"Prediction is very difficult, especially if it's about the future."

-Niels Bohr

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If you have followed Minnesota news media this week, you may be confused about pigs, fairs and influenza. On Monday, I recommended that county, regional and state fairs throughout North America eliminate swine exhibits for the remainder of the year owing to the threat of transmission of the novel influenza virus strain H3N2v, from pigs to other pigs and to humans.

State public health officials disagreed, citing the low risk for such infections to occur, particularly given steps to screen out sick pigs from coming to the fair and the increased attention to hand-washing. This is more than a scientific disagreement; it has potentially important public health implications.

To understand what's at stake, a bit of background will help. Dating back to Hippocrates, influenza has been one of the "lion kings" of infectious diseases. Influenza is caused by a respiratory virus and almost always is spread by humans to humans. The symptoms include fever, cough, muscle aches and just plain feeling horrible.

Influenza occurs in two different patterns: annual seasonal epidemics during our winter months or global pandemics, which can occur during any season and last more than a year. An estimated 3,000 to 49,000 individuals in the United States die every year from seasonal influenza, and thousands more require hospitalization for influenza-related disease.

Influenza pandemics occur when novel influenza viruses in animals undergo genetic changes that allow the viruses to infect humans and in turn humans to transmit the new human-adapted virus to others. Four pandemics have occurred in the last 94 years: 1918, 1957, 1968 and 2009.

Influenza pandemics can vary in severity; in 1918, an estimated 50 million to 100 million people died worldwide. In the 1957 and 1968 pandemics, an estimated 1.5 million and 750,000 people died, respectively. An official estimate of worldwide deaths from the 2009 pandemic is not expected until later this year.

The current situation with the H3N2v influenza virus, which contains parts of the H3N2 virus and the 2009 pandemic virus, has many leading international influenza experts concerned that we may be watching an animal influenza virus undergoing the genetic changes needed to make it a virus transmitted to and by humans.

The frequency of animal-to-human transmission of this novel virus is unprecedented and is largely associated with human contact with pigs at county and state fairs in the United States. Could this be the perfect animal/human interface for creating the next pandemic virus? We don't know.

While there are only three documented instances at this time where humans infected by pigs have transmitted this virus to other humans, each new transmission between pigs and humans is another throw at the genetic roulette table.

Research conducted by our NIH-supported Minnesota Center of Excellence for Influenza Research and Surveillance (MCEIRS) and also by investigators at the Ohio State University has demonstrated this summer that the H3N2v virus is being widely transmitted at fairs. Within several days of the fair opening, many of the pigs become infected via airborne transmission.

Visual examination of pigs for illness before admittance to the fair has not prevented transmission as pigs without symptoms shed the virus. Pigs and humans caught in airborne clouds of virus in the fair barns are becoming infected. While hand-washing is always a great idea to reduce the transmission of disease-causing bugs like E. coli and salmonella, it has little impact on airborne influenza transmission.

The bottom line is that none of us know if the H3N2v will lead to a new pandemic, but it might. And the consequences if that were to happen could be catastrophic. That's not just my opinion; it reflects the conclusions of some of the leading influenza researchers from Asia, Europe and North America.

Now is the time to consider what questions will be asked one day if the next pandemic strain of influenza occurs on our watch and we did not take reasonable steps to try to stop it. Pork is safe. But we should do what we can to limit the general public's contact with pigs at this time.


Michael Osterholm is director of Center for Infectious Disease and Research and Policy Director of the Minnesota Center of Excellence for Influenza Research and Surveillance.