CHANGSHA, Aug. 4 (Xinhua) -- After being kept in quarantine for 45 days, 980 out of 1,012 breeding hogs imported from the United States were allowed to leave the isolation field in central China's Hunan Province for farms across China.

They included durocs, landraces and large whites - breeds that grow faster and are superior to China's domestic breeds in many aspects.

The pigs departed from Chicago and arrived in Hunan on a chartered plane in June. They were isolated in a farm with precise temperature control, purified water and 24-hour watch. Professional animal nurses, treatment and massage were also provided.

The 980 that passed health test will be distributed to six farms in provinces of Hunan, Guangdong and Henan as well as Tianjin Municipality.

In 2012, Hunan imported more than 1,000 boars from the U.S. for the first time. Importing boars improved the quality of local breeding swine for Hunan, one of the major pig raising provinces in the country.

The United States is China's largest breeding swine exporter, accounting for 30 to 60 percent of China's total import over the past decade. Due to the outbreak of porcine epizootic diarrhea virus (PEDV) in the U.S. in 2013, the imports gradually fell to zero as of 2016.

But the trade picked up soon this year. In addition to Hunan, southwest China's Yunnan Province also imported 1,057 boars from the United States in June.

Pork is the staple meat for Chinese.