INTRODUCTION
African swine fever (ASF) is an acute, severe, and highly contagious infectious disease. It is caused by African swine fever virus (ASFV) (Hakizimana et al., 2020). ASF was first described in the 1920s in Kenya (Lopez et al., 2020; Miao et al., 2019; Ros-Lucas et al., 2020). In August 2018, the first ASF outbreak in China was recorded (Zhu et al., 2020), and the disease has rapidly spread to all parts of China since then (Teklue et al., 2020). ASFV is a large, enveloped virus from the Asfarviridae family, with icosahedral morphology and an average diameter of 200 nm (Alejo et al., 2018; Alonso et al., 2018; Barrado-Gil et al., 2020; Wang et al., 2019). The only member of Asfarviridae is the genus Asfivirus , the only known arbovirus (Franzoni et al., 2020; Zhang et al., 2020; Zhu et al., 2020b). The ASFV genome ranges in length from approximately 170 to 193 kb, depending on the isolate, and contains approximately 160 open reading frames (ORFs) (de Villiers et al., 2010). Studies have shown that more than 50 proteins, including pp220, pp62, p72, p54, p30, and CD2v, are packaged into virus particles and play a role in viral infections. The p30 protein is among the viral proteins expressed early and is encoded by the CP204L gene (Hübner et al., 2018). It has a relative molecular weight of 30 kDa; it is among the most antigenic structural proteins involved in ASFV entry and infection and has the strongest immunogenicity (Jia et al., 2017; Petrovan et al., 2019). The p72 protein with a relative molecular weight of 73.2 kDa is the major ASFV structural protein, and it is the crucial antigenic protein encoded by the B646L (VP72) gene (Kazakova et al., 2017; Portugal et al., 2012). In addition, it is the main component of the viral icosahedral symmetrica; therefore, it has a high degree of antigen activity and immunogenicity. Both domestic pigs and wild boars are susceptible to ASFV infection. Depending on the virulence of the strain, ASF manifests as acute, subacute, and chronic infections in clinical practice. Acute infections typically cause high fever, skin cyanosis, and severe lymph node bleeding (Chen et al., 2016) with a mortality rate as high as 100% (Jackman et al., 2020; Mee et al., 2020). Compared with the former, subacute infections are not fatal. Although chronic infection is not fatal, it develops into a recessive or persistent infection. ASFV spreads rapidly, there are no effective treatment drugs and vaccines for immunization (Jackman et al., 2020), and culling is the most effective means of controlling the disease (Gaudreault et al., 2020; Le et al., 2019). Therefore, ASF causes significant losses to the global pig industry (Cappai et al., 2020). The World Organization for Animal Health (OIE) lists ASFV as a statutory animal disease (Lopez et al., 2020) and a critical animal disease to be prevented in China (Fan et al., 2020).
In the absence of ASFV vaccination, a positive serological antibody test can confirm an ongoing or past infection. In epidemic areas, the antibodies in surviving animals last for months or years after subacute infection. Therefore, antibody testing can be applied for large-scale screening of chronic or non-infected animals. At present, the commonly used ASF serological antibody detection methods in laboratories include enzyme-linked immunosorbent assay (ELISA; Cubillos et al., 2013), fluorescent antibody test (FAT; Gallardo et al., 2015), and colloidal gold immunochromatography strips (ICS; Sastre et al., 2016), among others. Although ELISA can effectively detect ASFV-specific antibodies in pigs that survive ASFV infection (Bergeron et al., 2017), this method is time-consuming and requires professional technicians and specific equipment. Therefore, it is challenging to meet the requirements of a rapid, cheap, and efficient monitoring system for clinical antibody levels.
ICS is a new type of diagnostic technology that uses colloidal gold as a tracer and is combined with an antigen-antibody immune response. It is simple, fast, easy to interpret, does not require any equipment, and has a low cost (Yang et al., 2020). Therefore, it is suitable for on-site testing. Since the p30 and p72 proteins play essential roles in the early and late stages of infection, respectively, they have important application value in immunological diagnosis and development of new vaccines. This study aimed to establish an colloidal gold dual immunochromatographic test strip that can rapidly detect the level of antibodies after ASFV infection to efficiently monitor ASF. This would provide technical support for controlling the spread and prevalence of ASF.