The epidemic spread to almost half of China in 2006 and was atypical of previous outbreaks of the disease, the scientists said in a paper published in the June issue of the Public Library of Science.

It affected more than two million pigs and killed 400 000 of them, they said.

However, China's top vet said earlier this month that a variation of the Porcine Reproductive and Respiratory Syndrome (PRRS), or blue ear disease, killed a million pigs in 2006 and more than 18,000 hogs in the first five months of 2007.

The scientists revealed details of the new virus strain and said it may be responsible for several new and unusually severe symptoms, even in adult pigs.

Although a serious swine disease, PRRS in the past mainly caused reproductive failure in pregnant sows or respiratory tract distress in suckling pigs. Usually, it was the piglets that succumbed.

But this new virus strain spread very quickly, resulting in "unparalleled large-scale outbreaks" and caused high fever (40-42 degrees Celsius), shivering and a reddish rash brought on by congestion in the blood capillaries, the scientists wrote.

Autopsies confirmed that multiple organs, including the brain, spleen, lymph nodes, liver, heart, tonsil and kidney were infected, they said.