Covering four consecutive flu seasons between 2008 and 2012, the team examined spatiotemporal relationships between swine production and human flu for all 100 of North Carolina counties.

The state is the nation's second largest swine producer and is home to nearly 10 million pigs.

Flu activity peaked earlier during the 2009-2010 and 2010-2011 seasons, both of which were dominated by the 2009 H1N1 pandemic strain. The team didn't see the same association in seasons before and after those two seasons when swine-derived flu viruses weren't circulating.

Concentrated swine-feeding operations amplified flu transmission when H1N1 was circulating, researchers said.

However, they theorized that the finding might not be related to swine, but perhaps to other factors, such as other disease-driving dynamics—such as high-population density—in more urban areas. They wrote that the findings suggest a benefit for vaccine strategies that target swine workers and underscore the importance of virus surveillance in areas with high concentrations of swine production.