Boulders (United States). The H5N1 avian influenza outbreak that began in 2021 has become the largest avian influenza outbreak in history in the US and worldwide. In the US, the virus has destroyed millions of commercially raised chickens, turkeys, ducks and geese and killed thousands of wild birds. Many herpetologists are concerned that the virus could spread to humans and cause a new human pandemic. Is this virus a serious threat to humans?
H5N1 is a specific type of influenza virus that circulates mainly in birds and was first detected in 1996 on a duck farm in China. It has recently begun to infect an explosive diversity of bird and mammal species around the world. The virus causes severe symptoms in birds, which can even cause death.
But its effect on humans is complex. Human infections have been detected relatively rarely (fewer than 900 have been documented worldwide over several decades), but about half of those infected died. The good news about H5N1 for humans is that it currently does not spread from person to person. Most people who contract H5N1 contract it through direct contact with infected poultry, especially chickens, turkeys, and ducks, which are often raised in large numbers on large commercial farms. There are only a few examples of person-to-person spread.
Since the infection does not spread from human to human, and direct infection of humans via infected birds is still relatively rare, H5N1 has not yet become a human pandemic or pandemic. Why is there so much attention on this outbreak all of a sudden? The first reason bird flu is getting so much attention right now is that H5N1 is causing the largest “avian pandemic” ever recorded. A certain viral variant that originated in 2020, called H5N1 2.3.4.4b, is driving the outbreak. In poultry flocks, if any birds are found to be infected with H5N1, the entire flock is culled, regardless of symptoms or infection status.
One result is high prices for poultry eggs and meat in the US The Biden administration is considering vaccinating poultry flocks, but the process could be complicated. Another reason for the increased attention is that H5N1 is now infecting more species of birds and mammals than ever before.
The virus has been detected in a wide range of wild birds and various mammals, including badgers, black bears, bobcats, coyotes, ferrets, fishing cats, foxes, leopards, opossums, pigs, skunks, and sea lions. As we see, H5N1 is infecting more species, increasing its geographic range and generating more virulent variants that may have new biological properties. The third, and most worrying, reason why this virus is receiving so much attention is that H5N1 is now known to be transmitted between at least one mammalian species.
In late 2022, mammal-to-mammal spread occurred in mink farms in Spain. H5N1 spreads very efficiently among mink and causes clinical signs of illness and death in mink populations. Sea lions in Peru are also falling victim to the H5N1 virus in large numbers. It has not been definitively confirmed whether sea lions transmit the virus to each other or if they contract it from H5N1-infected birds or water. The key question here is: if H5N1 can spread to mink and possibly sea lions, why not humans?
We too are mammals. It is true that, like chickens in poultry farms, farmed mink are also kept together in large numbers, increasing the possibility of their spread. But since humans also live in high density in many cities around the world, the virus can produce a human-compatible variant if given a favorable environment. What things can help H5N1 spread to humans? Birds experience influenza as a gastrointestinal infection and transmit it primarily through feces in water. In contrast, humans experience influenza as a respiratory infection and spread it through breathing and coughing. Over the centuries, some of these avian influenza viruses have spread from birds to humans and other mammalian species, although this occurs less frequently.
This is because the avian influenza virus would have to mutate in several ways in order to effectively infect mammals. The most important mutational changes affect the tissue virulence of the virus, which is related to its ability to infect a specific part of the body. Avian influenza viruses have evolved to infect cells in the intestinal tract, while human influenza viruses have evolved to infect cells in the respiratory tract. However, sometimes a flu virus can acquire mutations that allow it to infect cells in a different part of the body.
The cells that the flu infects are determined in part by the specific receptor it binds to. Receptors are molecules on the surface of host cells that a virus uses to enter cells. Once the viruses are in cells, they can make copies of themselves, at which point infection occurs. Both human and bird influenza viruses use receptors called sialic acids that are common on the surface of cells. Avian influenza viruses, such as H5N1, use a variant called A2,3-linked sialic acid, while human influenza viruses use A2,6-linked sialic acid.
Therefore, to become efficient at infecting humans, H5N1 would need to mutate to use A2,6-linked sialic acid as its receptor. This is concerning because studies have shown that only one or two mutations in the viral genome are sufficient to change receptor binding from A2,3-linked sialic acid to human A2,6-linked sialic acid. It does not appear to be a genetic barrier. Why don’t we make a vaccine for that already? With avian influenza viruses, it is not possible to create effective human vaccines in advance, because we do not know exactly what the genetics of the virus would be if it started circulating in humans.
Remember that the seasonal flu vaccine must be remade each year, even though the common types of flu viruses it protects against are the same, because the specific genetic variants that affect humans change from year to year. . At this time, the best way to protect yourself from H5N1 is to avoid contact with infected birds. For more information on prevention, especially for those who keep birds as pets or are bird watchers, the Centers for Disease Control has a list of guidelines for avoiding H5N1 and other bird flu viruses.
