The world's first human death from H5N5 bird flu has been recorded. Here's what you need to know.

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In early November, a resident of Grays Harbor County, on the southwestern Pacific coast of Washington state, became severely ill with flu-like symptoms, including high fever, respiratory failure, and confusion.

He was hospitalized, and on November 14, it was officially confirmed that tests revealed an H5N5 avian influenza virus infection. The patient, an elderly person with underlying conditions, was treated in the hospital but died on November 21.

This was the first reported human infection with the H5N5 influenza virus.

What is the H5N5 influenza virus?

H5N5 influenza viruses are a type of avian influenza (influenza A virus) that, as the name suggests, primarily affects birds. In science, such viruses are classified as "highly pathogenic" or "low pathogenic" depending on how severe symptoms they cause specifically in domestic poultry (their severity also varies in other bird species).

This H5N5 strain, like its well-known "relative" H5N1, belongs to highly pathogenic forms - it causes severe symptoms and high mortality in birds.

Where did it come from?

It is not known exactly. But the patient kept a flock of domestic poultry on his farm that came into contact with wild birds.

H5N5 is found in wild birds around the world, and its transmission to domestic poultry flocks is quite common. However, this is the first time that the H5N5 influenza virus has gone a step further and infected a human.

What does the name mean? Is it similar to H5N1?

The classification of influenza A viruses is based on two proteins found on the surface of the viral particle: hemagglutinin (HA) and neuraminidase (NA). These proteins are targets for our immune system, so viruses are constantly mutating, creating new subtypes.

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The names H5N1 and H5N5 indicate the structure of these proteins. In both viruses, the HA protein belongs to the same type (H5), but the NA proteins are different (N1 versus N5). Genetic studies show that the H5N5 virus is a separate strain that differs from the currently dominant H5N1. This is similar to how people can get different strains of seasonal flu (e.g., H1N1 and H3N2) in the same year.

Should we worry about what happens next?

H5N5 is an environmental and agricultural threat. This is complicated enough, but avian influenza also requires our attention, as the virus can cause new pandemics.

In the long term, this risk is very significant. But it is worth remembering that although influenza is better than any other virus at changing its host and creating pandemics, it is still incredibly difficult for the virus to do this.

The vast majority of cases of avian influenza jumping to humans are isolated events. And their consequences are unpredictable. Most of them are quite mild - for example, they cause conjunctivitis - but some can be very severe, as in the case of the first registered case of H5N5. But after infecting one person, most avian influenza viruses do not spread further.

Scientists will be monitoring for several worrying signs that the virus may be adapting to humans, especially for any hints of human-to-human transmission. There is no evidence that this has happened now.

At least for now, the risk to humanity from H5N5 remains low, and there is no reason to think that this was anything other than a tragic isolated case. However, influenza viruses will have many opportunities to try again. As H5N5 and other subtypes of avian influenza virus continue to circulate, it is important that we continue to monitor this virus closely.

The author of the article is Ed Hutchinson. He is a professor at the MRC-University of Glasgow Centre for Virus Research. The article, translated from the original (with minor reductions) with the permission of the author, was published in The Conversation magazine under the title "First human bird-flu death from H5N5 — what you need to know". The article is distributed under the CC-BY-ND 4.0 license.

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