From Farm Journals PORK written by Jennifer Shike.
A newly discovered low virulent, genotype 1, African swine fever (ASF) virus is causing chronic infections in China’s pig herd, says ASF expert Dan Rock at the University of Illinois. 
Research confirms the detection of a second ASF virus strain present in two Chinese provinces, Rock wrote in the latest Swine Health Information Center newsletter. The ASF viruses described are genotype 1 viruses, distinct from the currently circulating genotype 2 virus Georgia-07 and its derivatives.
“These virus isolates (hemadsorption negative) are of lower virulence characterized by a chronic disease presentation including necrotic skin lesions and joint swelling,” Rock explains. “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.”
Fortunately, current diagnostic tools PCR (p72-based) or serologic (ELISA-based) should be adequate for detection of infected animals, the researchers say. Because of this virus’ reduced virulence and transmissibility characteristics, it is reasonable to assume it may be present in other regions of China and Southeast Asia, Rock adds.
“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,” Rock explains.
In the past, both NH/P68 and OURT88/3 were evaluated as ASF vaccine candidates. The article said 25% to 47% of animals inoculated with a naturally occurring attenuated ASFV isolate, ASFV/NH/P68 (likely a vaccine-derived virus) developed chronic lesions and disease characterized by late fever and viremia and by high levels of anti-ASFV antibodies with marked hypergammaglobulinemia. Research also confirms less severe post-vaccinal reactions involving fever and joint swelling were described for the ASF live-attenuated vaccine candidate, OUR T88/3.
“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 ASF virus strains,” Rock notes. “Given the active Chinese ‘Belt and Road Initiative,’ with hundreds of thousands of Chinese working in Africa and many direct flights from African cities to China, there is a possibility that other ASF virus strains already may be present or will emerge in China.”
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Expert Demystifies New African Swine Fever Virus in China