A common misconception is that viruses become milder over time as they become endemic within a population. Yet new research, led by Penn State and the University of Sydney, reveals that a virus — called myxoma — that affects rabbits has become more deadly over time. The findings highlight the need for rigorous monitoring of human viruses, including SARS-CoV-2, monkeypox and polio, for increased virulence.
“During the COVID-19 pandemic, many people have incorrectly assumed that as the SARS-CoV-2 virus becomes endemic, it will also become milder,” said Read. “However, we know that the Delta variant was more contagious and caused more severe illness than the original strain of the virus, and Omicron is even more transmissible than Delta. Our new research shows that a rabbit virus has evolved to become more deadly, and there is no reason why this couldn’t happen with SARS-CoV-2 or other viruses that affect humans.”
According to Read, myxoma was introduced to Australia in the early 1950s to quell an out-of-control non-native rabbit population. Known as ‘myxomytosis,’ the disease it caused resulted in puffy, fluid-filled skin lesions, swollen heads and eyelids, drooping ears and blocked airways, among other symptoms. The virus was so deadly that it killed an estimated 99.8 percent of the rabbits it infected within two weeks.
Over time, however, the virus became milder, killing only 60% of the rabbits it infected and taking longer to do so.
“Scientists at the time believed this outcome was inevitable,” said Read. “What they called the ‘law of declining virulence’ suggested that viruses naturally become milder over time to ensure that they do not kill their hosts before they’ve had a chance to be transmitted to other individuals.”
Yet, when Read and his team began to study the myxoma virus in rabbits in 2014, they found that the virus had regained the upper hand and was once again killing rabbits at a higher rate. In their most recent study, which published on Oct. 5 in the Journal of Virology, they examined several myxoma virus variants collected between 2012-2015 in the laboratory to determine their virulence. The team determined that the viruses fell into three lineages: a, b and c.
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