Some avian or porcine influenza viruses can infect humans, and sporadic cases are regularly detected on several continents. Lately, attention has focused on avian flu, or avian influenza, a viral infection that spreads among birds. It is found worldwide, affecting wild species and domestic poultry. More specifically, avian influenza is an infection caused by type A influenza viruses, as for certain human influenzas, and in particular by the H5, H7 and H9 subtypes. This disease is a zoonosis, that is, it can be transmitted between animals and humans. As the Institut Pasteur points out, the H5N1 avian flu virus was first detected in 1997, during an epidemic in Hong Kong, causing the death of six people. But this transmission is rare and occurs within farms through aerosols (the respiratory excretions of infected birds, or their pulverized dried droppings), especially since there is no transmission through meat or food products from birds.
But this question is once again being debated as recently two human cases of infection by an A(H5N1) virus were detected in Cambodia (including one death). So should we be worried? No, believes Public Health, which publishes on this occasion an epidemiological update on the circulation of avian influenza viruses. The agency first explains that ” Avian influenza means the disease caused in humans by influenza viruses infecting birds (avian influenza viruses, AI) which cross the species barrier and infect humans. Humans can be infected by several avian influenza viruses (particularly the H5N1, H7N9 and H9N2 subtypes). » According to its estimates, more than 300 outbreaks in livestock have been detected in France since August 2022, more than three quarters of which are concentrated in the Pays de la Loire region in an area with a high density of poultry farms. This points out that the risk of transmission does indeed exist for avian influenza H5N1 of clade 2.3.4.4b in around twenty different species of terrestrial and marine mammals, wild and domestic.
The real risk of this Influenza A (H5N1) virus is that it evolves
In addition, mammal-to-mammal transmission has been suggested in infections of seals in the United States (2022), mink in Spain (2023) and sea lions in Peru. In France, at the end of December 2022, a sick cat tested positive for the highly pathogenic H5N1 avian influenza virus in Deux-Sèvres. L’Anses* I investigated, which made it possible to identify the neighboring duck farm as being the source of the contamination of the cat. ” Investigations have also taken place with people who have been exposed to the infected cat, to ensure that they have not been contaminated. The first analyzes did not reveal any acute infection among the exposed people. Serological analyzes are in progress to confirm the absence of contamination of exposed persons. “, specifies Public Health France. However, the latter points out that it is indeed the increased frequency of transmission of these viruses to mammals of various species that increases the risk of emergence of a new influenza virus better adapted to humans and capable of human-to-human transmission. .
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The conclusion is the same for the Public Health Information Service which emphasizes that Influenza A viruses, and in particular H5N1, have a great capacity to mutate over time, but also to exchange their genes with influenza viruses belonging to other species (pigs, mink, humans, etc.). The organization believes that a “ such a phenomenon could occur during an infection of a human or an animal by two viruses simultaneously, for example an avian virus and a virus infecting mammals. In this case, within the same cell, the two viruses will multiply, producing many copies of their DNA. During the assembly of the new viruses, “hybrid” viruses having incorporated DNA segments from one and the other of the parental viruses may be formed. According to the latter, “these “hybrid” viruses thus formed could have acquired the capacity to be transmitted from human to human as efficiently as our seasonal flu. The risk is none other than, because of its avian origin, this virus could not be sensitive to the immunity that humans have against seasonal flu.
Can bird flu infect humans?
But could the latter be the cause of a vast epidemic potentially dangerous for the human species, like the one recently known with Covid-19? Faced with the risk of an influenza pandemic, the World Health Organization (WHO) emphasizes theimportance of monitoring the occurrence of outbreaks in poultry and migratory bird populations and respiratory disease in individuals exposed to infected poultry, and taking rapid control measures. Which explains why “ when an animal outbreak is identified, the measures consist of quarantine followed by the slaughter of infected animals and potentially exposed animals. », notes the Pasteur Institute. Mass slaughter remains the best strategy (as well as wearing a mask and respecting barrier gestures when on a farm, disinfection of farms and equipment) to systematically seek to reduce the number of bird flu virus multiplying throughout the world and thus limiting the exposure of mammals (including humans) to this virus and therefore reducing the risk of hybridization.
It should be noted that, in the same logic, people who work on farms (breeders, technicians, veterinarians) must be vaccinated against seasonal flu every winter. ” It is again a question of avoiding the encounter between a human Influenza virus and an animal Influenza virus. “, attests the Public Health Information Service. Added to this is the fact that not all avian influenza viruses are equally dangerous, ie the ability of an avian influenza virus to infect humans varies between variants. According to the organization, it is more important for the H5N1 strains (about 1000 cases and 500 deaths worldwide since 2003, none in France) and H7N9 (about 1600 cases and 600 deaths worldwide since 2013). “ These figures are modest considering the number of birds that fell victim to avian flu during this period (probably several hundred million). “, he says. Furthermore, the fact that avian influenza does not spread easily or quickly among humans is a characteristic that does not create the conditions favorable for a pandemic.
The Public Health Agency France also wants to be reassuring on this subject, stating that the “ number of human cases detected remains low given the intensity of the global epizootic linked to these viruses. And concerning the two human cases of infection by an A(H5N1) virus in Cambodia, the WHO considers that the viral strain in question belongs to a clade (2.3.2.1c) endemic to Cambodia for several years. Thus, if the circulation of certain avian virus subtypes, such as A(H5) and A(H7N9) in poultry, is a public health problem and the risk of the appearance of a virus capable of being transmitted from man to man must be taken into consideration, such a possibility is subject to extreme surveillance by the health authorities and preventive measures already in place (slaughter of infected animals and potentially exposed animals). Finally, the WHO considers that, to date, human-to-human transmission of these viruses is rare and occurs in the event of very close and prolonged contact between a patient and the people caring for him, and that there is no long-lasting human-to-human transmission.
