Boulder: The H5N1 avian influenza outbreak that began in 2021 has become the largest bird flu outbreak in history in the US and around the world. 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, which was first detected in 1996 on a duck farm in China. Recently it has started infecting an explosive diversity of bird and mammal species around the world. The virus causes severe symptoms in birds, which can even lead to death.
But its effect on humans is complex. Human infections have been relatively rare with fewer than 900 documented cases globally over several decades but about half of those infected individuals died. The good news about H5N1 for humans is that it does not currently spread from person to person. Most people who contract H5N1 get it from 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 human-to-human spread.
Since the infection does not spread from human to human and direct infection from infected birds to humans is still relatively rare, H5N1 has not yet emerged as a human pandemic or pandemic. Why is there suddenly so much attention being paid to this outbreak? The first reason bird flu is getting so much attention right now is that H5N1 is currently causing the largest “bird pandemic” ever recorded. A certain viral variant that originated in 2020, called H5N1 2.3.4.4b, is driving the outbreak. In farm poultry flocks, if some birds are found to be infected with H5N1, the entire flock is culled, regardless of symptoms or infection status.
High prices for eggs and poultry meat in the US are one result. The Biden administration is considering vaccinating farmed poultry flocks, but the process could be complicated. Another reason for the increased attention is that H5N1 is now infecting more bird and mammal species 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, fisher 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 getting so much attention, is that H5N1 is now known to transmit 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 prey to the H5N1 virus in large numbers. It has not been definitively confirmed whether sea lions are spreading the virus to each other or contracting it from birds or H5N1-infected water. The key question here is: If H5N1 can spread to mink and possibly sea lions, why not humans?
We are also mammals. It is true that like chickens on poultry farms, farm minks are also kept together in large numbers, which increases the possibility of its spread. But as 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 spread the flu primarily through feces in the 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 passed from birds to humans and other mammalian species, although this occurs less frequently.
This is because the bird 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 relates to its ability to infect a specific part of the body. Avian flu viruses have evolved to infect cells of the intestinal tract, while human flu viruses have evolved to infect cells of 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 influenza infects are partly determined by the specific receptor it binds to. Receptors are molecules on the surface of host cells that are used by a virus to enter cells. Once viruses are in cells, they may be able to produce copies of themselves, at which point infection has occurred. Both human and bird influenza viruses use receptors called sialic acids that are common on the surfaces of cells. Bird influenza viruses, such as H5N1, use a variant called A2,3-linked sialic acid, while human flu viruses use A2,6-linked sialic acid.
Thus, to become efficient at infecting humans, H5N1 would 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 sufficient to switch receptor binding from A2,3-linked sialic acid to human A2,6-linked sialic acid. It doesn’t appear to be a genetic barrier. Why don’t we make a vaccine for it 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 began to circulate in humans.
Remember that the seasonal flu vaccine needs to be made again every year, even though the common types of flu virus it protects against are the same because the specific genetic variants that affect humans change from year to year. Right now, 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 pet birds or are bird watchers, the Centers for Disease Control has a list of guidelines for avoiding H5N1 and other bird flu viruses.
