BEIJING (Reuters) - China reported a new African swine fever case on a farm with nearly 20,000 pigs on Monday, the largest farm yet to report the highly contagious disease in the world’s top pork producer.

The new case, one of numerous to be reported in northeast Liaoning province in recent days, underlines the escalating threat to the country’s $1 trillion pig industry from the disease despite a slew of initial measures imposed to curb its spread.

Pig prices have plunged in China’s northeast after farmers were unable to transport their herds out of infected provinces, while prices in the south have surged.

Farms owned by large companies typically have stronger biosecurity, or measures to guard against the spread of the disease, than China’s hundreds of thousands of small farms, and have until now not reported any outbreaks.

But one of three new cases confirmed in Liaoning on Monday occurred on a 19,938-head farm, according to a statement published on the website of the Ministry of Agriculture and Rural Affairs.

For a graphic on Swine fever in China, see - tmsnrt.rs/2PDt6Ud

“The fact that the disease was confirmed on a big pig farm showed that it got more serious,” said Yao Guiling, an analyst with consultancy China-America Commodity Data Analytics.

“Big companies usually have better biosecurity measures. (The new outbreak) highlights the difficulty in controlling the disease. Companies will probably rethink or slow their output expansion plans,” Yao said.

The statement did not identify the farm nor its owner where 221 pigs died but said it was in Jinzhou city.

Another farm of 1,571 pigs in Panjin city also reported an outbreak, which killed 109 pigs, and a smaller farm, also in Panjin, said 129 out of its 270 pigs died from the disease.

The disease was also reported in Tianjin city last week, fuelling fears that it remained far from control.

News of the spread of the disease sent shares at top domestic pork producer Henan Shuanghui Investment & Development Co Ltd closing down 7 percent on Monday.