Genetic diversity, spatiotemporal dynamics and evolution of porcine reproductive and respiratory syndrome virus 2 (PRRSV-2) lineage 1 in China

Viruses are constantly changing, but few are as successful at it as the one that causes porcine reproductive and respiratory syndrome, a disease that devastates pig farms around the world. This virus, known as PRRSV-2, is an RNA virus, meaning its genetic code is written in a molecule that is prone to making mistakes as it copies itself. These mistakes, or mutations, allow the virus to evolve rapidly, often outsmarting the immune systems of the pigs it infects and rendering vaccines less effective over time. For the global swine industry, which relies on healthy herds to feed billions of people, understanding how this virus moves and changes is not just a matter of academic curiosity; it is a critical necessity for economic survival. The virus exists in different genetic families, or lineages, and while many have appeared over the decades, one specific family has recently taken over, becoming the dominant strain in both the United States and China. The question facing scientists is not just which strain is winning, but how it manages to spread so effectively and why it seems to be winning in some places while losing in others.

A team of researchers from Yangzhou University in China set out to answer these questions by looking at the virus on a massive scale. They gathered and analyzed more than 32,000 genetic sequences of the virus from around the world, spanning from 1990 to 2025. Their goal was to map the history of the most successful family of this virus, known as lineage 1, with a special focus on how it behaves in China. By comparing the genetic code of these thousands of samples, the researchers could build a family tree that showed how the different strains are related, where they came from, and how they traveled across the country. They also looked for specific changes in the virus's genetic code that might give it an advantage, such as the ability to hide from the pig's immune system or to mix and match parts of its own DNA with other strains to create new, more dangerous versions.

The study revealed a striking difference between how the virus behaves in the United States compared to China. In the United States, the virus exists in a state of constant competition, with three different sub-groups of the lineage taking turns being the most common. One group might dominate for a few years, only to be replaced by another, creating a shifting landscape of viral diversity. In China, however, the situation is much more one-sided. One specific sub-group, known as L1C, has achieved total dominance. This single strain accounts for more than 86 percent of all the lineage 1 viruses found in China. The researchers found that this strain likely entered the country through a few introduction events and then spread rapidly, outcompeting all other versions. It is as if a single species of plant was introduced to a new garden and, due to perfect conditions, grew so aggressively that it choked out every other plant, leaving a field of nothing but that one type.

To understand how this dominant strain spread so quickly, the researchers traced its movement across the Chinese provinces. They identified a clear pattern of transmission, with the central province of Henan acting as the main hub. From Henan, the virus radiated outward to other major pig-producing areas, with Shandong province serving as a secondary center of spread. The study showed that the virus did not just move slowly from neighbor to neighbor; it traveled long distances, likely carried by the movement of live pigs between farms and markets. This high level of connectivity in China's farming system created a perfect highway for the virus to travel nationwide. The researchers also found that while this dominant strain was spreading, it was not standing still. It was evolving.

The evolution of this virus is driven by two main mechanisms that the researchers uncovered in detail. First, the virus is under constant pressure to change its appearance to avoid detection by the pig's immune system. The study found that the dominant strain has accumulated a large number of specific changes in the genes that code for the proteins on its surface. These changes are concentrated in the areas that the pig's immune system uses to recognize and attack the virus. By altering these spots, the virus effectively changes its "uniform," making it harder for the pig's defenses to recognize it. This is particularly true for the strain that dominates in China, which has more of these protective changes than its relatives in other parts of the world.

Second, the virus is engaging in a process called recombination, where it swaps chunks of its genetic code with other viruses it encounters. The researchers found that this swapping happens most frequently in specific parts of the virus's genome, particularly in the genes responsible for making the virus's outer shell and the genes that help it replicate inside the cell. This mixing and matching allows the virus to create new combinations of traits, potentially making it more infectious or better at evading vaccines. The study showed that the provinces where the virus is most active and where pigs are moved the most are also the places where this genetic mixing is most likely to occur.

The findings of this research paint a clear picture of a virus that has found a highly efficient way to thrive in China's intensive farming environment. The dominance of this single strain is not an accident; it is the result of a cycle where the virus spreads quickly through the movement of animals, changes its genetic makeup to avoid immune detection, and mixes its genes to create new variations. This cycle creates a powerful engine for evolution that keeps the virus one step ahead of control measures. The researchers suggest that to manage this disease effectively, surveillance efforts need to focus on the specific regions where the virus is most active and on the specific parts of the virus's genome where it is most likely to change. By understanding the precise routes the virus takes and the specific ways it evolves, scientists and farmers can develop better strategies to track and control the spread of this persistent and damaging pathogen.