSF vaccine (LOM strain) on naïve pigs in 2014 [42][43][44]. The alignment sequences of eld virulent CSFV strains in the Jeju island (Jeju LOM strain) and the LOM vaccine strain had a high identity on the amino acid sequence (98.7-99.0%) [44].…”

“…In addition, CSF reemerged accidentally in the Jeju island of South Korea after the unintentional vaccination of live attenuated CSF vaccine (LOM strain) on naïve pigs in 2014 [42][43][44]. The alignment sequences of eld virulent CSFV strains in the Jeju island (Jeju LOM strain) and the LOM vaccine strain had a high identity on the amino acid sequence (98.7-99.0%) [44]. Inoculation trials in SPF pigs and pregnant sows indicate that the CSFV Jeju LOM strain had the most characteristics of the LOM vaccine strains and may cause persistent infection in fetuses [42].…”

“…Incomplete vaccine e cacy may lead to virus escape from immunized pigs and act as a positive selection pressure to accelerate virus evolution [21,22,[44][45][46]. Other major drawbacks, including variant vaccination protocols in piglets due to deviation of MDA in sows, adverse effects in weak piglets, and interference by concurrent infection of PRRSV (porcine reproductive and respiratory virus), PCV2 (porcine circovirus type 2), and/or bacterial pathogens during immunization, all greatly impact vaccine e cacy in pig farms.…”