African swine fever (ASF) is an acute, hemorrhagic, highly contagious, infectious disease of domestic and wild pigs, caused by African swine fever virus (ASFV) (Galindo and Alonso, 2017, Costard et al., 2009, Sanchez-Vizcaino et al., 2015). ASFV is the sole member of the Asfivirus genus within the Asfarviridae family and is the only known DNA arbovirus (Dixon et al., 2013), which is a large enveloped double-stranded DNA virus. Morphologically, ASFV is icosahedrally symmetric and has a large genome size ranging from 170 to 190 kb, containing 150–167 open reading frames, encoding 68 structural proteins and more than 100 nonstructural proteins (Alonso et al., 2018, Blasco et al., 1989, Gaudreault et al., 2020, Wang et al., 2019). There are at least 24 different ASFV genotypes identified worldwide according to the B646L gene, which encodes the major capsid protein p72 (Quembo et al., 2018).

In China, ASFV was first introduced in 2018 and has continued to evolve and remain endemic. In 2020, low-virulence genotype II ASFV strain (HLJ/HRB1/20) lacking the hemadsorption (HAD) activity emerged in the fields of China (Sun et al., 2021a). Similarly, a low-virulence genotype I ASFV strain (SD/DY-I/21) and a naturally occurring high-virulence recombinant strain (JS/LG/21) of genotype I and II ASFV were reported in 2021 (Sun et al., 2021b, Zhao et al., 2023). The low-virulence strains of ASFV are characterized by a long incubation period, high stealthiness, inconspicuous clinical symptoms, irregular viral discharge, and a tendency for nucleic acid detection to be overlooked, leading to the spread of the epidemic (Ding et al., 2023, Ding et al., 2024). Therefore, WOAH recommended that antibody monitoring needs to be carried out in areas with low virulence or low prevalence of ASFVs, with the aim of early detection of pigs infected with ASFV through regular antibody screening, which provides a significant guidance for the prevention and control of ASF. The serological detection methods recommended by WOAH and the current national standard (GB/T18648-2020) for diagnostic technology of African swine fever in China consist of enzyme-linked immunosorbent assay (ELISA), indirect immunofluorescence assay (IFA), and immunoperoxidase monolayer assay (IPMA), of which IPMA and IFA are the gold standard of serological detection methods recommended by WOAH (African swine fever, 2019). Recently our group has established a credible IFA method for ASFV antibody detection (Wang et al., 2023), though the IFA requires expensive fluorescence microscope and its application in the field is limited.

In this study, a cell-adapted ASFV strain with highly attenuated virulence, designated as ASFV HLJ18/BK33, was obtained by serially passaging ASFV Pig/HLJ/18 strain for 33 generations on wild boar kidney cells (BK2258) (Wang et al., 2025). Ultimately, we established an IPMA method for ASFV antibodies detection based on ASFV HLJ18/BK33 strain, which was highly sensitive, specific, easy to operate, and can be observed and determined under an ordinary optical microscope. This method is suitable for widespread use at the local level for ASF monitoring, prevention, and control.