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WASHINGTON (AP) ― Preliminary tests show three pigs in Minnesota may have contracted the swine flu virus making them the first potential U.S. cases in swine, agricultural officials said Friday. They stressed the finding does not threaten food safety.

The samples were taken from pigs shown at the Minnesota State Fair between Aug. 26 and Sept. 1 as part of a university research project. Officials expect results next week to confirm whether the pigs were infected with swine flu virus, also known as H1N1.

The pigs did not show signs of sickness and officials said they likely contracted the virus from some of the nearly 1.8 million people who visited the fair.

The Department of Agriculture's veterinary lab in Ames, Iowa, is conducting tests to confirm the results, Agriculture Secretary Tom Vilsack said in a statement.

In a conference call with reporters Friday, Minnesota Agriculture Commissioner Gene Hugoson said officials don't know what happened to the three pigs, but that they probably were sent to slaughter soon after they were shown at the fair, which ended on Labor Day.

"This is not an issue of food safety," Hugoson said. "Pork products are and continue to be safe to eat."

Authorities aren't planning special measures if the tests confirm the Minnesota pigs had the virus. Hugoson said farmers will continue watching herds for flu symptoms, and slaughterhouse inspectors will continue rejecting pigs showing disease symptoms.

Agriculture officials had long expected the virus to reach domestic pigs this year and have guidelines to slow its spread, said Jeff Bender, director of the University of Minnesota's Center for Animal Health and Food Safety. Herd infections previously were reported in Canada, Australia, Argentina, Ireland, the United Kingdom and Norway. A hog vaccine for the virus is being developed but isn't yet available.

Jose Diez, a USDA veterinary official on Friday's call, said killing pigs to stop the spread of the virus has not been considered. He and other officials said swine usually recover quickly from influenza, and generally are sent to slaughter after they're healthy and ready for market.

While the chance of a pig infecting a person is considered remote, the animals can act as mixing vessels if they happen to catch two different strains at the same time, allowing mutation of a new one. Officials said there's no evidence that's happened.

Still, the news was clearly unwelcome for the pork industry, which has worked to distance itself from the swine flu virus.

"At the end of the day this is not a big deal," said Mike Wegner, a spokesman for the National Pork Board. "There is no reason for anyone to be concerned about the safety of pork, either eating or handling."

Hugoson said it's too early to gauge the impact of Friday's news on the industry, which already was struggling with decreasing exports fueled by virus fears.

Duane Woebbeking, a hog producer outside of Gladbrook, Iowa, said Friday's news presented a potential "public relations risk" to pork producers.

"I'm more concerned with the public fear," he said. "How many thousands of people die a year from the flu? Most years nobody thinks about it, but now everyone is up in arms because of this H1N1 thing."

Minnesota is the country's No. 3 pork-producing state behind Iowa and North Carolina. Minnesota pork producers had 7.3 million hogs and pigs as of Sept. 1, according to USDA figures, while the national inventory was 66.6 million head. The pork industry contributes nearly $1.5 billion and more than 21,000 jobs to the state's economy, according to the National Pork Producers Council.

More than 100 students from two 4-H programs were sent home from the Minnesota fair Sept. 2 after health officials confirmed four students had come down with swine flu. Friday's USDA announcement said current information suggests the children were not sickened by the pigs. The state officials said odds are that they didn't pass the virus to the animals, either.

The National Pork Producers Council issued a statement on the testing that said: "pork is safe to eat and handle and that, according to the Centers for Disease Control and Prevention, flu viruses cannot be transmitted through food, including pork."

(© 2009 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)