The bird flu virus that has been circulating for 26 years, why has the lethality not weakened?

In recent years, the new coronavirus has spread in the human world and has attracted much attention. It can be said that it has attracted almost all of our attention.

In fact, there is an avian influenza virus that is spreading among birds with amazing transmissibility and lethality recently. Since October last year, this avian influenza virus has caused nearly 3,000 poultry outbreaks in dozens of countries.

In order to stop the spread of the epidemic, more than 77 million poultry have been culled around the world so far, and 400,000 wild birds have died in recorded outbreaks.

Compared with the last bird flu epidemic, the death data of wild birds this time is twice the previous one, and the journal Nature uses "unprecedented (unprecedented)" to describe this wave of bird flu.

However, the manufacturer of this wave of epidemics is a strain we are more familiar with - the H5N1 strain. It is estimated that many people are familiar with it. This guy basically comes out every once in a while to create a wave of poultry epidemics, and sometimes even spread to people.

The H5N1 strain was first described in 1996, and it hasn't really disappeared since then, and there are reports of it every year, with reports of larger outbreaks every few years.

Although there are not many human infections so far, the mortality rate of human infections has reached as high as 60%, and for some birds, the H5N1 strain always causes severe illness or death in most infected people.

Then the question arises. Since 1996, this bird flu virus has been circulating for 26 years. Why is the virus still so deadly to birds?

Viruses don't necessarily get weaker

Since the virus needs to coexist with the host, if it is too deadly, it is actually very unfavorable for transmission, because the host may die before it has had a chance to spread the virus, so the virus itself cannot spread and survive. It's a huge victory for the virus.

Therefore, in our conventional cognition, we often think that the lethality of a virus will gradually become less lethal over time in order to achieve coexistence with the host.

Although this is indeed the case in many cases, for example, some severe coronaviruses will turn into ordinary flu, but in fact such a thing is not absolute.

Australia has a very typical case:

Australia has a very serious rabbit infestation, but interestingly, all Australian rabbits today are descendants of 13 rabbits brought by a wealthy immigrant named Thomas Austin in 1859.

As an invasive species, rabbits' super reproductive ability and adaptability have allowed them to grow wildly in Australia, which was originally isolated from the world, reaching as many as hundreds of millions.

Such a large population, even if they taste good, they have become terrible "pests", destroying the original ecology of Australia, causing the extinction of native animals and plants, and frustrating agriculture.

In order to deal with rabbits, Australian immigrants have exhausted their methods, including the release of myxoma virus, which is very effective against rabbits, and 99.8% of the infected rabbits will die at first. Over time, this virus has only about a 90% mortality rate, which is basically in line with the characteristics of the virus that needs to coexist with the host.

However, according to the limited data, the myxoma virus has not been decreasing in lethality all the time;

By the 1980s, myxoma virus had become much more lethal to Australian rabbits, with a larger percentage of circulating viruses being highly lethal than in the previous decade.

The reason why the lethality of myxoma virus to rabbits has not continued to decrease is actually very simple. Like other life, their genetic mutations are completely directionless.

Although it is impossible for the virus to only follow the path of weakening, there are some "powers" that act on the virus and the host at the same time!

Complex trade-offs between host and virus

Rabies is known to be 100% lethal to the human host, and according to known records, the virus has been circulating in humans as early as 4,000 years ago, but its lethality has never diminished.

If you just look at the spread of rabies in humans, it does seem difficult for them to be sustainable, but do they have to evolve to be less deadly to humans?

Obviously not needed. They have changed over thousands of years, if need be.

In fact, the reason why rabies viruses do not need to be "improved" is also very simple. For human hosts, they do not need to think about the host at all, because they can more easily survive and spread in animal hosts, such as dogs, bats and raccoons.

The same is true for the H5N1 strain of avian influenza. If the virus has spread well in some birds, they do not need to consider the lethality of other birds. This is also the case. This epidemic has affected some birds. It is far more lethal than other birds.

This is one of the points. Although there is a contradiction between the lethality of the virus and its ability to spread, it does not necessarily infect only one type of host.

The second point is that the ability of the virus to spread is determined by many factors. In some cases, it does not need to lose its own lethality in exchange for the ability to spread.

A simple example, one virus is very lethal, but it spreads in a room full of hosts, and the second virus is very low lethal, and the host can carry it everywhere, but it In a room with few other hosts.

Of the two viruses, which one do you think is likely to replicate more of itself? The answer must be the first deadly virus.

Third, the lethality of a virus is not necessarily the ability to kill the host, it can also make the host recover more slowly, so as to stay in the host for a longer time, thereby replicating more viruses, and spreading to more 's host.

It is not difficult to see from this point that becoming a deadly virus has both costs and benefits.

In fact, it is the same no matter from which point of view, the lethality and transmissibility of a virus is a matter of the cost of virus replication. Although a virus is just a piece of protein wrapping a piece of genetic material, its purpose is the same as that of other life forms—— Copy yourself.

Even if the genetic mutation has no direction, and the virus has no idea, they always try to find a solution with the lowest cost to replicate themselves, so as to replicate more of themselves.

Of course, sometimes the solution doesn't necessarily need to lose its lethality, it can even become more lethal.

Many researchers are now trying to mathematically find the final direction of a virus's spread in a host, and if all variables are taken into account, the direction of a virus is indeed predictable.

However, the trade-off relationship between the host and the virus is much more complex than the three cases we mentioned above, and it is extremely difficult to build a model.

Therefore, the development of the new crown can only be said that it may become a common flu, or a stronger strain may appear. Regarding the new crown, it is estimated that everyone has heard such an answer.

at last

So far, there have been two human infections of the H5N1 strain of avian influenza, but don't worry, the possibility of this bird flu being considered "human-to-human" is very low, and almost all human infected people are infected through contact with sick birds.

In addition, like human flu, bird flu in birds is actually very common. I don't know if you have heard of "chicken plague". Sometimes it is bird flu.

Even if this bird flu is "unprecedented", it has not reached the point where it will threaten our health. The most likely damage is those rare and endangered birds, because the migration of birds is very frequent and it is difficult to control the spread of the virus.