A four-year survey of swine influenza cases in Denmark reveals a surge of an especially worrisome novel reassortant virus. Researchers found that this virus has an increased risk of infecting humans and greater potential for human-to-human transmission.
In 2009, a pivotal event for global public health occurred in Mexico. A type of influenza virus previously confined to pigs made the jump to humans. The resulting 2009 swine influenza pandemic spread rapidly, infecting an estimated 11–21% of the world’s population and causing close to 575,000 deaths within a year.
This highlights why scientists closely monitor influenza in pigs. Now, a team of researchers from University of Copenhagen and Statens Serum Institut has tracked the development of swine influenza in pigs in Denmark between 2019 and 2022.
Their findings reveal that H1pdm09N1av, a specific novel reassortant subtype of swine influenza virus, is surging among Denmark’s pigs. Novel reassortant viruses are created through reassortment: the process by which related segmented viruses exchange genome segments. H1pdm09N1av is especially worrisome because it has an increased risk of jumping to humans and be transmitted between people.
“Our study suggests that if the H1pdm09N1av subtype jumps from pigs to people, it could spread rapidly, since we have very little resistance to it. This type of research helps to obtain insight into what may be on the horizon, enabling us to prepare for it by, for example, already thinking about developing vaccines,” explains a researcher behind the study, Pia Ryt-Hansen, Assistant Professor, Department of Veterinary and Animal Sciences, University of Copenhagen, Denmark.
The research has been published in Emerging Microbes & Infections.
Monitoring swine influenza since 2011
Swine influenza – or indeed, any type of influenza – is far more complex than it might seem.
Influenza viruses are constantly evolving, often by acquiring advantageous genes from one another through reassortment. This enables them to adapt more effectively to their current hosts and increases their ability to infect new ones – and over time, may even enhance their capacity to jump between species.
H1pdm09N1av, was first detected in Denmark in 2018. Because Denmark has monitored influenza viruses in pigs since 2011, the researchers tracked the proportion of total swine influenza cases attributed to H1pdm09N1av from 2018 to 2022.
“We are interested in H1pdm09N1av because it has previously jumped from pigs to a human. Transmission between pigs and humans is rare and has only occurred twice in Denmark, but on one occasion, the virus was H1pdm09N1av,” says Pia Ryt-Hansen.
She highlights that the concern with influenza viruses extends beyond pig-to-pig transmission because these viruses can also be transmitted from pigs to people – and unlike human influenza, swine influenza is not primarily confined to the winter months.
“Modern pig farming is more intensive than in the past, creating optimal conditions for influenza to circulate throughout the year. This year-round susceptibility not only affects their health but also increases the likelihood of transmission to people, heightening the overall risk of pig-to-human transmission,” notes Pia Ryt-Hansen.
Airborne transmission
The analysis of data collected on H1pdm09N1av between 2018 and 2022 reveals a considerable rise in the transmission of this specific type of swine influenza, increasing from a single case in 2018 to accounting for 27% of all swine influenza cases by 2022.
Pia Ryt-Hansen explains that since 2022, researchers have observed only a slight increase in the proportion of infections caused by H1pdm09N1av, suggesting that its spread may have stabilised.
In addition, the researchers conducted an experiment to evaluate the potential for transmitting H1pdm09N1av. This study involved ferrets, which are frequently used as a model for studying human influenza.
The researchers tested whether ferrets housed in the same cage could transmit the virus after one had been deliberately infected. They found that ferrets sharing the same cage could transmit the virus. Further, they discovered that ferrets could also transmit the virus through the air, even without direct contact or sharing a cage.
“This is worrisome,” says Pia Ryt-Hansen. “Although pigs have sporadically transmitted to people, we have not yet seen human-to-human transmission. However, if the virus can transmit through the air, we assume that the risk of transmission between humans increases.”
No human immunity
Finally, the researchers conducted an experiment to analyse blood samples from individuals across age groups, aiming to determine whether certain segments of the population had stronger antibody protection against H1pdm09N1av. All age groups were almost entirely defenceless, with only 3% of individuals possessing antibodies to H1pdm09N1av.
“This indicates that H1pdm09N1av jumping to humans could pose significant challenges because it could spread unrestricted, since our lack of immunity would not slow it. This situation would resemble the events of 2009, when a similar leap from pigs to humans led to a global pandemic. Without intervention, such an outbreak would result in widespread infections and many deaths. To mitigate this risk, we must improve preparedness. This includes, for instance, developing and producing vaccines that can be administered to vulnerable populations in the event of a pandemic,” concludes Pia Ryt-Hansen.
