Humans could transmit avian flu

A recent American study reveals that the avian flu virus could mutate to the point where it can be transmitted from human to human.

Bruno Knellwolf / ch media

The bird lay dead ten days ago on the shore of Lake Constance near Triboltingen (TG). The Yellow-legged Gull had been infected with the H5N1 avian flu virus. The Federal Office for Food Safety and Veterinary Affairs (FSVO) has therefore ordered preventive measures for the area concerned.

Since 2021, H5N1 viruses of the sub-variety called “clade 2.3.4.4b” have become a dominant virus strain in wild birds, poultry and dairy cows. Other species of terrestrial and marine mammals as well as humans have also been infected by this virus. In the United States in particular, H5N1 is currently wreaking havoc. For several months, we have not been able to stop avian flu epidemics in cattle. The virus has been detected in around 700 cattle farms. In Oregon, two pigs were also infected.

Fifty-eight people have been infected, most of them from sick cows and poultry on farms. In the United States, the course was relatively benign, the symptoms: conjunctivitis, cough and fever. But for the first time, a teenager has become seriously ill in Canada. Virologists are worried because the virus changed in the young man's body after infection. The virus mutated and adapted to its human host. And this, at an important location in the virus, at the level of the viral hemagglutinin (HA) protein.

New pandemic strain?

A team of researchers led by Ian Wilson of the Scripps Research Institute in La Jolla took a detailed look at hemagglutinin found in cattle in a study published in the peer-reviewed journal “Science.” The study authors write that as the virus spreads from pets to humans, “it could also emerge as a new pandemic strain in an immunologically unprotected human population.”

In the laboratory, Californian researchers were able to show that the avian flu virus which circulates among cattle adapts relatively easily to humans. It would only take a single modified amino acid for hemagglutinin to bind specifically to receptors on human host cells. This is considered a prerequisite for human-to-human transmission of the virus. And such receptors are very common in the upper respiratory tract of humans.

Ian Wilson's team of researchers also notes that a second mutation further strengthens the effect. With this, the HA protein binds even better to human receptors.

For a pandemic, it would still be necessary overcome biological obstacles

“Wilson's work highlights the risks associated with influenza viruses, and this discovery is indeed a source of concern” according to biologist Nathalie Rochat of the Institute of Virology and Immunology (IVI) in Bern. But she tempers:

She mentions a publication from the German University of Freiburg which has just been published and which shows that the H5N1 virus continues to be inhibited by an antiviral component.

“But the pandemic risk would increase considerably if the virus managed to exchange genetic material with human viruses or swine influenza viruses,” explains Nathalie Rochat. This process could occur if humans, pigs or other intermediate hosts such as mink were simultaneously infected with influenza viruses of different origins.

American virologists also write that, in the beginning flu season, simultaneous infection with H5N1 and seasonal flu viruses could lead to the emergence of a hybrid virus better adapted to human infection:

The goal should be to give the virus as few opportunities as possible to reproduce in different hosts and thus continue to adapt.

No vaccine against H5N1 yet in Switzerland

As a precautionary measure, the United Kingdom has procured five million doses of bird flu vaccines. Several H5N1 avian flu vaccines are already authorized worldwide, with others in development, including mRNA vaccines. Finland intends to start vaccinating at-risk groups this summer, especially fur farm workers.

“There is no authorized vaccine against avian flu in Switzerland and no one has submitted an authorization request to Swissmedic so far,” says Barbara Camenisch from Ofag. In 2023, there was a research project at Basel Zoo and the Dählhölzli wildlife park in Bern to vaccinate birds against H5N1. With success, as Nathalie Rochat says:

Despite this, Switzerland is not yet relying on vaccination, because avian flu requires transnational solutions. However, these do not exist at the moment. This is why it was decided, in close consultation with the sector, not to take the lead in terms of vaccination.

What is the situation in Switzerland with bird flu?

No person has yet been infected in Switzerland, transmission is extremely rare so far. “Poultry products like chicken and eggs can be eaten without fear. For safety reasons, wild birds found dead should generally not be touched,” recalls Barbara Camenisch. Ofag urges all poultry farmers to protect their animals against bird flu. Above all, they should prevent their animals from coming into contact with wild birds, for example by placing a tight-mesh net over the enclosure.

The number of cases per year depends on several factors. The number of migratory birds plays a role, as does the type of wild birds involved. Swans and cranes spread the virus little. “Both came forward last winter. The number of migratory birds was lower than in other years. In addition, it was mainly the swans and cranes that fell ill,” explains Barbara Camenisch. There was only one case of avian flu in Switzerland last winter, in a swan. Larger bird flu outbreaks occurred in November 2016 and January 2023.

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