The highly pathogenic H5N1 avian influenza virus arrived in France in August 2021 via migratory birds. The first cases were identified in wildlife before a laying hen farm in the North was affected in November 2021. It was the first case since winter 2020, and since then the virus has never stopped circulate. It gradually spread from the north to the south-west, then to the regions of the Great West. In the spring of 2022, outbreaks were identified further inland, particularly in Dordogne and Corrèze. In total, the Ministry of Agriculture has identified 315 outbreaks of avian flu in livestock since 1is August 2021. About 28 million poultry had to be slaughtered to limit the spread of the disease (between October 2021 and May 2022).
Today, the situation has calmed down, as explained by Jean-Luc Guérin, professor of poultry farming and avian pathology, and head of the UMR Host-Pathogen Interaction (IHAP) at the Toulouse Veterinary School: ” bird flu today in breeding is in sharp decline. I wouldn’t call it an extinction, but there are very few new outbreaks. In wildlife, the situation is more confused since there are still significant mortalities in certain populations of wild birds. For example, black-headed gulls are particularly affected in different parts of the territory, including inland “. The Ministry of Agriculture reports 16 outbreaks identified from February to March 2023, and the epidemic is still active in nine French departments.
Le clade 2.3.4.4b
The influenza viruses responsible for this panzootic, the equivalent of pandemics in animals, come from Southeast Asia. ” These are Asian lineage viruses, distant descendants of the H5N1 strain that emerged in 1996 precisely. Several strains are circulating on a European scale and in the world, they belong to a clade present since 2016 and which is at the origin of all our problems, the clade 2.3.4.4b. »
This clade is ” little or very little zoonotic “, according to Jean-Luc Guérin. In other words, its ability to cross the species barrier is limited. Influenza viruses of clade 2.3.4.4b are highly adapted to birds and are not genetically armed to effectively infect mammals. ” The risk of the transition to humans and mammals is very low “, he specifies.
Contaminations of mammals explained
How then to explain the impressive list of mammals that have contracted avian flu around the world? Foxes and otters in England, sea lions in Chile and most recently in New England, all of these mammals have a diet made up in part of wild birds. ” If, for example, a fox or a mink has a risk in 1,000 or 10,000 of becoming infected if it eats a dead seagull of bird flu, if 50 birds die in the territory, the probability of having a case in mammals is zero. But if there are a million, there are cases. »
The only case in which there might have been a mammal-to-mammal transmission is that of mink farming in Spain. ” Mink, and mustelids in general, have cell receptors with higher affinity for avian viruses than humans. They are therefore not very representative of the general situation in mammals. »
Significant reworking of the hemagglutinin gene, and gene segments, of H5N1 strains would be required for them to be able to infect mammals and humans. Hemagglutinin is one of the surface proteins, along with neuraminidase, of influenza viruses which serve as their key to entry into host cells. ” The virus is now highly adapted to birds and as long as it is, the risk that it will at the same time be highly adapted to humans is very low. A few mutations have been observed in recent cases in mammals, but these mutations alone cannot explain anything other than very sporadic infections. »
The handful of human cases caused by 2.3.4.4b
In fact, human cases of bird flu are very rare. The WHO recently published a report on the subject which reports 868 proven cases since 2003, all clades combined. Human cases due to clade 2.3.4.4b can be counted on the fingers of one hand: an octogenarian in England (January 2022), a 9-year-old girl in Ecuador (January 2023), two women aged 38 and 53 in China, a 5-year-old girl in Vietnam. The case of the girl in Cambodia that we discussed in this article is due to clade 2.3.4.4c, another offshoot from the historic H5N1 that circulates locally in Southeast Asia, and not to clade 2.3.4.4b.
Despite sporadic human cases, several manufacturers, including Moderna and Sanofi, have recently said to be ready to produce human avian flu vaccines. So, does vaccinating humans have an interest in this panzootic?
“There is no epidemic in humans”
« None. Today the level of exposure is very lowsays Jean-Luc Guérin. In a pinch, it might be useful for very exposed people like me, breeders or veterinarians who really go to the front in homes. But to protect them, there are other means and in the first place personal protective equipment. There is no epidemic in humans, and to date these cases remain completely anecdotal. »
Vaccinate millions of poultry?
On the other hand, vaccinating poultry makes sense. This would reduce the number of viral particles in circulation and slow down the evolution of the epidemic by limiting the appearance of new reassortants. Influenza viruses have the particularity of being able to mix their eight genetic segments. The result of this mixing are reassortants, virions that have mosaic genomes, sorts of chimeras.
« The French government made decisions more than a year ago, and the roadmap set by the current minister is to be able to vaccinate poultry from the start of the school year, in September. Vaccines exist, but vaccination once morest avian influenza in farms is not easy to implement. The most complicated question is that in France, there are duck, chicken and turkey farms and what is under discussion is: should all ducks, hens, turkeys…? In which regions and at what time of year? And it’s complicated because the epidemic dynamics are not the same depending on the species. »
Intensive farming in question?
Southeast Asia, the cradle of H5N1, is conducive to the emergence of infectious diseases because very dense human populations coexist with equally dense avian and pig populations. This dynamic promotes close contact between humans and animals and increases the risk of zoonotic virus transmission from one to the other. But has the size of farms, especially the most intensive ones, had an impact on the panzootic of H5N1?
« In Southeast Asia, there are very large farms, but also a lot of very small farms and it is this dynamic that is difficult to monitor. However, on a global scale, the very significant increase in poultry production, particularly in Asia, is not neutral, because it is in a way fuel for the virus. And this can favor, arithmetically, the probability of emergence of new strains », explains Jean-Luc Guérin.
But the scientist believes that farming techniques in Europe have very little effect on the dynamics of the H5N1 panzootic. ” The way poultry are kept in Europe has a negligible effect on the epidemic on a continental and intercontinental scale. The virus came to Europe through migratory birds that were not infected in Europe. Many species are found in migration corridors where the virus can easily spread. In Europe, wild birds can transmit the H5N1 virus both to an intensive poultry farm and to a small chicken coop at the bottom of a private garden.
