ASF in China: mutations confirmed by Chinese scientists

Chinese scientists have confirmed that milder mutations of the African swine fever (ASF) virus are common in China. Such conclusions were made by scientists of the Harbin Veterinary Research Institute. The study was published in Life Sciences, Science China, and follows earlier reports of ASF mutations emerging in China.

In their article, the researchers wrote: "The emergence of natural mutations with lower virulence complicates the early diagnosis of ASF and creates new challenges for the fight against ASF." They added: "Given the huge pig population in China and the endemic period of more than 2 years, it is not surprising to find complex genetic diversity among field ASF viruses in China."

Scientists conducted an observation in 7 provinces of China from June to December 2020. They took 3,660 samples from farms, slaughterhouses and waste disposal facilities in the provinces of Hebei, Heilongjiang, Hubei, Inner Mongolia, Jilin, Liaoning and Shanxi. The experts isolated and characterized 22 ASFv genotype II viruses. All 22 viruses had mutations, deletions or substitutions compared to "HLJ/18", the earliest isolate in China.

The study notes: “11 isolates had 4 different types of naturally occurring mutations or deletions in the EP402R gene and exhibited a non-haemadsorbing (non-HAD) phenotype. An additional 4 isolates were tested for virulence in pigs; 2 were found to be as deadly as HLJ/18. However, 2 non-HAD isolates showed lower virulence but were highly transmissible; infection with a dose of 106 TCID50 was partially lethal and caused acute or subacute disease, while a dose of 103 TCID50 caused non-lethal, subacute or chronic disease and chronic infection.

The researchers also noted that there are no commercial vaccines against ASF, emphasizing that "consequently, the control of the disease depends on the rapid diagnosis and culling of infected animals." The emergence of non-HAD mutations will cause more problems and pose a greater challenge to ASF control in China, as they cause much slower progression and mild, chronic symptoms of the disease while constantly spreading through the oral and rectal routes.

The Harbin Institute is working on an ASF vaccine based on the deletion of 7 genes. Scientists are racing to find out if their vaccine provides protection against newly discovered low-virulence strains.

A few weeks ago, Chinese veterinarian Dr. E. Wayne Johnson said that a mutated version of the ASF virus had found its way into some Chinese pig farms, most likely due to the use of inappropriate smuggled vaccines. In his observations, the MGF360 gene changed - this gene was not mentioned in the Harbin study.