University of Florida infectious disease experts have released a study revealing that several pigs exhibited at U.S. state fair competitions in 2009 were infected with influenza virus, despite their healthy appearance.

Up to 20 percent of pigs at the 2009 Minnesota State Fair youth show were infected, and an infected animal was also found at the 2009 South Dakota State Fair. The study is reported in the September issue of CDC’s Emerging Infectious Diseases journal.

“The new H3N2 variant viruses that are circulating now in pigs and apparently affecting people at pig shows are offsprings of the 2009 pandemic virus that spread throughout the world,” said lead investigator Gregory Gray, M.D., M.P.H., chairman of the UF College of Public Health and Health Professions department of environmental and global health, and a member of UF’s Emerging Pathogens Institute. “It mixed with the viruses that were already present in pigs and out has come a new progeny virus.”

For the 2009 study, Gray, then the director of the University of Iowa’s Center for Emerging Infectious Diseases, and his team evaluated pigs and their handlers at the Minnesota and South Dakota state fairs. Researchers collected completed questionnaires from handlers, and swab samples from pigs’ nostrils. A veterinarian had deemed all of the pigs healthy before entering the show.

Of 57 pigs examined at the Minnesota fair, 11 tested positive for the flu virus; one out of 45 pigs examined at the South Dakota fair, tested positive. Most of the flu-positive pigs had nasal samples collected within 24 hours of arrival at the fair, so the results suggest the animals were infected previously.  

Researchers point out that the presence of influenza A in healthy-looking pigs is consistent with a 2011 Canadian study that demonstrated up to 90 percent of infected pigs may not show clinical signs of viral infection.

In addition, six of seven viruses that researchers isolated from pigs in the 2009 study matched the pandemic H1N1 virus that had first been confirmed in humans in April of that year.

“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,” said Juergen A. Richt, D.V.M., at Kansas State University’s College of Veterinary Medicine, who 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 emphasizes that it’s important to increase public awareness that animals, such as pigs, poultry, cattle and sheep can carry viruses that might be of concern to people—and vice versa.

CDC recommends the public these guidelines: Wash your hands before and after exposure to animals, don’t eat or drink in areas where animals are kept and avoid contact with animals if you are experiencing flu-like symptoms or if the animals appear sick. CDC also advises women who are pregnant, children under 5 years old or individuals with a weakened immune systems should avoid exposure to swine.

Hog farm workers, veterinarians and swine exhibitors should receive seasonal flu vaccinations and consider using gloves. Gray adds that swine show organizers should consider using inexpensive rapid-diagnostic tests to determine onsite whether show pigs are carrying viruses that could spread to humans.

“Public health, veterinary health, environmental health and the pork industry must work closely together to understand influenza transmission as some of these viruses can cause significant health problems in man and pigs, as well as major economic harm to agribusinesses,” Gray says.