Pigs in Minnesota may have tested positive for the H1N1 virus in a preliminary test, which would be the first U.S. cases in swine, Agriculture Department officials said Friday.

The officials cautioned that further tests were needed to confirm that the pigs had been infected with H1N1, also known as swine flu virus. The pigs did not exhibit signs of sickness and may have been infected with the virus by a group of children carrying it, they said.

Samples from the pigs that may have tested positive were collected at the Minnesota State Fair between Aug. 26 and Sept. 1. USDA officials did not say how many pigs might have tested positive.

Agriculture Secretary Tom Vilsack said in a statement that testing was under way and results should be available in a matter of days. He said the USDA was working with the federal Centers for Disease Control and Prevention and his agency's National Veterinary Services Laboratories would conduct tests to confirm the results.

Vilsack asked for caution from consumers and said people should not react to the news by avoiding pork products.

"I want to remind people that people cannot get this flu from eating pork or pork products," he said.

Pigs regularly get influenza viruses and recover quickly. While the chance of a pig infecting a person is considered remote, they are of interest because they can act as mixing vessels if they happen to catch two different strains at the same time, which can allow mutation into a new virus. Agriculture officials already were working on a pig vaccine and would quarantine and monitor infected herds.

Still, the news clearly was unwelcome for the pork industry, which has worked assiduously to distance itself from the H1N1 virus.

Mike Wagner, a spokesman for the National Pork Board, stressed that no threat to public health exists. "Pigs get sick from the flu every winter, just like humans get sick from the flu every winter," he said.

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."

Around 36,000 Americans die of flu every year.

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.

Agriculture officials have said they expected H1N1 to reach domestic pigs this year. It has led pork producers to push for a hog vaccine for the virus. H1N1 infections of swine herds have previously been reported in Canada, Australia and Argentina but not previously the United States.

The potentially positive test was discovered by a CDC research project conducted by the University of Iowa and University of Minnesota, which is documenting instances of influenza viruses where humans and pigs regularly interact, such as state fairs.

A record crowd of nearly 1.8 million people attended the 2009 Minnesota State Fair, an annual event.

More than 100 students were sent home from the fair on Sept. 2 after health officials confirmed four students had come down with swine flu. Friday's USDA announcement said no link between the pigs and the children had been made and current information suggests the children were not sickened by the pigs.