Human technology

The rabbit virus has evolved to be deadlier

A common misconception is that viruses fade over time as they become endemic in 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 results highlight the need for rigorous monitoring of human viruses, including SARS-CoV-2, monkeypox and poliomyelitis, for increased virulence.

“During the COVID-19 pandemic, many people mistakenly assumed that as the SARS-CoV-2 virus becomes endemic, it will also become milder,” Read said. “However, we know that the Delta variant was more contagious and caused more severe disease 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’s no reason this shouldn’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 suppress an out-of-control non-native rabbit population. Known as “myxomytosis”, the disease it caused resulted in swollen, fluid-filled skin lesions, swollen heads and eyelids, droopy ears and blocked airways, among other symptoms. The virus was so deadly that it killed around 99.8% of the rabbits it infected in two weeks.

Over time, however, the virus subsided, killing only 60% of infected rabbits and taking longer to do so.

“Scientists at the time thought this outcome was inevitable,” Read said. “What they’ve called the ‘law of waning virulence’ suggests that viruses naturally attenuate over time to ensure they don’t kill their hosts before they’ve had a chance to be transmitted to other individuals.”

Yet when Read and his team began studying the myxoma virus in rabbits in 2014, they found that the virus had taken over and was once again killing rabbits at a higher rate. In their most recent study, published on October 5 in the Virology Journal, they examined several myxoma virus variants collected between 2012 and 2015 in the laboratory to determine their virulence. The team determined that the viruses belonged to three lineages: a, b and c.

Interestingly, Read said, the rabbits in their study showed different symptoms from those induced by viruses collected in the first decades after release.

“Instead of developing swollen, fluid-filled lesions, these rabbits developed flat lesions, suggesting a lack of reduced immune response,” Read said. “Furthermore, these rabbits had significantly more bacteria distributed in multiple tissues, which is also consistent with immunosuppression. We interpreted this ‘amyxomatous’ phenotype as an adaptation of the virus to overcome the evolution of resistance in the population. wild rabbits.”

Line c, however, produced a slightly different response in rabbits. Rabbits infected with lineage c had significantly more swelling at the base of the ears and around the eyelids, where mosquitoes usually bite. These areas also contained extremely high amounts of virus.

“Insect transmissibility depends on the presence of large amounts of virus in sites accessible to the vector,” Read said. “We hypothesize that c-lineage viruses are capable of increased dissemination to sites around the head where mosquitoes are more likely to feed and are capable of suppressing inflammatory responses at these sites. , allowing persistent replication of the virus in large quantities.”

Read said the team’s findings demonstrate that viruses don’t always evolve to become softer.

“By definition, an evolutionary arms race occurs when organisms develop adaptations and counter-adaptations against each other,” Read said. “With the myxoma, the virus has developed new tricks, which lead to greater mortality in rabbits. However, over time rabbits will likely develop a resistance to these tricks. A similar arms race may occur with SARS-CoV-2 and other human viruses as humans become more immune. That’s why it’s so important for vaccine manufacturers to keep up to date with the latest variants and for the public to keep up to date with their vaccines. Even better would be to develop a universal vaccine that would work against all variants and be effective for a longer period of time. »

Other authors of the paper include Peter Kerr, formerly of CSIRO Health and Biosecurity, now deceased; Isabella Cattadori, professor of biology, Penn State; Derek Sim, associate research professor of biology, Penn State; June Liu, Postdoctoral Fellow, CSIRO Health and Biosecurity; and Edward Holmes, Professor of Medicine and Health, University of Sydney.

The National Institute of Allergy and Infectious Diseases supported this research, which was initially spurred by a seed grant from Penn State’s Huck Institutes of Life Sciences.

Reference: Kerr PJ, Cattadori IM, Sim D, Liu J, Holmes EC, Read AF. Divergent evolutionary pathways of myxoma virus in Australia: virulence phenotypes in susceptible and partially resistant rabbits indicate possible selection for transmissibility. J. Virol. 2022:e00886-22. do I: 10.1128/jvi.00886-22

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