With the implementation of the C-strain vaccine, classical swine fever (CSF) has been under control in China, which is currently in a chronic atypical epidemic situation.  African swine fever (ASF) emerged in China in 2018 and spread quickly across the country. It is presently occurring sporadically due to the lack of commercial vaccines and farmers’ increased awareness of biosafety.  Atypical porcine pestivirus (APPV) was first detected in Guangdong Province, China, in 2016, which mainly harms piglets and has a local epidemic situation in southern China.  These three diseases have similar clinical symptoms in pig herds, which cause considerable losses to the pig industry.  They are difficult to be distinguished only by clinical diagnosis.  Therefore, developing an early and accurate simultaneous detection and differential diagnosis of the diseases induced by these viruses is essential.  In this study, three pairs of specific primers and Taq-man probes were designed from highly conserved genomic regions of CSFV (5´ UTR), African swine fever virus (ASFV) (B646L), and APPV (5´ UTR), followed by the optimization of reaction conditions to establish a multiplex real-time PCR detection assay.  The results showed that the method did not cross-react with other swine pathogens (porcine circovirus type 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), foot-and-mouth disease virus (FMDV), pseudorabies virus (PRV), porcine parvovirus (PPV), and bovine viral diarrhea virus BVDV).  The sensitivity results showed that CSFV, ASFV, and APPV could be detected as low as 1 copy mL^–1; the repeatability results showed that the intra-assay and inter-assay coefficient of variation of ASFV, CSFV, and APPV was less than 1%.  Twenty-two virus samples were detected by the multiplex real-time PCR, compared with national standard diagnostic and patented method assay for CSF (GB/T 27540–2011), ASF (GB/T 18648–2020), and APPV (CN108611442A), respectively.  The sensitivity of this triple real-time PCR for CSFV, ASFV, and APPV was almost the same, and the  compliance results were the same (100%).  A total of 451 clinical samples were detected, and the results showed that the positive rates of CSFV, ASFV, and APPV were 0.22% (1/451), 1.3% (6/451), and 0% (0/451), respectively.  This assay provides a valuale tool for rapid detection and accurate diagnosis of CSFV, ASFV, and APPV.

Classical swine fever caused by Classical swine fever virus (CSFV) is a serious problem for swine industries in developing countries, which successful control of the disease have been relying on vaccination. However, classical swine fever still occurs in some immunized swine herds for various reasons. In this study, we conducted animal experiments to examine the influence of single or mixed infection with Porcine parvo virus (PPV), Pseudorabies virus (PRV) and Porcine reproductive and respiratory syndrome virus (PRRSV) on the protective immunity induced by the Lapinized hog cholera virus (HCLV) vaccine and the pathogenicity of CSFV. In experiment 1, pigs were first inoculated with PPV, PRV or PRRSV, then immunized with HCLV, and finally challenged with a highly virulent CSFV Shimen strain. All of the pigs immunized with HCLV survived after the challenge, while all of the pigs in the non-immunized control group died after the challenge. The pigs in the group immunized with HCLV did not show any clinical symptoms of classical swine fever and were negative with CSFV after the challenge. The pigs infected with the non-CSFV before HCLV immunization did not display any clinical symptoms after the challenge with CSFV Shiman strain, but 11 of the 12 pigs were positive with CSFV. In experiment 2, pre-infections with PPV, PRV, and PRRSV were followed by inoculation with a low-virulence CSFV strain (CSFV 39), and then the pigs were challenged with the CSFV Shimen strain. Infections by either PPV, PRV or PRRSV did not enhance the virulence of CSFV-39, but pigs infected by a mixture of the 3 viruses developed clinical symptoms after inoculation with CSFV-39. The mixed infection also increased mortality caused by the challenge with the CSFV Shimen strain. Together, these results showed PPV, PRV and PRRSV infections in pigs can reduce the efficacy of the HCLV vaccine and enhance the pathogenicity of CSFV, which may partly explain the immunization failure against CSFV in some swine herds.