Spatiotemporal genomic epidemiology of Eurasian avian-like H1N1 swine influenza viruses

Viruses are constantly reshaping themselves, but few do so with the consequences for human health quite like influenza. These viruses carry their genetic instructions in eight separate pieces, like a deck of cards that can be shuffled and dealt to a new host. When a single animal, such as a pig, gets infected with two different flu viruses at the same time, these pieces can mix, creating a brand-new virus with a unique combination of traits. Pigs are particularly good at this mixing because they can catch flu strains from birds, humans, and other pigs. This ability to swap genetic material makes them a potential breeding ground for viruses that could jump to people and cause widespread illness. One specific family of flu viruses, known as Eurasian avian-like H1N1, has been circulating in pigs across Europe and Asia for decades. While it has been around for a long time, scientists have not fully understood how these viruses move between countries, how they change over time, or which factors drive their spread.

A team of researchers set out to map the journey of these viruses across two continents. They gathered a massive collection of genetic data, pulling together 1,703 complete virus genomes from pigs in 19 different countries between 2009 and 2025. By looking at the eight genetic segments of each virus, they could see exactly which pieces came from the original Eurasian strain and which had been swapped in from other flu viruses, such as the one that caused the 2009 pandemic or older human seasonal strains. This allowed them to group the viruses into distinct genetic families, or genotypes, and track how these families appeared, disappeared, or persisted over the years.

The researchers found that while many new combinations of virus genes appeared and vanished quickly, a few specific families managed to stick around for a long time. The most common family, which made up more than half of all the viruses they studied, was made entirely of the original Eurasian genetic pieces. This group was found consistently from 2009 all the way through 2025. However, other families that had swapped in genetic material from the 2009 pandemic virus or other sources also proved surprisingly durable, surviving for over a decade in pig populations. The study revealed that these different genetic families were not spread evenly across the globe. In Europe, the viruses tended to stay closer to their original Eurasian roots, while in Asia, there was a much higher mix of viruses that had swapped in genetic material from other sources. This suggests that the way pigs are raised and moved in these different regions creates distinct environments for the virus to evolve.

To understand how these viruses traveled between countries, the team built a detailed map of viral movement using the timing and location of the samples. They discovered that the spread of the virus was not random; it followed specific pathways with a clear direction. Western Europe emerged as a central hub, acting as a major source from which the virus moved outward to other parts of Europe and into Asia. The movement was not a two-way street but rather a flow, with Western Europe sending viruses to other regions much more often than it received them. When the researchers tested various factors to see what drove this movement, one stood out above all others: the trade of live pigs. The data showed a strong, direct link between the movement of live swine for trade and the movement of the virus. While factors like the size of the pig population or the distance between countries played a role, they were far less significant than the actual transport of animals. This finding suggests that the commercial movement of live pigs is the primary engine driving the spread of these flu viruses across borders.

The study also looked closely at the virus's surface protein, the part that helps it attach to cells and is the main target for the immune system. By analyzing the genetic code, the researchers identified specific spots on this protein that were changing rapidly, likely because the virus was trying to escape detection by the immune systems of the pigs it infects. Most of these changes were happening on the outer, exposed part of the protein, which makes sense as this is the area the immune system sees first. Some of these changes occurred in regions known to be important for how the virus binds to cells, hinting that the virus is constantly fine-tuning its ability to infect its host. The fact that these changes are happening in specific, recurring patterns suggests the virus is under constant pressure to adapt.

The picture that emerges is one of a virus that is both stable and dynamic. It maintains a core genetic identity while constantly shuffling its internal pieces to create new variations. Some of these new variations are short-lived, appearing briefly before disappearing, while others become established and persist for years. The spread of these viruses is tightly connected to human activity, specifically the trade of live pigs, which acts as a bridge carrying the virus from one region to another. The research highlights that Western Europe plays a pivotal role in this global network, serving as a key point of origin for viral spread. By combining genetic data with information about animal movement, the study provides a clearer view of how these viruses evolve and travel. This approach offers a practical way to improve surveillance, suggesting that monitoring the trade routes of live pigs could help scientists detect new, potentially dangerous virus combinations before they become a wider threat. The findings underscore the importance of watching not just the virus itself, but also the human systems that move the animals carrying it.