A China Harbin Laboratory paper describing their analysis of African swine fever virus (ASFV) variants emerging in China as well as media and on-the-ground reports of non-approved vaccine use causing pathology, chronic infection and vaccine virus shedding raised concern. The Swine Health Information Center (SHIC) asked Dr. Dan Rock, University of Illinois, to help give some interpretation and context to the Harbin Laboratory paper and attenuating mutations of the ASFV genome such as those that could be used in unapproved vaccines. Dr. Rock has extensive research experience focused on exotic viral diseases of high consequence and the molecular mechanisms that underlie viral virulence.
China ASFV Variants Review by Dr. Dan Rock
In the manuscript “Emergence and prevalence of naturally occurring lower virulent African swine fever viruses (ASFV) in domestic pigs in China 2020.” (Sun et al., 2020), the authors describe genetic changes in all 22 ASFV field isolates collected during a six-month sampling period (June to December 2020) when compared to the virulent field strain, HLJ/18 2020. This manuscript is especially relevant as there are reports of gene deleted vaccine viruses being illegally used in China resulting in chronic infection with increased virus transmissibility.
Notably, 11 of the viruses isolated from two provinces with high pig density contained mutations in the EP402R gene (encoding for the CD2v protein) and exhibited a non-hemadsorbing phenotype (non-HAD). Similar mutations (a single nucleotide substitution resulting in a stop codon and a truncated CD2v protein) were observed in a number of the viruses suggesting they are of the same lineage expressing a truncated CD2v protein of 44 AA. It is likely these represent spontaneous mutations in the CD2v gene occurring on pig-passage rather than engineered mutations in a vaccine virus. Clearly, these mutations are distinct from those previously described for Chinese vaccine virus candidates4; in the case of these engineered viruses, the CD2 gene would be deleted completely with a reporter gene cassette inserted in its place. Given the limited sampling of field virus, the presence of other engineered gene deleted vaccine viruses cannot be formally excluded.
Naturally occurring Non-HAD ASF viruses have been described previously.1, 5, 6, 9, 11 It remains unclear if CD2 mutations occur under selective pressure generated in the host or arise spontaneously and are easily recognized due to loss of the HA phenotype. In any event and consistent with spontaneous mutation, non-HAD ASF viruses appear to arise infrequently. As would be expected, mutations (without easily discernable phenotypes) have been observed throughout the ASFV genome from current European ASFV field isolates separated in time.8
In general, non-HAD ASF viruses including those described by Sun et al.,2020 exhibit altered pathogenesis and attenuation in the pig with the observed effect isolate/infection dose dependent.1-7, 9-11 Typical peracute/acute ASFV infection observed with virulent field isolates is replaced with delayed disease onset and a subclinical and chronic disease presentation characterized by the absence of or reduced mortality and decreased viremia and tissue titers. Duration of viral persistence (blood and tissue) in pigs is variable. The genetic background of the virus may be important for the degree of attenuation/persistence observed for non-HAD ASF viruses as there are examples where CD2v gene mutations/deletions had little to no effect on viral virulence.2, 3, 6, 9
Higher transmissibility was observed for non-HAD viruses evaluated by Sun et al., 2020. Again, this may be isolate/infection dose dependent as the opposite result of reduced transmissibility was observed for Portuguese non-HAD viruses.1 The actual significance of persistent infection with non-HAD viruses and the potential for enhanced transmission under field conditions remains unclear. From available data,1, 10 animals infected with non-HAD viruses seroconvert and are resistant to homologues virulent virus infection.
Non-HAD viruses ASF viruses with reduced virulence and possibly enhanced transmissibility pose new challenges for early ASFV detection and emergency response. Clinical signs observed are not as obvious as those seen with peracute/acute ASFV: therefore, greater vigilance for signs of subclinical and chronic disease is needed. Fortunately, current ASFV diagnostics (serologic and PCR-based) use highly conserved viral structural proteins as targets will detect non-HAD infected animals.
It is important to consider that other attenuating mutations of the ASFV genome such as those that could be used in unapproved vaccines and are yet to be described – not involving CD2v – also may pose problems for early viral detection, response, and disease control of ASF.
10 Sun, E., Zhang, Z., Wang, Z., He, X., Zhang, X., Wang, L., Wang, W., Huang, L., Xi, F., Huangfu, H., et al. (2021). Emergence and prevalence of naturally occurring lower virulent African swine fever viruses in domestic pigs in China in 2020. Sci China Life Sci 64, https://doi.org/10.1007/ s11427-021-1904-4