Chinese study discovers new hard-to-detect variants of African swine fever

New variants of the African swine fever (ASF) are reportedly circulating in China, which  appear to cause a milder form of the disease, making it less deadly but harder to detect and to bring under control, studies published this month have shown.

According to Reuters, 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.

A second paper by a team at the Harbin Veterinary Research Institute under the Chinese Academy of Sciences also looked at natural mutations of the virus which decimated China’s pig herd during 2018-19 and continues to harm pig herds around the world.

“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, noting that mutant viruses cause a “much more delayed course, and mild, chronic signs, while being continuously shed via the oral and rectal routes.”

The new findings are based on sampling from seven provinces during the second half of 2020. The team found 22 isolates with mutations, and tested four of those for virulence in pigs. Two isolates found were as lethal as the earliest virus circulating in China. Two further isolates showed lower virulence with symptoms varying from partially lethal to non-lethal depending on the dose given to pigs.

Last month there were reports that that at least two new strains of ASF had been found on Chinese pig farms, which appeared to be man-made. The strains are causing a chronic form of ASF that is impacting production on sow farms, industry insiders have said, with the disease also harder to detect.

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

“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,” Dixon said.

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