The researchers say their findings emphasize the need for stronger surveillance in areas with high density of pig populations and where pigs and humans are in close contact.
This was the conclusion of a new international study published in the journal eLife.
While swine flu viruses have long been considered a risk for human pandemics - they were the source of the 2009 pandemic H1N1 virus - researchers are starting to look at the problem from the other side: what happens to flu viruses that travel from humans to pigs.
A virus cannot replicate without a host cell. In the case of the influenza A virus, the host cell can be that of a pig or a human, and both our populations experience regular outbreaks of influenza A, mostly from H1 and H3 subtypes.
We rely on our immune system recognizing a virus and releasing an appropriate response to protect us from infection. The way the immune system does this is by identifying surface proteins on the virus called antigens.
An abundance of host cells creates opportunities for viruses to evolve as they replicate. This evolution affects not only genetic diversity but also antigen diversity.
'Once in pigs, flu viruses from humans continue to evolve their antigens'
If virus evolution occurs in human populations, then human immunity has a chance to keep up. But what happens if the virus moves to another species, evolves in that population, and then re-enters the human population?
The new study provides novel insights into this potential threat by looking at the antigen diversity of influenza A in pig populations. This aspect of flu virus diversity is not well understood; most previous studies have focused on genetic diversity.
First author Dr. Nicola Lewis, from the Department of Zoology at the University of Cambridge in the United Kingdom, explains what they found:
"Once in pigs, flu viruses from humans continue to evolve their surface proteins, generically referred to as antigens, resulting in a tremendous diversity of novel flu viruses that can be transmitted to other pigs and also to humans."
She says the new flu viruses "pose a serious threat to public health" because their antigens have evolved to the point where they are no longer similar to the ones our immune systems recognize.
"Understanding the dynamics and consequences of this two-way transmission is important for designing effective strategies to detect and respond to new strains of flu."
For their study, Dr. Lewis and colleagues created the largest and most geographically comprehensive database of antigen variation, featuring antigens from nearly 600 flu viruses dating from 1930-2013 and covering several continents, including Europe, the United States, and Asia. The collection includes 200 viruses never studied before.
The researchers say their discovery could be important not only for developing effective pig vaccines but also for improving surveillance and disease control to prevent spread of new flu variants.
The extent of antigen diversity they uncovered suggests it is "highly unlikely that one vaccine strain per subtype would be effective on a global scale, or even in a given region," says co-author Dr. Colin Russell, also from the University of Cambridge.
He adds that the findings emphasize the need for stronger surveillance in areas with high density of pig populations - such as China - and where pigs and humans are in close contact.