In the United States, a 55th human case of avian flu was detected in the San Francisco Bay, alongside multiple contaminations on cattle farms. Is such a phenomenon to be feared in France? Are there risks of human-to-human transmission? The professor of avian pathology at the National Veterinary School of Toulouse, Jean-Luc Guérin, answered “Marianne’s” questions.
The threat is contained in a few characters: H5N1. This is not a code name or a password, but the strain of avian flu that is worrying scientists around the world. Particularly in the United States where, in recent months, this virus has spread to cattle farms and then to humans. This Friday, November 23, American health authorities announced that a child, residing in Alameda County, in the San Francisco Bay area, had been infected with avian flu.
Across the Atlantic, this is the 55th human case since the start of the year. In France, should we be worried about such a phenomenon? Is the virus likely to mutate and ultimately be transmitted from man to man? Elements of response with Jean-Luc Guérin, professor of avian pathology at the National Veterinary School of Toulouse and unit director at the National Institute of Agronomic Research (INRAE).
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Marianne : Given the situation in the United States, should we be worried in France?
Jean-Luc Guérin: Today, in France, we are not at all in the situation that we have known in South-East Asia for 25 years now nor, much more recently, in the United States. For good reason, the strains circulating among us have a zoonotic potential – designating the risk of crossing the species barrier – which is much lower.
We note that the area in which all these viruses emerged was South-East Asia, and in particular China, in 1996. The first episodes of human cases subsequently appeared in Hong Kong in 1997, with the H5N1 viruses.
Since then, there has been very large-scale circulation within different bird species, which explains the wide variety of strains – also called clades – in this area. Among these clades, some are zoonotic from the outset and therefore at risk of transmission to humans. However, it has always remained rare and the phenomenon is confined to a few hundred cases.
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However, in Southeast Asia, these cases have been marked by very serious developments. The disease begins in the lungs and often results in pneumonia. Over the entire period, since 1997, there have been around 1,000 cases, with a 50% fatality rate. Remember that this phenomenon is very specific to this area, and continues today sporadically, particularly in Cambodia.
How can we explain the evolution of the situation in the United States, where 55 human cases have been detected?
A little over two years ago, strains from Europe crossed the Atlantic via Greenland and completely invaded the American continent. In recent months, we have noticed that cattle farms have been contaminated by the H5N1 virus. It turns out that in the udder of cows there are receptors allowing the virus to attach because they are very similar to those found in the respiratory system of birds. And we hadn't anticipated that well.
The virus was probably spread through milk, the handling of milking staff and through soiled equipment which contaminated the cows one after the other. More than 600 cattle herds in 15 states have tested positive since the spring. Furthermore, around fifty people were in fact contaminated, which suggests that the strains which drifted into the United States acquired particular mutations which give them a higher zoonotic potential.
Note that human cases remain very sporadic. Man is an epidemiological dead end. In fact, these strains are not pandemic: they are not transmitted effectively from human to human and are most of the time much less pathogenic.
Perhaps this is linked to the efficiency of the health system in the United States, with faster treatment of patients compared to what we observe in certain countries in Southeast Asia. This situation on the North American continent concerns scientists, but it does not exist in Europe or in France.
Do we have reason to believe that human cases of avian flu could also emerge in France?
Currently, no. In Europe, since 2016, the strain circulating is called clade 2.3.4.4b. It is very pathogenic for poultry, which has led to culling measures and the implementation of vaccination of ducks in France. But the luck we have is that the strains are rarely zoonotic.
However, a monitoring system must be put in place. All strains that are detected are systematically sequenced with particular attention paid to adaptation markers – which are mutations that could raise fears of adaptation.
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Furthermore, it is strongly recommended that personnel who are exposed to the virus and in direct contact with birds (breeders, veterinarians, etc.) be monitored and vaccinated, to ensure that we do not miss an infection. which would not be clinical, but which would allow the virus to multiply. In addition, for years we have been explaining to breeders that it is imperative to cull infected poultry. If we allow the virus to circulate, we take the risk of acquiring a mutation.
In France, the vaccination of ducks, implemented two years ago, is very effective because it allows producers to work calmly while avoiding the risk of the virus actively multiplying and continuing to evolve.
Are there treatments suitable for the H5N1 virus for humans?
We use treatments against influenza viruses which are quite generic, but not specific to H5N1. They help block the multiplication of these viruses. Furthermore, vaccination against seasonal flu will have no effect because the latter is caused by H1N1 or H3N2 viruses. In fact, the immune system will not recognize the H5N1 virus. On the other hand, we recommend this vaccination for breeders to avoid mixing strains, and them being infected at the same time with H1N1 and H5N1.
