Do you think that all viruses will soften over time? This is not a rabbit killer.

As Covid’s death rate around the world drops to its lowest level since the first weeks of the pandemic in 2020, the coronavirus may be tempted to conclude that it is becoming irreversibly mild. This is in line with popular belief that all viruses become disgusting over time and gradually evolve.

Aris Katzurakis, an evolutionary biologist at Oxford University, said: “There is a predominant story that natural forces will solve this pandemic for us.”

But there is no such natural law. The evolution of the virus often requires unexpected twists and turns. For many virologists, the best example of this surprise is the pathogen that has been killing rabbits in Australia for the past 72 years: the myxoma virus.

According to Andrew Reed, an evolutionary biologist at Pennsylvania State University, myxoma has killed hundreds of millions of rabbits and become the most dangerous vertebrate virus known to science. “This is the biggest massacre of spinal diseases,” he said.

Myxoma virus was less lethal to rabbits after its introduction in 1950, but Dr. Red and his colleagues discovered that he had changed direction in the 1990s. A recent study published this month by researchers shows that the virus is spreading faster from rabbit to rabbit.

“It’s still getting new tricks,” he said.

Researchers introduced the myxoma virus to Australia in the hope of eradicating the country’s invasive rabbit population. In 1859, a farmer named Thomas Austin brought twenty rabbits from England to hunt on his farm in Victoria. There are no natural predators or pathogens to trap them, they have multiplied by millions and eaten enough plants that pose a threat to the continent’s wildlife and sheep farms.

In the early 1900’s, researchers in Brazil proposed a solution to Australia. They found the myxoma virus in a species of cotton-tailed rabbit from South America. The virus, which is transmitted by mosquitoes and fleas, has caused little harm to animals. However, when scientists infected European rabbits in a laboratory, the myxoma virus caused an amazing death.

The rabbits’ skin became infected with viruses. The infection then spreads to other organs, and the animals usually die within a few days. This terrible disease is called myxomatosis.

Brazilian scientists sent samples of the myxoma virus to Australia, where scientists tested it for many years in laboratories to determine if it posed a threat only to rabbits and not to other species. Some scientists have injected myxoma viruses into themselves.

Once the virus was found to be safe, researchers sprayed it on several warrens to see what would happen. The rabbits died quickly, but not until the mosquitoes had bitten them and spread the virus to others. Hundreds of miles away, rabbits soon died.

Shortly after myxoma was introduced, an Australian virologist, Dr. Frank Fenner began a thorough, long-term study of his massacre. He said the virus had killed 100 million rabbits in the first six months alone. Dr. In laboratory experiments, Fenner found that 99.8 percent of rabbits infected with the myxoma virus usually died in less than two weeks.

However, the myxoma virus has not been able to eradicate Australian rabbits. Until the 1950s, Dr. Fenner found out why: Myxoma virus was less lethal. In his experiments, the most common strains of the virus killed 60 percent of rabbits. The submission of the rabbits that killed the strains took a long time.

This evolution corresponds to the popular ideas of the time. Many biologists believe that viruses and other parasites gradually evolve and soften, which has been called the law of diminishing returns.

Zoologist Gordon Ball wrote in 1943: “According to evolution, long-lived parasites are far less harmful to their hosts than recent ones.

Theoretically, newly acquired parasites die because they have not yet adapted to their owners. Keeping the host alive for a long time gave the parasites more time to multiply and spread to new hosts.

The law of diminishing virulence seemed to explain why myxoma viruses were less lethal in Australia and why they were harmless in Brazil. Because the virus is so prevalent in South American cotton-tailed rabbits, they no longer cause the disease.

In recent decades, however, evolutionary biologists have begun to question the logic of the law. For some pathogens, gentle growth may be the best strategy, but it is not the only one. “There are forces that can push a virulent in the other direction,” he said. Katzurakis said.

Dr. When Reed started his lab in Penn State in 2008, he decided to revise the story of the myxoma virus. “I knew it as a textbook,” he said. “What’s happening?” I began to think. ”

Myxoma virus has not been studied by anyone since the doctor’s appointment. Fenner stopped in the 1960s. (She had moved to help eradicate smallpox, so there was reason to refuse.)

Dr. Organized training for doctors. Fenner’s samples were sent to Pennsylvania, where he and his colleagues found the latest mixoma samples. Scientists have listed the DNA of viruses. Fenner failed to do so – and conducted an infectious study of laboratory rabbits.

When they tested the dominant viral lines in the 1950s, they found that they were less lethal than the original virus. Fenner’s findings. Mortality rates remained relatively low in the 1990’s.

But then things changed.

New viral strains have killed many laboratory rabbits. And they often do so in a new way: by shutting down the animal’s immune system. Intestinal bacteria in rabbits are usually harmless, multiply, and cause deadly infections.

“It was really scary when we first saw it,” the doctor said. Read

Interestingly, wild rabbits in Australia Dr. J. Training laboratory animals. He and his colleagues suspect that the new adaptation to the virus was a response to the strong defenses in rabbits. Studies have shown that Australian rabbits have new mutations in genes involved in the first line of defense against a disease known as innate immunity.

Because rabbits have a stronger innate immunity, Dr. Red and his colleagues are skeptical that natural selection, in turn, favored viruses that could overcome this defense. This evolutionary arms race has eliminated the short-lived advantage that wild rabbits have. However, these viruses have been shown to be even worse against rabbits that have not yet developed this resistance, e.g. Training laboratory.

And the arms race is still going on. About a decade ago, a new line of myxoma viruses appeared in southeastern Australia. This branch, called Lineage C, is developing much faster than other breeds.

According to a recent study, infectious experiments suggest that the new mutations will allow Lineage C to perform its host-to-host transition better. The study and its colleagues have not yet been published in a scientific journal. Many infected rabbits show a strange form of myxomatosis, with large tumors in their eyes and ears. This is where mosquitoes like to drink blood – and viruses have a better chance of reaching a new host.

Virologists see important lessons to be learned from the myxoma virus as the world struggles with the Covid pandemic. Both diseases are affected not only by the genetic makeup of the virus but also by the immune system.

As the pandemic continues for the third year in a row, people have been protected from unprecedented immunizations and immunizations.

However, the coronavirus, like myxoma, is not necessarily mild.

The Delta variant, which appeared in the United States last fall, was more deadly than the original version of the virus. Delta has been replaced by Omicron, which causes less severe disease for the average person. However, virologists at the University of Tokyo have conducted experiments and speculated that the Omicron variant is becoming a dangerous form.

“We don’t know what the next step in evolution will be,” he said. Katzurakis warned. “This section of the trajectory of the evolution of virulence has not yet been written.”

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