* Blood must be heated to 80 degrees C, stored for weeks
* Canada says rules disappointing, not scientific
* U.S. seeks clarity, plans no change to its rules (Updates with U.S., Canada and industry comment, background)
By Barbara Lewis and Tom Polansek
BRUSSELS/CHICAGO, May 6 (Reuters) - The European Commission has approved new rules aimed at limiting the spread of a virus that has killed millions of piglets in the United States, highlighting the risk of animal feed products as a potential transmission agent.
Porcine Epidemic Diarrhoea virus (PEDv) has wiped out more than 10 percent of the U.S. pig population since it appeared a year ago and has also struck in Canada, Mexico and Japan. The European Union has not been affected by this strain.
In the United States, the world's biggest pork exporter, losses from PEDv could cut pork production as much as 7 percent in 2014, according to research firm Rabobank, much steeper than government estimates of a 2 percent fall.
While the Commission stopped short of an outright ban of pig byproducts, which France considered, the new rules include a requirement that any pig blood products imported to the European Union for use in pig feed must have been treated at 80 degrees Celsius and then kept in storage for six weeks at room temperature to ensure any PEDv is deactivated.
The United States said its Animal and Plant Inspection Service (APHIS) would be working with the EU to get clarity on the rules and when they would take effect. APHIS said no changes were being considered to U.S. regulations for treating feed.
"Ongoing research will help determine the possible role of feed in the domestic transmission of PED," Abby Yigzaw, a spokeswoman for APHIS, said in an email.
The USDA finally responded to calls for more reliable data and classified PEDv as a reportable disease last month, a step that requires the pork industry to track its spread. The virus is not a risk to human health and is not a food safety issue.
The highly contagious virus is known to be transmitted among pigs through faeces, but the possibility of a link with pig blood products used in feed is not proven, and scientists continue to search for its origins and a cure.
Canada alarmed the farm and feed industries in February when it said it had determined the virus was present in samples of U.S.-origin plasma. It was unclear whether the feed was capable of causing the disease in piglets.
Canada's agriculture minister, Gerry Ritz, said it was "disappointing" that the EU had taken the measures, which he said were not founded in science.
Spray dried pig plasma is mixed with feed and fed to piglets as an extra source of protein. U.S. plasma producers are already largely using the temperature benchmark, but the storage requirements are more extensive than current practice.
"Six week storage before release is not typical," Louis Russell, president of APC Inc, which processes blood products, said in an email.
He added that U.S. manufacturers currently held porcine plasma for 14 days at room temperature.
Dan Shields, director of quality control and nutrition for Merrick's Animal Nutrition, which makes feed for baby animals, said six weeks seemed "extraordinarily long, but I can also see you want to err on the safe side."
The EU imports about 2.2 tonnes of pig blood from the United States annually, a spokesman for the European Commission said.
The European Commission said it decided against changing the rules governing imports of live pigs because regulations were already strict. U.S. and Canadian authorities also told EU officials that no live pigs were scheduled to be dispatched to the European Union, according to the EU statement. (Additional reporting by Sybille de la Hamaide in Paris, Meredith Davis in Chicago and Rod Nickel in Winnipeg; Editing by Foo Yun Chee, Greg Mahlich and Andre Grenon)