Whole-genome characterization of vaccine-like african swine fever virus strains detected in non-vaccinated pig herds in Thailand

Abstract

African swine fever (ASF) is a highly infectious and severe hemorrhagic disease of pigs that causes high mortality in domestic pigs. To date, 24 genotypes of African swine fever virus (ASFV) have been identified based on nucleotide variations in the partial B646L (p72) gene. Since its introduction into Asia in 2018 in China, ASF has transitioned from an epidemic to an endemic stage, with genotype II being the dominant strain. Recently, genetically diverse ASFV variants, including genotype I/II recombinants and naturally or artificially attenuated strains, have been reported in China and Vietnam. ASF was first reported in Thailand in 2022, with strains genetically identical to those initially detected in China, suggesting possible cross-border transmission. Recently, ASF outbreaks in Thailand have shown prolonged persistence and pigs exhibited clinical signs associated with the disease. Therefore, the objective of this study was to genetically characterize ASFV collected from these outbreaks. The complete whole-genome sequences of the two strains were generated. Comparative genomic analysis identified the deletion of six genes within the multigene family (MGF 505-1R to MGF 505-3R) and the insertion of an Escherichia coli GusA gene at the deletion site compared with the Georgia 2007 strain. This deletion pattern was similar to that of the live-vaccine candidate ASFV-G-ΔMGF. Additionally, 15 mutations were detected in ORFs across the genome, most of which were silent mutations located in non-coding regions. A three-nucleotide insertion resulted in one additional amino acid in the MGF110-10L and MGF110-14L proteins. The detection of a vaccine-like ASFV strain in herds with no history of vaccination suggests the possible circulation of vaccine-derived variants in swine farms.