The African swine fever virus (ASFV) genome encodes over 170 genes, but only a small number have been investigated for their roles in disease progression. Understanding the genetic and biological functions of these lesser-known genes accelerates the specification of open reading frames involved in pathogenesis in host animals and the development of ASF vaccines. In this study, we deleted the I196L gene of the highly virulent parental strain AQS-C-1-22 to generate the ASFV mutant AQSΔI196L. In vitro, AQSΔI196L replicated similarly to the parental virus in immortalized porcine kidney macrophage cultures. Pigs intramuscularly inoculated with 10² median tissue culture infectious dose (TCID50) AQSΔI196L developed similar clinical signs and consequences (e.g., high fever, anorexia, and death) as infections with the parental strain. However, viremia in AQSΔI196L-inoculated pigs was slightly reduced compared with pigs inoculated with the parental strain. These findings suggest that the I196L gene does not significantly affect the pathogenicity of the virus in pigs.

Background: Highly potent vaccines are essential for the effective control of classical swine fever (CSF). Since CSF re-emerged in 2018 in Japan, the live CSF virus (CSFV) vaccine—a guinea pig exaltation of Newcastle disease virus-negative strain vaccine (GPE⁻, genotype 1.1)—has been applied to domestic pigs, contributing to a reduction in outbreaks. Meanwhile, the persistence and continued expansion of CSFV in wild boar populations have raised concerns regarding potential antigenic divergence. Methods: We systematically evaluated the neutralizing reactivity of sera from GPE⁻-vaccinated pigs against CSFV strains (genotype 2.1) recently circulating in Japan against identified a representative strain that showed markedly reduced neutralization. We directly assessed the protective efficacy of the GPE⁻ vaccine against this strain in a controlled challenge experiment. At 4 weeks post-vaccination, both vaccinated and unvaccinated pigs were orally challenged with the representative Japanese strain and monitored for 3 weeks thereafter. Results: Among the Japanese CSFV strains, the JPN/SM/WB/2022 isolate exhibited markedly reduced neutralizing reactivity—over 32-fold lower than that against the vaccine strain—when tested with GPE⁻ vaccine-induced antisera. In the experimental infection in pigs, unvaccinated pigs exhibited typical clinical signs of CSF and viremia, and two pigs reached the humane endpoint. In contrast, none of the vaccinated pigs showed any clinical signs of infection. Robust humoral and cellular immune responses were induced in vaccinated pigs, which may correlate with the observed complete protection. Conclusions: The GPE^– live vaccine provides protective immunity against an antigenically distinct strain, prevents disease, and limits viral spread in domestic pigs.

African swine fever (ASF) is a highly fatal, febrile infectious disease of domestic pigs and wild boars caused by the African swine fever virus (ASFV). Recently, highly virulent recombinant ASFVs with chimeric genomes derived from p72 genotype I and II viruses have emerged in China, Vietnam, and Russia. These genotype I/II recombinants can evade immunity induced by genotype II–based vaccines, thereby complicating disease control efforts. To address this challenge, a novel duplex quantitative PCR (qPCR) assay was developed to simultaneously detect and differentiate genotypes I, II, and I/II recombinants in a single reaction. The assay exhibited high sensitivity and specificity, with a reliable detection limit of 10 copies/reaction for genotype I and II ASFV DNA. Validation using clinical samples collected in northern Vietnam in 2025 confirmed a robust performance in accurately distinguishing circulating genotype II viruses from recombinant genotype I/II viruses, including the detection of potential co-infection. Whole-genome sequencing of selected positive samples further corroborated these findings. Overall, this qPCR assay provides a precise and efficient tool for identifying currently circulating ASFV genotypes, thereby facilitating improved disease surveillance and supporting a comprehensive understanding of the evolving epidemiological landscape of ASF in regions with increasing viral genetic diversity.