How Influenza Reassortment May Make Bird Flu More Dangerous
The genes of the genes could let the species jump from the flu
Flu viruses like bird flu can mix and match their genomes, and this has played a role in at least three of the last four influenza pandemics
Aviary flu, an Orthomyxoviridae family virus. The influenza virus causes an infectious and contagious respiratory disease and often causes a pandemic and / or smaller seasonal epidemic.
James Cavallini / Science Science
Flu viruses are clever entities. They regularly accumulate small genetic changes, requiring annual updates to flu vaccines, because the tension of the previous year may not look much like the following year. But they can also jump suddenly by engaging in major genetic changes which can allow them to jump from one animal species to another or to humans.
A seemingly ingenious and sneaky way for viruses to make these jumps is to exchange genetic equipment with other flu stumps. Called restocking, this exchange occurs when a person or an animal is infected with two types of influenza viruses at the same time. While reproducing inside the host cell, viruses can grasp bits of the genetic code of the other and incorporate them into their own genes of genes.
The restocking is much less common than small mutations that modify the flu from year to year, but it is important: at least three of the last four pandemics of the human influenza have involved a restocking.
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“The Retaoriment has played a major and major role in the emergence of the pandemic flu,” explains Daniel Perez, professor of poultry medicine at the University of Georgia College of Veterinary Medicine, who studies how influenza moves between species.
The last century saw four influenza pandemics. The first was the notorious of the high flu of 1918, which killed around 50 million people. The second took place in 1957, when a new flu killed between one million and four million people worldwide. In 1968, a new flu emerged, killing another million to four million people. Finally, in 2009, a new pig flu appeared, killing between 151,000 and 575,000 people that year.
Flu viruses are classified by two types of protein on their surfaces, hemagglutinin (ha) and neuraminidase (NA). These proteins each have several subtypes, which is why you will see labels such as H1N1 or H5N1. The H refers to the type of Ha protein, and the N refers to the type of NA protein. The big flu that swept the globe during the First World War was an H1N1 flu which probably emerged in Kansas. His descendants circulated in humans and pigs until 1957, when he was suddenly replaced by man by a H2N2 flu. This new virus appeared for the first time in southern China. His main genetic skeleton belonged to the influenza of 1918, known as Perez, but he had acquired three new sequences of genes from an avian flu, exchanging his Ha and Na proteins for new subtypes. For reasons that are not fully understood, this new H2N2 has erased H1N1 in humans for decades – H1N1 would not be revised in people before 1977.
The 1968 pandemic was another restoration event. This time, the H2N2 which was circulating in humans exchanged genes with an avian flu H3N2, probably somewhere in China. (The first identified epidemic took place in Hong Kong.)
Then came the 2009 pandemic, a real “globalized pandemic”, says Perez. In the early 2000s, there had been some sporadic human infections in the United States with so-called triple income viruses that contained human, avian and pig flu. These cases were rare and especially in people who worked on pig farms; These viruses are not transmitted from man to man. This changed in 2009 when triple recorded viruses picked up new genes from Eurasian pork flu. “It is a perfect example of globalization,” says Perez, “because the virus contains not only gene segments of an avian flu, a swine flu [and] From a human flu but also from very different geographic places. »»
The restoration of the flu viruses which infects different species occurs fortunately relatively rarely, explains Charlotte Kristensen, postdoctoral researcher in veterinary clinical microbiology at the University of Copenhagen. “It must be two different viruses, infecting the same host cell, and the restocking must succeed. And it is not always as if the gene segments are compatible, ”she says.
Such a reassortment occurs all the time between the avian influenza strains that infect birds, explains Yuan Liang, also a postdoctoral researcher from the University of Copenhagen. “Especially since 2020, there have been a lot of new variants that have emerged because of the restorations” in birds, says Liang.
The different stumps of H5N1 now circulating in wild birds, domestic and dairy poultry have not yet caused a pandemic in people. It is difficult to say if the virus will remain mainly in animals or if we are now in a period like the one before the 2009 flu pandemic, when agricultural workers have sometimes fallen with a restocking virus which would later gain the sequences of genes which it was necessary to spread from one person to another. No one expected the H5N1 to settle in dairy cattle, says Liang, so the question is now what new unexpected step this virus could take.
“This whole situation really highlights how much we know and how complex,” explains Kristensen.
