(HealthDay) -- They were being shown off as healthy porcine specimens, but several of the show pigs at the Minnesota State Fair in 2009 actually were infected with swine flu, according to a new study.
Researchers said as many as 20 percent of the show pigs at the fair were infected, and another infected pig was discovered at the 2009 South Dakota State Fair.
Federal health officials have warned recently that strains of swine flu are on the march and could reach people through agricultural fairs.
"The new H3N2 variant viruses that are circulating now in pigs and apparently affecting people at pig shows are offspring of the 2009 pandemic virus that spread throughout the world," lead investigator Dr. Gregory Gray, chairman of the University of Florida College of Public Health and Health Professions, said in a university news release. "It mixed with the viruses that were already present in pigs, and out has come a new progeny virus."
More than 150 cases of infection with the variants in humans were confirmed in four states earlier this summer.
The new study looks at the results of examinations of show pigs in 2009; veterinarians had previously ruled them to be healthy.
Eleven of 57 pigs at the Minnesota fair turned out to be infected with swine flu, along with the one of the 45 pigs at the South Dakota fair. The findings suggest they were infected before reaching the fairs.
Several of the viruses in the pigs were identical to the pandemic H1N1 virus that was found in humans in 2009.
"Pandemic influenza is a good example of how certain strains of influenza can spread in both human and pig populations at the same time -- this is obviously of serious concern," Dr. Juergen Richt, a professor at the Kansas State University College of Veterinary Medicine, said in the news release. Richt was not involved in the study.
"The recent sharp increase in cases of influenza associated with the novel H3N2 variant at local and state fairs throughout the Midwest further confirms this phenomenon," he added. "It is important that we increase public awareness that animals, such as pigs, cattle and sheep, can carry viruses that might be of concern to the general public."
If you're visiting a fair, University of Florida investigators recommend these guidelines from the U.S. Centers for Disease Control and Prevention: Wash your hands before and after you're around animals, don't eat or drink near them and avoid animals if you feel sick or if they look ill. Pregnant women, children younger than 5 or anyone with a weakened immune system also should avoid exposure to pigs and pig barns.
The findings appear in the September issue of the CDC's Emerging Infectious Diseases journal.
Explore further: Flu cases from county fair traced to pigs
For more about swine flu, try the U.S. National Library of Medicine.