US researchers have discovered that the United States can be a breeding ground for certain flu strains, allowing them to spread to warmer regions. Previous assumptions that flu strains primarily originated in China and Southeast Asia have been challenged by a study from the University of Michigan, the Howard Hughes Medical Institute, and Florida State University. Some strains of flu in North America do not disappear at the end of flu season, instead traveling to South America and beyond. This phenomenon may have played a role in the H1N1 swine flu pandemic. Trevor Bedford of the University of Michigan, whose study was published in the Public Library of Science journal PLoS Pathogens, revealed that China and Southeast Asia are the main players in the influenza A migration network, but temperate regions, such as the USA, also play a significant role. Bedford and his team analyzed genetic sequences of seasonal flu viruses collected from patients worldwide between 1998 and 2009, creating a family tree to show the relationships among the viruses. This new understanding of flu may lead to changes in public health strategies for fighting the virus, as aggressive use of antiviral drugs like Tamiflu could potentially lead to drug resistance if flu strains persist in the United States. "We discovered that South America primarily receives flu strains from North America," Bedford stated. "Instead of providing South America with the same vaccine as the rest of the world, it may be more beneficial to create a vaccine using strains from North America from the previous season." The researchers believe that these findings can improve the tracking of flu strains. "Through this research, we gain a better understanding of the origins of flu outbreaks worldwide. While Southeast Asia is known to be a major source, we now realize that temperate regions can also play a role, highlighting the need for global surveillance," Bedford explained. The United States was where the first cases of H1N1 swine flu were identified. The exact origin of swine flu remains uncertain, but genetic research indicates it likely came from pigs and had been present for a long time before being discovered.