By analyzing the mutations in swine flu samples, scientists have assembled the most complete model to date of the pandemic's birth.
Traditional modeling — the pins-on-a-map approach — relies on formal reports of a disease. Although these convey the big picture, they don't always give a clear picture of how the disease spread.
But because flu viruses constantly pick up genetic mutations as they multiply, scientists can deduce a family tree by comparing the shared ancestry of their genomes.
"This helps reveal hidden information about the spatial spread of the virus," said Marc Suchard, a University of California at Los Angeles biomathematician and co-author of the analysis, which was published last week in Public Library of Science Currents.
Other co-authors were Rega Institute molecular epidemiologist Philippe Lemey and Andrew Rambaut, the University of Edinburgh virologist whose genetic analyses provided the earliest insights into swine flu's evolution into a human-infectious form.
The researchers ran 242 viral genomes, collected around the world between late March and mid-July, through algorithms that determined their most likely evolutionary path. From hundreds of trillions of possible configurations, the program arrived at the models above and below.
The model "helps us learn about the process by which the epidemic evolves," said Suchard. "We can learn about the underlying epidemic process, and apply it to the future."
Note: The video below is newer, but doesn't clearly show swine flu spread outside North America. The video above is older and contains fewer data points, but gives a more complete sense of the virus' global jumps.
Citation: "Reconstructing the initial global spread of a human influenza pandemic: A Bayesian spatial-temporal model for the global spread of H1N1pdm." By Andrew Rambaut, Philippe Lemey and Marc Suchard. PLoS Currents, September 2, 2009.
Video: Philippe Lemey
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