It may be tempting to conclude that the coronaviruses is becoming milder as the Covid death rate has fallen to its lowest level since the early weeks of the Pandemic. That idea is in line with the belief that all viruses start off nasty and eventually evolve to be less nasty over time.
Aris is an evolutionary biologist at the University of Oxford.
There is no law like that. The evolution of a virus can be unexpected. The myxoma virus, which has been killing rabbits in Australia for 72 years, is the best example of this unpredictability.
Andrew Read, an evolutionary Biologist at Pennsylvania State University, said that myxoma is the most deadlyviruses known to science. He said it was the largest carnage of any disease.
After the introduction of myxoma virus in 1950, it became less lethal to rabbits, but in the 1990s it reversed course. The researchers found that the virus appeared to be evolving to spread from rabbit to rabbit.
He said that it was still receiving new tricks.
The aim of introducing the myxoma virus to Australia was to wipe out the country's rabbits. Austin imported rabbits from England in order to hunt them on his farm. Without natural predator or pathogen to hold them back, they were able to eat enough vegetation to threaten native wildlife and sheep ranches.
Researchers in Brazil came up with a solution for Australia in the early 1900s. The cottontail rabbit is native to South America. The animals were not harmed by the viruses. The myxoma virus killed European rabbits in the lab.
The rabbits had skin that was filled with Viruses. The animals were usually killed in a day or two. The disease was called myxomatosis.
Scientists in Australia spent years testing the myxoma virus in labs to make sure it wasn't a threat to other animals. Scientists injected myxomaviruses into themselves.
Researchers sprayed the virus into a few warrens to see if it worked. The rabbits died, but not before the mosquitoes bit them and spread the disease. rabbits hundreds of miles away were also dying
Dr. Frank Fenner started studying myxoma after it was introduced. The virus killed 100 million rabbits in the first half of the year. In laboratory tests, Dr. Fenner found that the myxoma virus killed almost all of the rabbits it was in.
The Australian rabbits were not wiped out by the myxomaviruses. The myxoma virus became less deadly through the 1950s. The majority of the rabbits were killed by the most common strain of the virus. The rabbits took a long time to die.
At the time, this evolution was popular. The law of declining virulence was believed to be the reason for the evolution of viruses and other parasites.
The zoologist Gordon Ball wrote in 1943 that long standing parasites have less of a harmful effect on the host than recently acquired parasites.
The theory says that parasites were deadly because they hadn't adapted to their hosts. Keeping a host alive was thought to give parasites more time to grow and spread.
The law of declining virulence seems to have explained why myxoma viruses were harmless in Brazil and less lethal in Australia. South American cottontail rabbits have no disease at all due to the fact that the viruses have been evolving for a long time.
Evolutionary biologists have questioned the logic of the law in the past. Growing milder is the best strategy for some diseases. The virulence can be pushed in the other direction.
After starting his laboratory at Penn State, Dr. Read decided to revisit the myxoma virus saga. He knew it was a textbook case. I started to wonder what was going to happen next.
After Dr. Fenner stopped studying the myxoma virus, there was no systematic study of it. He had moved on to help eradicate the disease.
Dr. Read was able to arrange for Dr. Fenner's samples to be shipped to Pennsylvania, as well as track down more recent myxoma samples. Dr. Fenner could not do the work that the researchers did, but they did the work that they could.
They confirmed Dr. Fenner's findings when they tested the viruses that had been dominant in the 1950s. The fatality rate was low through the 90s.
Things changed afterwards.
The lab rabbits were killed by newer viruses. They shut down the animals' immune systems. The rabbits' gutbacteria caused fatal infections.
When we first saw that, it was frightening.
Wild rabbits in Australia have not been killed like Dr. Read's lab animals. The new adaptation in the viruses is thought to be the result of stronger defenses in the rabbits. Australian rabbits have gained new genes that are involved in innate immunity.
Dr. Read and his colleagues believe that natural selection favored viruses that could overcome the rabbits' innate immunity. The advantage the rabbits had was erased by this arms race. The rabbits in Dr. Read's laboratory were not resistant to these viruses.
The arms race is continuing. A new group of myxoma viruses appeared in Australia a decade ago. The other lineages are evolving slower than this branch.
According to the latest study by Dr. Read and his colleagues, there are new genes that allow Lineage C to do a better job of getting from host to host. Many rabbits show a form of myxomatosis that causes huge swellings on their eyes and ears. These are the places where mosquitoes like to drink blood and where the viruses may have a better chance of getting to a new host.
Some important lessons can be learned from the myxoma virus. Both diseases are affected by the host's defense against the virus.
People are more protected than ever because of the immunity that has developed from vaccinations.
The coronaviruses have not been on a path to mildness.
The Delta variant was more dangerous than the original version. Delta caused a lot of disease for the average person. Experiments done at the University of Tokyo suggest that the Omicron variant is becoming more dangerous.
There is no idea what the next step in evolution will be. The chapter in the trajectory of virulence evolution has not been written.
All Rights Reserved © 2024
