BEIJING (Reuters) - China is culling over 1,300 pigs as authorities rush to control a fourth outbreak of African swine fever and try to trace the origins of the virus that first struck the world’s largest hog herd three weeks ago.

Authorities in the eastern province of Zhejiang said they would slaughter 1,332 hogs and seal off an area within 3 km (1.8 miles) from the pig farming community in the city of Yueqing, where the outbreak was discovered on Thursday.

The local government said in a statement that it had also banned the movement of animals that could be easily infected inside and outside the affected area, and closed some live hog trading markets and slaughterhouses following the outbreak.

The latest cull will bring the total number of pigs deliberately killed to prevent he spread of the highly contagious disease to over 25,000.

The discovery and two other cases reported in Henan in central China and Jiangsu province to the north of Zhejiang are thousands of kilometers away from the first outbreak.

That highlights the challenge the government faces in preventing the spread of disease across the nation and into Southeast Asia.

“It is unclear how this disease is moving through the country,” said the Swine Health Information Center, a U.S. research body.

“It is appearing in a range of farms across swine-dense areas. Trade in pigs, semen and pork products are all possibilities.”

The disease is transmitted by ticks and direct contact between animals, and can also travel via contaminated food, animal feed and people moving from one place to another. There is no vaccine, but it is not harmful to humans.

GRAPHIC: Locations of African swine fever outbreaks across China - reut.rs/2MwkDov

SIMILAR TO RUSSIA

The strain that has struck China is similar to the one that has spread across Russia, Georgia and Estonia over the past decade, a senior veterinarian at the United Nations’ Food and Agriculture Organization (FAO) told Reuters on Friday.

An outbreak in Irkutsk in March 2017 marked the disease’s first long-jump from central-eastern Europe to eastern Russia, 1,000 km from the Chinese border.

In China, experts are still trying to establish the source of the first infection in the city of Shenyang in the northeastern province of Liaoning on Aug. 3, said Wantanee Kalpravidh, project regional manager at the FAO’s Emergency Centre for Transboundary Animal Diseases in Bangkok, Thailand.

They have traced the movement of pigs as far back as March, but they have not found evidence of the virus, she said.

“It is still very inconclusive. The team in China is still working on it.”

The virus which causes African swine fever is hardy and has been shown to remain infectious for at least 30 days in uninhabited pig pens, over four months in pork products, including salted dried hams, and indefinitely in frozen pig carcasses, the Swine Health Information Center said.

“The hardiness of this virus, along with the fact it is found throughout the pig as well as its faeces and saliva, means these leaps of the disease across large distances are possible.”