Lhe viral surge that has affected the United States since 2020 worries scientists a little more every week. However, the H5N1 virus has been traveling the world for almost thirty years. With massive infections in poultry farms and sporadic human cases, the specter of a pandemic even regularly arises in the sights of global health authorities.
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“This virus has concerned experts since it was first found in humans. It was in Hong Kong in 1997,” explains Professor Antoine Flahault, epidemiologist, director of the Institute of Global Health in Geneva. During all these years, several hundred human cases, some of which were fatal, were recorded mainly in Asia. All of these patients were contaminated through contact with an animal. For a year, concern has nevertheless been growing around the unprecedented profile of the virus circulating on American territory, reinforced this Wednesday, December 18 by the announcement of a first patient in a ” grave » in Louisiana, while California declared a state of emergency. Six-point explanations.
Mammals are more easily affected
For the first time, the avian flu virus has infected dairy cows. “The intensity and acceleration of the current North American bovine epizootic and the proximity of these farms to humans reinforce the level of concern regarding the risk of transmission of this avian virus within the human species,” points out Antoine Flahault. American authorities have recorded a total of more than eight hundred and forty infected herds in sixteen states with very rapid progression in recent weeks, which pushed California Governor Gavin Newsom to declare a state of emergency this Wednesday.
In October 2024, a mutation facilitating viral replication in mammals was identified in a virus originating from an agricultural worker infected by his herd of cattle. Minks, seals or even sea lions… In total, nearly thirty species of land and marine mammals were affected.
“This H5N1 virus is already circulating widely around the world in wild avifauna. There was even a very strong “panzootic” [pandémie chez l’animal, NDLR] decimating colonies of seabirds in particular and also affecting mammals, farmed mink in Spain, wild sea lions in South America. On the other hand, dairy cows remain a purely North American peculiarity to this day,” continues the specialist.
American health authorities did not react soon enough
He’s running, he’s running the virus. For almost a year, numerous criticisms have been raised in the United States and around the world against the administration’s lack of responsiveness. Surveillance systems in the dairy sector have proven to be largely faulty, contributing to the spread of the virus on farms by transporting animals to states that are still free of the virus. “Workers in the sector have not been systematically tested, the genetic sequences of the viruses taken have not always been carried out or shared. The interests of the dairy industry have been prioritized at the cost of endangering public health,” laments Antoine Flahault.
Human cases that intrigue doctors
At the last available count, more than 60 human cases have been reported in the United States, with no human-to-human transmission identified. On Wednesday, December 18, the American authorities communicated about a first “serious” case in Louisiana. “To my knowledge, we do not know their precise mode of contamination, but it is often difficult to trace the origin of viral contamination. We saw it for Covid, it is often the same for the flu, particularly H5N1. In any case, no chains of human-to-human transmission have been found. The contamination of certain cases, such as young Canadians, is most likely due to contact with a living infected bird or possibly with a living infected mammal,” explains Antoine Flahault.
ALSO READ United States: first case of avian flu detected without animal contactThe other alert that worries doctors dates from November 2024, when a pig farm was affected. The appearance of avian flu in pigs – already documented during previous epizootics – increases the risks of the emergence of a virus adapted to humans. Indeed, the cells of the pig’s respiratory system have the particularity of having receptors that are as well adapted to mammalian viruses, including humans, as to bird viruses.
The simultaneous infection of farm pigs by the H5N1 virus circulating in American cows and by that of the human seasonal flu circulating this winter increases the opportunities for genetic exchanges between the two pathogens. The fear is therefore to see the emergence of a new influenza virus that is potentially very contagious and dangerous for the human species. As there has been no epidemic of human influenza with an influenza virus of subtype H5, population immunity would be completely naive in the face of such a virus.
The latest strains analyzed raise fears of adaptation to humans
On December 5, a study published in the journal Science indicates that a single genetic mutation “would be enough to cause [le H5N1] from avian to human specificity”. According to this study, “the appearance of this single mutation could be an indicator of a risk of a pandemic”. “In total, a few final mutations in the genome of the H5N1 virus would still be needed for it to truly cause an epidemic,” specifies Antoine Flahault. They may not occur quickly, or may never occur. The whole challenge is to slow down as much as possible the process which would lead to favoring these mutations.
“One of them concerns an enzyme of the virus called polymerase and which would allow the replication of the virus in human cells; this could occur in the fairly short term,” fears the scientist. The other two concern hemagglutinin. One would allow the virus to attach to the virus’s surface receptor and the other would allow the virus to resist in a more acidic aerosol environment than the one in which it currently evolves. “Without this last mutation, effective respiratory transmission of H5N1 influenza cannot take place, greatly limiting the epidemic or even pandemic potential of the virus. But regarding this latest mutation, we are still far from it, it seems, which leaves us precious time to act,” warns Antoine Flahault.
Why is raw milk incriminated?
A study published in Nature in October 2024 showed that since the start of the epizootic in the United States, contamination of cattle among themselves seems mainly linked to contaminated milk. The virus is believed to be transmitted by passing milking equipment from one animal to another. “The usual route of contamination of the influenza virus is respiratory and not digestive; moreover, the epizootic in dairy cows due to H5N1 has only been observed over the past year in North America. However, transmission between these cows seems to take place via poorly disinfected milking machines and not by respiratory or digestive routes. Moreover, contaminated cows suffer from mastitis, that is to say infections of their udders, and not of their lungs,” explains Antoine Flahault, who emphasizes that “there is “no known risk.” to this day in Europe” with raw milk.
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Will vaccination be quickly available?
In the event of a pandemic, the situation would be very different from that experienced with Covid-19. “We already have vaccines against the H5N1 virus circulating in the United States using messenger RNA technology. And since the pandemic, we even have all the infrastructure to mass manufacture it and deploy it quickly if necessary,” reassures Antoine Flahault who concludes: “No one knows the day or hour when we might need it and even if we will need it one day. »
