New Chinese study finds milder variants of African swine fever virus

BEIJING (Reuters) - New variants of the African swine fever virus circulating in China appear to cause a milder form of the disease, making it less deadly but harder to detect and to bring under control, a study published this week showed.

The paper by a team at the Harbin Veterinary Research Institute under the Chinese Academy of Sciences is the second this month to report on natural mutations in the virus which ravaged China’s pig herd during 2018 and 2019 and continues to kill pigs in the world’s biggest pork producer.

The first study from the Military Veterinary Institute in Changchun reported finding a virus that had a partial deletion of genes, previously shown to protect pigs against the African swine fever (ASF) when deleted. However that study did not investigate the virulence of the variant.

It comes amid growing concerns in the industry over the evolution of a disease with no approved vaccine.

"The emergence of lower virulent natural mutants brings greater difficulty to early detection and poses new challenges for the control of ASF," Sun Encheng and colleagues wrote in the Life Sciences journal. (

They noted the mutants cause a “much more delayed course, and mild, chronic signs, while being continuously shed via the oral and rectal routes.”

The new findings come from sampling from seven provinces during the second half of last year. The team found 22 isolates with mutations, and they later tested four of those for virulence in pigs.

Two isolates were just as lethal as the earliest virus circulating in China. But two showed lower virulence with symptoms varying from partially lethal to non-lethal depending on the dose given to pigs.

Some analysts estimate about 20% of sows in northern China have been impacted by the disease this winter.

Reuters reported last month that at least two new strains of African swine fever had been found on Chinese pig farms, which appeared to be man-made.

The strains are causing a chronic form of African swine fever that is impacting production on sow farms, industry insiders have said, with the disease also harder to detect.

It is not clear how common the new variants identified by the Harbin team are on pig farms. Samples were taken from farms, slaughterhouses and disposal plants in Heilongjiang, Jilin, Liaoning, Shanxi, Inner Mongolia, Hebei and Hubei, they said.

The study is “very important” for understanding the epidemiology of African swine fever in China, said Linda Dixon, an expert on the disease at Britain’s Pirbright Institute.

It should be extended to determine how much the variants are circulating and if they are increasing or decreasing relative to the highly virulent isolates, she added.

“It would be beneficial for other countries, particularly in Asia, to monitor for circulation of variants of reduced virulence to ensure their control measures are appropriate,” said Dixon.

Unlike the strains described by industry insiders to Reuters, the new variants identified in the study found the MGF505 and MGF360 genes to be unchanged.

The researchers added that verifying the efficacy of a vaccine currently under development at Harbin needs to be “urgently evaluated” against the new strains.

Reporting by Dominique Patton; Editing by Ana Nicolaci da Costa and David Evans