A recent report by Sun et al. (2021) revealed a second African swine fever virus (ASFV) strain has been discovered in two Chinese provinces. The ASF viruses are described as genotype 1 viruses, distinct from the currently circulating genotype 2 virus Georgia-07 and its derivatives. According to the Swine Health Information Center (SHIC), these virus isolates (hemadsorption negative) are of lower virulence characterized by a chronic disease presentation including necrotic skin lesions and joint swelling.
Data presented suggest the viruses are readily transmissible to contact animals. Notably, pigs infected with these viruses could easily be missed early in a disease outbreak due to their reduced virulence. However, current diagnostic tools PCR (p72-based) or serologic (ELISA-based) should be adequate for detection of infected animals [Sun et al., 2021]. Given their reduced virulence and transmissibility characteristics, it is reasonable to assume these viruses also may be present in other regions of China and Southeast Asia.
SHIC noted that the source of these viruses and the nature of their introduction into China is unclear. While they may represent a new introduction of virus from an African source, the striking degree of genetic similarity with NH/P68 and OURT88/3, two genotype I ASFVs isolated in Portugal in the 1960s suggest they may have originated from a European source – possibly imported legally or illegally to be evaluated as potential ASF vaccine candidates in China.
SHIC said both NH/P68 and OURT88/3 were evaluated as ASF vaccine candidates in the past. Twenty-five to 47% of animals inoculated with a naturally occurring attenuated ASFV isolate, ASFV/NH/P68 (likely a vaccine-derived virus [Portugal et al., 2015]) developed chronic lesions and disease characterized by late fever and viremia and by high levels of anti-ASFV antibodies with marked hypergammaglobulinemia [Leitão et al., 2001]. Less severe post-vaccinal reactions involving fever and joint swelling were described for the ASF live-attenuated vaccine candidate, OUR T88/3 [King et al., 2011].
Currently, ASF vaccines are being developed against Georgia-07 and derivatives (a genotype 2, serogroup 8 virus) [Rock, 2021]. It is highly unlikely these vaccine candidates will provide protection against the viruses (genotype 1, serogroup 4) described here due to the lack of cross protection observed among heterologous ASFV strains.
The active Chinese “Belt and Road Initiative,” an initiative that seeks to connect Asia with Africa and Europe via land and maritime networks, has resulted in hundreds of thousands of Chinese working in Africa and many direct flights from African cities to China. Because of this, SHIC said there is a possibility that other ASF strains already may be present or will emerge in China.