African swine fever is a highly contagious virus which is fatal to pigs. China was ravaged by the disease in 2018 and 2019.
And now a new variant has reportedly emerged at a farm in Hong Kong.
Veterinarian Dirk Pfeiffer told German media outlet Deutschlandfunk: “We just had our first outbreak in Hong Kong on a pig farm.”
When the pigs became ill at the beginning of February it was not thought to be African swine fever as the symptoms were too mild.
Mr Pfeiffer said: “Our people struggled to diagnose this because it looked unusual.”
A similar case reportedly occurred in January 2021 at a factory in mainland China.
The sows only had mild symptoms but gave birth to fewer piglets.
An investigation found two new variants of the virus.
The variants lacked certain genes, which made the disease less fatal.
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He said: “The farmers are just desperate and some of them, it was said, were playing around with vaccines, weren’t they?”
The Chinese government has warned against the use of unauthorised vaccines.
Another possible explanation for the new variant of African swine fever is thought to be natural evolution.
A paper from the Harbin Veterinary Research Institute, which was published last month, said researchers had discovered 22 new naturally occurring variants after collecting 3,660 samples in seven Chinese provinces
Pigs infected with two of the new mutations had a weaker version of the disease.
But experts have warned mild symptoms could make the virus more difficult to control.
Additional reporting by Monika Pallenberg.