As Covid’s death rate Globally, it has fallen to its lowest levels since the early weeks of the 2020 pandemic, and it may be tempting to conclude that the coronavirus is irreversibly mild. This concept fits in with the broad belief that all viruses start with the nasty ones and evolve to inevitably become milder over time.
Aris Katzourakis, an evolutionary biologist at Oxford University, said:
However, there is no such law of nature. The evolution of viruses is often accompanied by unexpected twists and turns. For many virologists, the best example of this unpredictability is the myxoma virus, the pathogen that has attacked rabbits in Australia for the past 72 years.
Myxoma has killed hundreds of millions of rabbits and has become the most deadly vertebrate virus known in science, said Andrew Reed, an evolutionary biologist at Pennsylvania State University. “It’s by far the biggest genocide of vertebrate disease,” he said.
After being introduced in 1950, the myxoma virus became less lethal to rabbits, but Dr. Reed and his colleagues discovered that it turned the course in the 1990s.And the latest of researchers studyReleased this month, has discovered that the virus appears to be evolving to spread more rapidly from rabbit to rabbit.
“It’s still getting new tricks,” he said.
Scientists have deliberately introduced the myxoma virus into Australia in hopes of clearing Australia’s invasive rabbit population. In 1859, a farmer named Thomas Austin imported 20 rabbits from England so that he could hunt rabbits on his farm in Victoria. Without natural predators and pathogens stopping them, they increased millions and ate enough vegetation to threaten native wildlife and sheep farms throughout the continent.
In the early 1900s, Brazilian researchers provided a solution to Australia. They found the myxoma virus in cottontail rabbit species native to South America. The virus spread by mosquitoes and fleas did little harm to animals. But when scientists infected European rabbits in their lab, the myxoma virus turned out to be surprisingly deadly.
Rabbits developed skin nodules filled with the virus. The infection then spreads to other organs, usually killing the animal in a few days. This horrific disease has become known as myxoma.
Brazilian scientists have shipped samples of the myxoma virus to Australia. In Australia, the virus has been a threat in the lab over the years, and other species have been confirmed to be non-threatening. Several scientists even injected themselves with the myxoma virus.
After the virus proved safe, researchers sprayed the virus on several Warrens to see what would happen. The rabbit died soon, but not before the mosquito bites the rabbit and spreads the virus to others. Soon, a rabbit hundreds of miles away also died.
Immediately after the introduction of myxoma, Australian virologist Dr. Frank Fenner has begun a cautious and long-term study of the genocide. He estimated that the virus killed 100 million rabbits in the first six months alone. In a laboratory experiment, Dr. Fenner determined that 99.8 percent of rabbits infected with the myxoma virus were killed, usually within two weeks.
Still, the myxoma virus did not eradicate Australian rabbits. Throughout the 1950s, Dr. Fenner discovered the reason. The myxoma virus was not so deadly. In his experiments, the most common strain of virus killed only 60 percent of rabbits. And the rabbit killed by the strain took time to succumb.
This evolution fits in with the ideas that were popular at the time. Many biologists believed that viruses and other parasites would inevitably evolve and become milder. Known As a law to reduce pathogenicity.
“Evolutionary long-standing parasites have far less adverse effects on the host than recently acquired parasites,” wrote zoologist Gordonball in 1943.
Theoretically, the newly acquired parasite was fatal because it had not yet adapted to the host. By keeping the host alive longer, the idea went on and gave more time for the parasites to multiply and spread to new hosts.
The Law of Pathogenicity seems to explain why the myxoma virus has become less lethal in Australia and why it has become harmless in Brazil. The virus has evolved much longer in South American cottontail rabbits and has not caused any disease.
But evolutionary biologists have begun to question the logic of law in recent decades. Grow growth may be the best strategy for some pathogens, but it’s not the only one. “It has the power to push pathogenicity in other directions,” said Dr. Katzourakis.
Dr. Reed decided to revisit the story of the myxoma virus when he started his lab at Pennsylvania State University in 2008. “I knew it as a textbook case,” he said. “I started thinking,’Well, what’s going on next?'”
No one has systematically studied the myxoma virus since Dr. Fenner stopped in the 1960s. (He moved to help, so he had a good reason to give it up Eradicate smallpox. )
Dr. Reed arranged to ship Dr. Fenner’s sample to Pennsylvania, and he and his colleagues also tracked more recent myxoma samples. Researchers sequenced the viral DNA that Dr. Fenner couldn’t, and conducted an experimental rabbit infection study.
When they tested the predominant virus lineage in the 1950s, they found it to be less lethal than the first virus, confirming Dr. Fenner’s discovery. And case fatality remained relatively low throughout the 1990s.
But then things have changed.
The new viral strain killed more experimental rabbits. And they often did so in new ways: by shutting down the animal’s immune system. Rabbit gut bacteria are usually harmless and multiply and cause fatal infections.
“It was really scary when we first saw it,” said Dr. Reed.
Curiously, Australian wild rabbits do not suffer from the terrifying fate of Dr. Reed’s laboratory animals. He and his colleagues suspect that the new adaptation of the virus was a response to the rabbit’s stronger defenses. the study Rabbits in Australia have revealed that they have acquired new mutations in genes involved in the front line of defense of a disease known as innate immunity.
As rabbits develop stronger innate immunity, Dr. Reed and his colleagues suspect that natural selection may have endorsed a virus that could overcome this defense. This evolutionary arms race has wiped out the benefits that wild rabbits temporarily enjoyed. However, these viruses have proven to be exacerbated against rabbits that have not evolved this resistance, such as the rabbits in Dr. Reed’s lab.
And the arms race is still going on. About 10 years ago, a new strain of myxoma virus emerged in southeastern Australia. This branch, called Lineage C, is evolving much faster than any other lineage.
According to a recent study by Dr. Reed and his colleagues, which has not yet been published in a scientific journal, infection experiments allow new mutations to do a better job of moving Lineage C from host to host. It suggests that. Many infected rabbits show a strange form of myxoma, which causes large swelling of the eyes and ears. These are exactly where mosquitoes like to drink blood, where the virus is likely to reach new hosts.
Virologists see some important lessons that the myxoma virus can provide as the world works on the Covid pandemic. Both diseases are affected not only by the genetic makeup of the virus, but also by the defense of its host.
In the third year of the pandemic, people are more protected than ever, thanks to the immunity that comes from vaccinations and infectious diseases.
However, coronaviruses such as myxoma have not taken the inevitable path to mild.
The Delta variant, which surged in the United States last fall, was more deadly than the original version of the virus. Delta has been replaced by Omicron. This caused a less serious illness for the average person.However, the virologist at the University of Tokyo experiment It suggests that Omicron variants have evolved into more dangerous forms.
“I don’t know what the next step in evolution will be,” Dr. Katzourakis warned. “That chapter of the pathogenic evolutionary trajectory has not yet been written.”
