H5N1 Bird Flu Cases Spike in Europe. How Worried Should We Be?

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This article is republished from The Conversation under a Creative Commons license.

On November 13, Spain’s Ministry of Agriculture, Fisheries and Food imposed the confinement of all free-range poultry farms to curb the spread of avian influenza—more commonly known as “bird flu”—caused by the H5N1 virus. This drastic, though not unprecedented, measure was justified by climbing numbers of outbreaks in Europe, as well as the heightened risk posed by the southward migration of wild birds in Europe.

This type of news understandably causes alarm among the general public, but has the risk of the H5N1 virus triggering a pandemic among humans actually increased? And why is it causing increasing devastation not only among birds, but also among many species of mammals?

Masters of Variability

Flu viruses come in many forms. There are four strains (A, B, C, and D), each with many different sub-types.

Type A viruses are the most significant for human and animal health, and they are all descended from avian viruses. The flu A virus has two essential proteins in its envelope: haemagglutinin (H) and neuraminidase (N). There are 18 types of H and 11 types of N, which can be found in any combination: H1N1, H1N2, and so on up to H18N11.

There is also considerable genetic diversity within each type. This means strains belonging to the same type may differ in their ability to cause disease, their range of possible hosts, transmissibility, and so on.

The H5N1-A Pandemic

The H5N1 virus is a type of highly pathogenic influenza A virus with a high mortality rate among birds. Wild species, especially those in aquatic environments, are the virus’ natural reservoir.

H5N1 was first detected in domestic geese in China in 1997, and it has since spread widely across several continents via migratory birds, diversifying into different genetic groups or clades. The virus has spread to a large number of bird and mammal species across the globe, and is now the cause of a very real pandemic in the animal world, which is known as a panzootic.

In 2020, the 2.3.4.4b clade emerged, reaching North America in late 2021. H5N1 viruses in this clade have been able to replicate massively in wild and domestic birds, affecting more than 380 different bird species belonging to 52 families and 25 orders.

The impact on birds has been enormous in many countries, and millions have died from this pathogen in recent years. It is estimated, for example, that the virus caused the death of around 40% of all pelicans in Peru in less than five months.

Huge Geographic Spread

Avian influenza virus cases tend to rise during the colder months, as is the case with human influenza. However, since 2020, H5N1 has also been appearing during spring and summer. This has contributed to its enormous geographical spread, and it is now present on every continent except Oceania.

After causing thousands of outbreaks in Europe, the H5N1 virus was able to cross the Atlantic Ocean in late 2021 and reach North America, wreaking havoc on both the poultry industry and wild birds in Canada, the United States and Mexico. From there, the pathogen spread throughout Latin America, from Peru to Chile and Argentina. In 2024, it was detected in several bird species in Antarctica.

Bird Flu Spreads to Mammals

One of the most worrying changes is virus’ ability to infect multiple mammals, both wild and domestic (such as dogs and cats). Since 2021, it has spread to more than 50 species, from bears, otters and skunks to elephant seals, dolphins and walruses. In some cases, it has been highly virulent: tens of thousands of sea lions and elephant seals have died in the Americas, for instance.

Furthermore, the virus has evolved and adapted to these new hosts and is capable of spreading between them, as has been demonstrated among elephant seals and in fur farms in Europe.

In animals, the H5N1 virus usually causes symptoms ranging from pneumonia and meningoencephalitis to neurological effects such as tremors, convulsions, and ataxia. High viral loads have been documented in the brains of some species.

H5N1 and Cattle in the US

At the end of March 2024, the first case of H5N1 2.3.4.4b infection in dairy cows was reported in the United States. They were a completely unexpected reservoir for the virus.

This jump to cattle represents a significant change in the virus’ behavior, as it revealed a new ability to multiply in mammary tissue, with high viral loads detected in milk. This shows it has adapted to mammal hosts, and highlights the potential for zoonotic transmission to humans. Genomic analyses identified new mutations that increase the virus’s ability to bind to mammalian cell receptors and facilitate its spread.

To date, it remains a mystery why H5N1’s spread to cattle has only been detected in the United States and not in other countries.

H5N1 in Humans

Despite the spread of the H5N1 virus in animals and exposure to humans, relatively few human infections have been reported to date. Since its first detection in China, only about 900 cases have been reported, the vast majority in individuals who work on poultry farms or handle birds. Although the virus’s mortality rate can be up to 50% in humans, most cases reported in recent years have been mild.

Current data indicates that these viruses have not acquired the ability to sustain transmission between people, so the risk to the general population remains low.

The Next Human Pandemic?

There is no doubt that the H5N1 virus also poses a threat to our species, but for this virus to become pandemic, it would need to undergo several changes. These would include an improved ability to spread through the air between humans and to bind to human cell receptors, and a greater capacity to enter our cells and multiply within them. The virus would also need to be able to evade our immune systems.

It is difficult for the right combination of all these mutations to occur, but not impossible. The flu virus is a master of variability, mutation and recombination. Other influenza viruses caused the major pandemics of the 20th century. This means the massive global circulation of the H5N1 virus in the animal world is very bad news indeed.

To preserve human health, it is essential to monitor the animal world. The best way to do this is the One Health approach. This involves improving biosecurity on farms, intensifying veterinary surveillance (not only in poultry but also in cattle and pigs), and promoting effective coordination between the public health and animal health sectors through a collaborative approach. While we are not facing a new pandemic right now, the H5N1 virus is getting closer and closer.

Ignacio López-Goñi, Catedrático de Microbiología. Miembro de la Sociedad Española de Microbiología (SEM), Universidad de Navarra 

This article is republished from The Conversation under a Creative Commons license. Read the original article.