Despite evidence that mortality as a result of African swine fever is declining in China and that herds are being restored to pre-ASF levels, the infection is endemic with outbreaks reported from numerous provinces. Prices of weaning, market hogs and pork are more reliable indicators of the status of health in hog herd in China then official reports by the Ministry of Agriculture.
A study conducted from June through December* 2020 revealed 22 isolates of genotype II ASF virus that had undergone mutations distinguishing them from the lethal HLJ/18 strain, the earliest isolated in China in 2018. Eleven isolates were non-hemadsorbing. Of these variants two were lethal and two non-HAD strains displayed reduced virulence but were highly transmissible. The authors concluded that ASF viruses that lose the EP402R gene do not express CD2v protein and display lower pathogenicity. This has implications for recognition of outbreaks that initially are based on a rise in mortality with confirmation applying PCR or other molecular diagnostic procedures.
There are no currently approved ASF vaccines in use although the issue of clandestine development and illegal deployment of gene-deleted preparations has been raised. The research group involved in the study has produced a candidate vaccine based on removal of 7 genes from HLJ/18 that has proven safe under laboratory evaluation.
Encheng Sun, Zhenjiang Zhang, Zilong Wang, Xijun He, et al.: Emergence and prevalence of naturally occurring lower virulent African swine fever viruses in domestic pigs in China in 2020. Science China Life
Sciences. 2021; doi: <https://doi.org/10.1007/s11427-021-1904-4>.