Приведет ли крупнейшая в истории вспышка птичьего гриппа к пандемии среди людей?

Вспышка птичьего гриппа H5N1, начавшаяся в 2021 году, стала крупнейшей вспышкой птичьего гриппа в истории как в США, так и во всем мире. В США

” data-gt-translate-атрибуты = “[{” 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 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.

Peru decreed a 90-day health emergency in December 2022 after more than 13,000 pelicans died on its beaches, possibly infected with H5N1.

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.

Всемирная организация здравоохранения внимательно следит и анализирует распространение H5N1 среди млекопитающих.

3. Какие особенности могли бы помочь H5N1 хорошо распространяться среди людей?

Птицы переносят грипп как желудочно-кишечную инфекцию и распространяют грипп преимущественно через дефекация в воде. Напротив, люди переносят грипп как респираторную инфекцию и распространяют ее при дыхании и кашле.

На протяжении веков некоторые из этих вирусов птичьего гриппа передавались от птиц человеку и другим видам млекопитающих, хотя это происходит относительно редко.

Это связано с тем, что вирусы птичьего гриппа должны мутировать несколькими способами. эффективно заражать млекопитающих. Наиболее важные мутационные изменения затрагивают тканевой тропизм вируса – его способность поражать конкретную часть организма.

Вирусы птичьего гриппа эволюционировали, чтобы инфицировать клетки кишечника, в то время как вирусы человеческого гриппа эволюционировали, чтобы инфицировать клетки дыхательных путей. Однако иногда вирус гриппа может приобретать мутации, которые позволяют ему заражать клетки в другой части тела.

Какие клетки инфицирует грипп, частично зависит от специфического рецептора, который он связывает. Рецепторы — это молекулы на поверхности клеток-хозяев, которые вирус использует для проникновения в эти клетки. Как только вирусы попадают в клетки, они могут производить копии самих себяв этот момент инфекция была достигнута.

Заражение птичьим гриппом у людей встречается редко, но возможно. Большинство зарегистрированных случаев заражения людей птичьим гриппом произошло после незащищенного контакта с инфицированными птицами или загрязненными поверхностями. Предоставлено: Центр контроля заболеваний США.

Вирусы как человеческого, так и птичьего гриппа используют рецепторы, называемые сиаловыми кислотами, которые распространены на поверхности клеток. Вирусы птичьего гриппа, такие как H5N1, используют версию, называемую а2,3-связанной сиаловой кислотой.

” data-gt-translate-атрибуты = “[{” 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.