史上最大の鳥インフルエンザの流行は人類のパンデミックにつながるのでしょうか?

2021 年に始まった H5N1 鳥インフルエンザの発生は、米国と世界の両方で史上最大の鳥インフルエンザの発生となりました。 米国では、[{” attribute=””>virus has led to the destruction of millions of commercially raised chickens, turkeys, ducks, and geese, and has killed thousands of wild birds.

Many virologists are concerned that this virus could spill over to humans and cause a new human pandemic. University of Colorado Boulder virologists Sara Sawyer, Emma Worden-Sapper and Sharon Wu summarize the compelling story of H5N1 and why scientists are closely watching the outbreak.

1. Is this virus a serious threat to humans?

H5N1 is a specific type of influenza virus, predominantly harbored by birds, that was first detected on a goose farm in China in 1996. Recently it has begun infecting an exploding diversity of bird and mammalian species around the globe.

The virus is highly pathogenic to birds, meaning that infections often cause extreme symptoms, including death. But its impact on humans is complicated. There have been relatively few human infections detected – fewer than 900 documented globally over several decades – but about half of those infected individuals have died.

The good news about H5N1 for humans is that it currently doesn’t spread well between people. Most people who have contracted H5N1 have gotten it directly from interacting with infected poultry – specifically chickens, turkeys, ducks and geese, which often are raised in close quarters on large commercial farms.

There are only a small handful of examples of human-to-human spread. Because H5N1 doesn’t spread well between people, and because direct infection of humans by infected birds is still relatively rare, H5N1 has not yet erupted into a human epidemic or pandemic.

2. Why is this outbreak suddenly getting so much attention?

The first reason that so much attention is being paid to bird flu right now is that currently H5N1 is causing the largest “bird pandemic” ever recorded. A certain viral variant that arose in 2020, called H5N1 2.3.4.4b, is driving this outbreak.

In agricultural poultry flocks, if a few birds test positive for H5N1, the whole flock is killed regardless of symptoms or infection status. Higher prices for eggs and poultry meat in the U.S. are one result. The Biden administration is considering vaccinating farmed poultry flocks, but the logistics could be quite complicated.

The second reason for increased attention is that H5N1 is now infecting more bird and mammalian species than ever before. The virus has been detected in a broad array of wild birds and in diverse mammals, including badgers, black bears, bobcats, coyotes, ferrets, fisher cats, foxes, leopards, opossums, pigs, skunks and sea lions.

As H5N1 infects more species, it also increases its geographical range and produces more viral variants that could have new biological properties.

The third and most worrisome reason that this virus is getting so much press is that H5N1 now seems to be transmitting well between individuals of at least one mammalian species. In late 2022, mammal-to-mammal spread occurred in Spain in farmed minks. H5N1 spread very efficiently between the minks and caused clinical signs of illness and death in the mink populations where it was detected.

Sea lions in Peru are also succumbing to H5N1 virus in massive numbers. It hasn’t been confirmed definitively whether the sea lions are spreading the virus to each other or are contracting it from birds or H5N1-infected water.

Here’s the key question: If H5N1 can achieve spread in minks and possibly sea lions, why not humans? We are also mammals. It is true that the farmed minks were confined in close quarters, like chickens on a poultry farm, so that may have contributed. But humans also live in high densities in many cities around the world, providing the virus similar tinder should a human-compatible variant arise.

世界保健機関は、哺乳類における H5N1 の広がりを綿密に監視および分析しています。

3. H5N1 がヒトにうまく拡散するのに役立つのはどのような特徴ですか?

鳥は胃腸感染症としてインフルエンザを経験し、主に水中で排泄することによってインフルエンザを広げます。 対照的に、人間はインフルエンザを呼吸器感染症として経験し、呼吸や咳によって広がります。

何世紀にもわたって、これらの鳥インフルエンザ ウイルスの一部は、鳥から人間や他の哺乳類に感染してきましたが、これは比較的まれな出来事です。

これは、鳥インフルエンザウイルスが哺乳動物に効率的に感染するためにいくつかの方法で変異する必要があるためです. 最も重要な突然変異の変化は、ウイルスの組織指向性、つまり体の特定の部分に感染する能力に影響を与えます。

鳥インフルエンザウイルスは腸の細胞に感染するように進化し、ヒトインフルエンザウイルスは気道の細胞に感染するように進化しました. ただし、インフルエンザウイルスは、体の別の部分の細胞に感染することを可能にする突然変異を獲得することがあります.

インフルエンザがどの細胞に感染するかは、それが結合する特定の受容体によって部分的に決定されます。 受容体は、ウイルスがそれらの細胞に侵入するために利用する宿主細胞の表面上の分子です。 ウイルスが細胞内に入ると、ウイルスは自分自身のコピーを生成できるようになる可能性があり、その時点で感染が達成されます。

ヒトと鳥の両方のインフルエンザ ウイルスは、細胞の表面に共通するシアル酸と呼ばれる受容体を使用します。 H5N1 などの鳥インフルエンザ ウイルスは、α2,3-結合シアル酸と呼ばれるバージョンを使用します。[{” attribute=””>acid, while human flu viruses use a2,6-linked sialic acid – the predominant variant in the human upper respiratory tract. Thus, to become efficient at infecting humans, H5N1 would likely need to mutate to use a2,6-linked sialic acid as its receptor.

This is a concern because studies have shown that only one or two mutations in the viral genome are enough to switch receptor binding from a2,3-linked sialic acid to the human a2,6-linked sialic acid. That doesn’t seem like much of a genetic obstacle.

4. Why don’t we make a vaccine just in case?

With avian influenza viruses, it is not possible to make effective human vaccines in advance, because we don’t know exactly what the genetics of the virus will be if it starts to spread well in humans. Remember that the seasonal flu vaccine must be remade every year, even though the general types of flu viruses that it protects against are the same, because the specific genetic variants that affect humans change from year to year.

Right now, the best way people can protect themselves from H5N1 is to avoid contact with infected birds. For more information about prevention, especially for people who keep domesticated birds or are bird-watching hobbyists, the Centers for Disease Control has a list of guidelines for avoiding H5N1 and other bird flu viruses.

Written by:

• Sara Sawyer, Professor of Molecular, Cellular and Developmental Biology, University of Colorado Boulder
• Emma Worden-Sapper, PhD Student in Molecular, Cellular and Developmental Biology, University of Colorado Boulder
• Sharon Wu, PhD Student in Interdisciplinary Quantitative Biology and Molecular, Cellular, and Developmental Biology, University of Colorado Boulder

This article was first published in The Conversation.