The H9N2 strain shows mutations that increase its affinity for humans. Experts are calling for more global surveillance
For over twenty years, when we talk about avian flu we are referring to H5N1 virusthe mutation first identified in 1996 and responsible for most of the epidemics recorded worldwide in recent years. If global attention is continuing to focus on this mutation, responsible for serious infections in animals and, in rare cases, also in humans, there is, however, another strain hitherto considered minor which could pose a more subtle threat in the coming years.
A minor strain of bird flu could trigger a pandemic
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It is about theH9N2, a virus that has been circulating in poultry farms in Asia for years and which, according to a new study from the University of Hong Kong, is developing mutations that they would make it more capable of infecting humans. The alarm was raised during Pandemic Research Alliance International Symposium of Melbourne and the results have been published in the magazine Emerging Microbes & Infections.
A virus not to be underestimated
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To date, H9N2 has been classified as a low pathogenic virusbecause it causes only mild or moderate symptoms in birds. But according to Kelvin To, a clinical microbiologist at the University of Hong Kong, this label risks underestimating a growing danger. “H9N2 is the second most common strain of avian influenza that infects humans, after H5N1. At least 173 human cases have been recorded since 1998, almost all in China, but the real number could be much higher,” To explained.
This happens, according to Michelle Wille, virologist at the Peter Doherty Institute for Infection and Immunity in Melbourne, because many infections go unnoticed: “They often do not cause serious illness or hospital admissions, and in many countries diagnostic tests focus on H5N1, leaving out other strains such as H9N2.”
Mutations that increase affinity with humans
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The study conducted by To’s group shows that, starting from 2015, the H9N2 virus has accumulated significant genetic changes which increased its ability to bind to receptors present in the human respiratory tract. In the laboratory, a version of the virus isolated in 2024 infected many more human cells than a historical sample from 1999. This, the authors explain, is due to changes in the virus’s two surface proteins — haemagglutinin (HA) e neuraminidase (NA) — which function as “molecular keys” to enter host cells.
“The researchers analyzed the genes coding for HA and NA nucleotide by nucleotide. They discovered that these mutations allow the virus to attach to human respiratory mucosal receptors, as well as those of birds. It is a clear signal of evolutionary adaptation”, he underlined Gian Carlo Icardidirector of Hygiene at the San Martino Polyclinic and professor at the University of Genoa, to the newspaper The Republic.
The risk of “species leap”
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The good news, for now, is that nThere is no evidence that H9N2 transmits sustainably from person to person. If the so-called species jump has not yet occurred, however, it does not mean that this will not happen in the future: “For an influenza virus to become pandemic it must be able to stably bind to human receptors, adapt to growing at temperatures and pH different from those of birds and spread easily between individuals of the same species. H9N2 is not yet at that point, but it has already taken important steps in the direction of adaptation”, warned Icardi.
A further risk factor is represented by the so-called genetic reassortmentthe exchange of genetic material between different viruses within the same infected host. If, for example, an animal contracts multiple flu strains at the same time, Virus genes can recombine, giving rise to new variants that are potentially more infectious for humans. “Genetic fragments of H9N2 have already been found in other human influenza viruses of the past”, recalls To.
“This shows that it actively participates in the evolutionary mechanisms that generate new strains”, concluded Professor To, according to whom it is essential right from the start strengthen virological surveillance in poultry farms and among mammals in close contact with birdssuch as pigs, cats or mink and force countries to report all cases linked to low pathogenic avian strains like H9N2, which to date still does not happen.
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