Despite their healthy appearance, several pigs on exhibit at a 2009 U.S. state fair competition were infected with swine flu, according to a new study by University of Florida  infectious disease experts.
Up to 20% of show pigs at the 2009 Minnesota State Fair were infected, and an infected animal was also found at the 2009 South Dakota fair, the researchers report in the September issue of the Centers for Disease Control (CDC) and Prevention’s Emerging Infectious Diseases journal.
The findings come in the wake of recent CDC warnings to fairgoers and reports of new swine flu strains, called H3N2 variants, in people who had direct or indirect contact with pigs at agricultural fairs.
“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,” says lead investigator Gregory Gray, 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.”
Between July 12 and Aug. 10, health authorities have confirmed 153 cases of H3N2 variant infections in four states. And with county and state fair season in full swing, the number of infections is expected to rise.
For the summer 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. The researchers collected completed questionnaires from handlers, and swab samples from pigs’ nostrils. All the pigs had been deemed healthy by a veterinarian before being permitted to enter the show.
Of 57 pigs examined at the Minnesota fair, 11 tested positive for swine flu, and of 45 pigs examined at the South Dakota fair, one 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 before they got there.
The presence of influenza A in healthy-looking pigs is consistent with a 2011 Canadian study that demonstrated that as many as 90% of infected pigs may not show clinical signs of viral infection.
In addition, six of seven viruses the researchers isolated from pigs in the 2009 study were identical to the pandemic H1N1 virus that had first been confirmed in humans just five months earlier, in April of that year. In a follow-up telephone survey, two human study participants reported influenza-like illness.
“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,” says Juergen A. Richt, the Regents Distinguished Professor at the Kansas State University 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. 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.”
The UF investigators recommend that swine show visitors follow the CDC’s 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. People who are pregnant, under 5 years of age or who have weakened immune systems should avoid exposure to pigs and pig barns.
Eating pork from a pig infected with influenza has not been shown to cause infection in humans, according to the CDC.
Workers involved in swine farming, food animal veterinarians and people who raise show pigs should receive seasonal flu vaccinations and consider using gloves in their work, Gray says. And swine show organizers should consider using inexpensive rapid diagnostic tests to determine onsite whether the show pigs are carrying viruses that could spread to humans. Such testing is especially important since studies show that accurate infection surveillance cannot be based solely on observation of signs in animals, Gray said.
“We really need a much better understanding of how common these infections are in U.S. pigs and how they are spreading,” Gray says. “Public health, veterinary health, environmental health and the pork industry must work closely together in understanding influenza transmission as some of these viruses can cause significant health problems in man and pigs, as well as major economic harm to agribusinesses.”