Do viruses always become less deadly over time?

The textbook example of a virus which became less deadly in order to survive may have recently become more severe again.

 A technician poses for the media with a test tube for testing against pandemic influenza A (H1N1) virus in the national reference laboratory at the Robert Koch scientific institute in Berlin, October 2, 2009 (photo credit: Fabrizio Bensch/Reuters)
A technician poses for the media with a test tube for testing against pandemic influenza A (H1N1) virus in the national reference laboratory at the Robert Koch scientific institute in Berlin, October 2, 2009
(photo credit: Fabrizio Bensch/Reuters)

The evolution of a rabbit virus in Australia shows that viruses don't necessarily become less deadly over time, according to a new study published last week.

The study, published in the peer-reviewed Journal of Virology, follows a virus called myxoma, which was released in Australia in 1950 in order to control the rabbit population. It was also released into the wild rabbit population in France in 1952.

Myxoma has been used as the textbook example of how viruses tend to become less deadly over time in order to avoid killing off their hosts, but the new findings published in the study question that conclusion.

While the virus was very deadly at first, it eventually became less severe over a matter of years. However, samples taken in the 1990s show that the virus became more lethal again, attacking the rabbits' bodies in new ways.

Samples recently taken in Great Britain presented similar results, with scientists positing that this suggests convergent evolution, meaning that a number of separate lineages evolving separately appear to be evolving in similar ways.

 Bunny rabbit at Alligator Bay, Beauvoir, France. (credit: CREATIVE COMMONS) Bunny rabbit at Alligator Bay, Beauvoir, France. (credit: CREATIVE COMMONS)

Myxoma causes lesions and swelling on the head. The virus, which is primarily transmitted by mosquitoes or fleas, is very immunosuppressive and typically causes death within 10 to 15 days after infection.

Over time, the symptoms of the virus changed. While at first it caused raised domed lesions, it evolved in Australia to cause flat lesions instead. In Europe, the virus started causing a disease referred to as amyxomatous due to the fact that infected rabbits no longer developed lesions, although they did still suffer from swelling.

Recent samples of the virus showed new symptoms, including hemorrhaging, pulmonary edema and large masses of bacteria without any visible inflammatory response.

A similar development of symptoms was seen in both Australia and the UK, despite there being almost no shared mutations between the two lineages. The researchers pointed out that this suggests that the viral genomes offer multiple pathways to the same traits.

The researchers wrote that they believe the new, more deadly version of the virus evolved in order to overcome evolving resistance in the rabbit population, a sort of "biological arms race."

Researchers stress that novel coronavirus isn't guaranteed to become less severe

While the researchers in the study had found indications of this trend in the past, this latest study comes as questions remain about the future of the SARS-CoV-2 pandemic, including whether or not the virus will continue to become less severe.

“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 Andrew Read, co-author of the study and director of the Huck Institutes of the Life Sciences at Penn State, in an article on the university's website.

"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,” he said.

"By definition, an evolutionary arms race occurs when organisms develop adaptations and counter-adaptations against each other,” said Read. “With myxoma, the virus has developed new tricks, which are resulting in greater rabbit mortality. However, over time, the rabbits will likely evolve resistance to these tricks.

"This is why it’s so important for vaccine manufacturers to keep up with the latest variants and for the public to stay up to date on their vaccines. Better still would be to develop a universal vaccine that would work against all variants and be effective for a longer period of time,” said the Penn State researcher.

Evolving Omicron subvariants spark concern

In recent weeks, multiple preprint studies have warned that Omicron subvariants are evolving to better evade immunity.

A number of new subvariants have been developing in multiple locations and converging on similar mutations despite being geographically isolated from one another.

The authors of one of the recent preprint studies wrote that "such rapid and simultaneous emergence of variants with enormous advantages is unprecedented."

Immunologist Yunlong Richard Cao, one of the authors of that study, called BA.2.75.2 and BQ.1.1 the "most antibody-evasive convergent variants tested, far exceeding BA.5 and approaching [the level of] SARS-CoV-1," the original SARS virus which caused an outbreak in the early 2000's.

That study noted that the new variants could escape the majority of neutralizing antibodies, which may make it hard for patients' bodies to neutralize the virus and could lead to more severe symptoms.

Health Maintenance Organizations in Israel and in multiple other countries have recently begun offering a booster shot tailored for the Omicron vaccine. In Israel, anyone over the age of 12 can make an appointment at their HMO to get the shot.