Wednesday, November 29, 2023

Mixing of virus types – bird flu continues to be rampant

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For a long time it seemed quiet around the topic – but perceptions are misleading. There are changes, especially with seagulls.

Things seem to have become a little quieter about bird flu in this country, but this perception is misleading. Authorities and researchers are very close to the topic, says Gertraud Schüpbach, professor at the Vetsuisse Faculty at the University of Bern. “In Switzerland, the virus has established itself in some bird populations and has become endemic.” Endemic means that the disease occurs consistently in a population.

Seagulls particularly affected

The highly contagious H5N1 virus has established itself particularly in black-headed gulls and other seagull species. “That was expectable. It has also been observed in other countries that seagulls are quite susceptible to the virus.

However, bird flu is not as devastating for seagulls as it is for other water birds. Entire populations were never threatened, says Schüpbach. “Apparently they are not very sensitive and do not die in large numbers. But of course there were also dead people, otherwise the outbreak would not have been noticed.”


A black-headed gull is illuminated in flight by the low evening sun in Hamburg.

Keystone/Daniel Reinhardt

Susceptible to the virus – but not that sensitive: What does that mean exactly biologically? Virologist Gert Zimmer, bird flu specialist at the Federal Institute for Virology and Immunology IVI, knows the answers. Zimmer explains: There are basically two different types of bird flu viruses: They are either highly pathogenic or low pathogenic.

Virus types have mixed

Low-pathogenic bird flu viruses have apparently been circulating among seagulls for a long time. They then came into contact with the highly pathogenic H5N1. The comparatively mild course of the seagulls can be explained in this way. “The H5N1 virus has exchanged gene segments with so-called low-pathogenic bird flu viruses adapted to seagulls.”

Seagulls are not only restricted to coastal regions but also breed inland. In this way, the virus can also spread domestically.”

In other words: the two virus types have mixed genetically. “This resulted in highly pathogenic H5N1 viruses that replicate well in seagulls.”

That’s worrying, says Zimmer. “Seagulls are not only limited to coastal regions, but also breed inland. In this way, the virus can also spread domestically.” There could be outbreaks again in chickens and other domestic poultry in Swiss stables.

When it comes to wild birds, however, the situation in this country and elsewhere in Europe has eased somewhat. Researchers suspect that the same genetic mechanisms play a role as in seagulls. “Many wild birds are probably relatively resistant to highly pathogenic H5N1 viruses because they have already had contact with similar viruses,” says Zimmer.

Low risk to humans

Other wild bird species have built up immunity to the virus simply through infection with H5N1. For example, the northern gannets: goose-sized seabirds that breed in the north and were almost wiped out by bird flu.

Almost: “In those animals that survived, the iris turned black. In this way, you can clearly see from the outside which animals are immune to H5N1 and which are not.”

And the risk to humans? Gert Zimmer gives the all-clear: There is no evidence that the circulating H5N1 virus threatens humans. Individual cases of infections through contact with infected chickens were mild. But people have never become infected with one another.

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