Evolving viruses in pigs pose new swine flu threat

While swine flu viruses have long been considered a risk for human pandemics, and were the source of the 2009 pandemic H1N1 virus, attention has recently turned to the transmission of flu viruses from humans to pigs.

"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 [back] to humans," explains Nicola Lewis, the lead researcher of a new study published in the journal eLife.

"These flu viruses pose a serious threat to public health because they are no longer similar enough to the current human flu strains for our immune systems to recognize them and mount an effective defense. 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."

Humans and pigs both experience regular outbreaks of influenza A viruses, most commonly from H1 and H3 subtypes, and their genetic diversity is well characterized. However, the diversity of their antigens, which shapes their pandemic potential, is poorly understood, mainly due to lack of data, a recent article explains.

To help improve this understanding, Lewis and her team created the largest and most geographically comprehensive data set of antigenic variation. They amassed and characterized antigens from nearly 600 flu viruses dating back from 1930 through to 2013 and collected from multiple continents, including Europe, North America and Asia.

Analysis of their data reveals that the amount of antigenic diversity in swine flu viruses resembles the diversity of H1 and H3 viruses seen in humans over the last 40 years, driven by the frequent introduction of human viruses to pigs.

"Since most of the current swine flu viruses are the result of human seasonal flu virus introductions into pigs, we anticipate at least some cross-protective immunity in the human population, which could potentially interfere with a re-introduction of these viruses. For example, the H1N1pdm09 viruses circulating in both humans and pigs are antigenically similar and therefore likely induce some immunity in both hosts," Lewis said in the article.

"However, for the H1 1C, H3 3A, and H3 3B human seasonal lineages in pigs, the risk of re-introduction into the human population increases with the number of people born after the circulation of the human precursor virus, and is increased by the antigenic evolution of these viruses in pigs. Earlier introduced lineages of human H1 and H3 viruses therefore pose the greatest current risk to humans, due to the low or negligible predicted levels of cross-immunity in individuals born since the 1970s."

While vaccination to control flu in pigs is used extensively in the U.S. and occasionally in other regions, there is no formal system for matching vaccine strains with circulating strains and no validated protocols for standardization and effective vaccine use.

"The significant antigenic diversity that we see in our data means it is highly unlikely that one vaccine strain per subtype would be effective on a global scale, or even in a given region," study co-author Colin Russell, said.

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.

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