“The infection rate and lethalness of the virus are terrifying,” said Wu Jinhong, a shrimp farmer in Da’ao township in Jiangmen city.

“It only takes two or three days from detecting the first infection for all shrimp in the pond to be killed.”

The first signs of infection are typically when shrimp begin turning a reddish colour, before their shells soften and they sink to the bottom of ponds, local farmers said.

The virus does not discriminate between species and infects “big or small, the Pacific white shrimp or giant freshwater prawn”, said Zhong Qiang, another shrimp farmer in Zhuhai city.

“Once one pond is infected with the virus, there is little our farmers can do given the very high risk of nearby ponds getting infected a few days later,” he said.

Scientists from the Chinese Academy of Fishery Sciences first identified the mysterious virus in Pacific white shrimp, the main species farmed in China, in Zhejiang province in December 2014.

Div1 has received little public attention since then, despite grave concern in the industry that it could spread across the country.

By 2018, the virus had been found in shrimp farms and breeding operations in 11 provinces, Qiu Liang, a researcher with Yellow Sea Fisheries Research Institute, said.

The outbreak eased during the summer and autumn months when temperatures were higher, but made a comeback again in February. Farmers say that temperatures above 30 degrees keep the virus at bay.

“This virus is horrifying shrimp farmers, like avian flu did to poultry farmers and African swine fever to those raising pigs,” said Dai Jinzhi, who found out recently his six hectares of ponds were infected.

After he was forced to drain his enclosures, which held about 3700kg worth of shrimp, he was left with only 200kg of the crustaceans alive – something that cost him more than 100,000 yuan (US$14,000).

“We can’t do anything but remove the shrimp and sell them at dirt cheap prices, then dry out the pond and leave it idle for at least two months,” Dai said.

“Some [other farmers] suffered another hit by the virus after restarting farming in the same pond [too soon]. So I don’t dare start over until the end of next month, when the weather has warmed up.”

The source of the virus and how it is transmitted remain unclear, according to industry insiders and experts.

Because there is no effective way to prevent it spreading, a growing number of shrimp farmers in Guangdong will not allow outsiders, including friends and family, to get close to their ponds – similar to the way pig farmers barred people from getting near their farms due to swine fever.

While new infections are thought to occur “mainly through water and the local environment”, Qiu said there was a possibility it could be carried onto farms by humans.

However, scientists and industry groups admit they know very little about the virus.

“As far as we know, besides China, the virus has also appeared in Southeast Asian waters,” said Huang Jie, director general of the Network of Aquaculture Centres in Asia-Pacific.

He added more attention needed to be paid to it given the threat it poses to shrimp production in China.

“A widespread outbreak could be possible if there isn’t enough concern in the industry and among the relevant [government] departments.”

It is difficult to accurately count the losses caused by the virus as there is no official or third-party data. Usually, a shrimp pond can raise about four batches of shrimps per year, so if one becomes infected with the virus, it means that the annual output of the pond would be reduced by at least a quarter.

As living standards rise in China, demand for shrimp, prawns, lobsters and crayfish has surged in recent years. But domestic output of some crustaceans has been affected by Div1.

The disease contributed to a decline in the annual output of Pacific white shrimp, for instance, to 1.2 million tonnes in 2018 from 1.5 million tonnes in 2013, the 2019 China Fishery Statistical Yearbook said.