Hebei, China (CNN) Sun Dawu's pigs started dying from a mysterious virus last December. Four months later, all 20,000 in one farm were dead.

Of those, 15,000 were killed by the virus. The other 5,000 were destroyed as a precaution.

"It began with a few pigs a day, then it was hundreds," said Sun, a pig farmer and agricultural entrepreneur from Hebei province, China. "In the end, 800 pigs would die in a single day."

African swine fever (ASF) is decimating China's pork industry, by far the biggest in the world. Dutch bank Rabobank, which lends to the global agricultural sector, estimates the country's pig population could shrink by a third in 2019 -- up to 200 million animals -- through a combination of the disease and culling.

To put it in context, that's almost as many pigs as in the US and Europe combined.

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