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#17,053
The track record of `conventional wisdom' on pandemics and emerging viruses has taken a bit of beating over the past couple of decades, with many long held beliefs either shattered, or badly undermined.
As we approach the 3rd anniversary of COVID - old, oft repeated truisms - like a pandemic could last `up to 18 months', have been shown to be naively optimistic at best.
Twenty years ago, the only respiratory virus thought to have serious pandemic potential was novel influenza. Coronaviruses - while known for decades - were believed to be mild, almost inconsequential pathogens.
Then a deadly SARS-CoV emerged in China (see SARS and Remembrance) in late 2002 - albeit hidden by Chinese officials for months - before spreading internationally in 2003.
SARS was eventually contained, but it was followed up by the emergence of another coronavirus - MERS-CoV - in 2012. While neither have produced a pandemic (yet), the third time was was the charm with the emergence of SARS-CoV-2 in 2019.
Now, coronaviruses are considered very much in the same league as novel influenza viruses.
During the opening months of COVID, `conventional wisdom' led us astray again, as we were assured
1) Herd immunity would grow and rapidly end the pandemic (see GAO: A Herd Immunity For COVID-19 Primer);
2) That SARS-CoV-2 wasn't `airborne' (see COVID-19: The Airborne Division);
3) That Face masks wouldn't protect the public, and that
All of which have been proven wrong. But these `truths' died slowly, and surprisingly, some are still widely held.
Another bit of conventional wisdom that gets trotted out regularly is that viruses eventually weaken or attenuate. First popularized in the late 1800s by noted bacteriologist Dr. Theobald Smith (1859–1934), what is often called the ‘law of declining virulence’ suggests that hosts and pathogens develop a "mutually benign relationship" over time.
And while that may happen sometimes, it isn't writ in stone.
The Delta variant that emerged in 2021 was much deadlier than that `wildtype' COVID of 2020. Admittedly, it was eventually replaced by a `milder' Omicron lineage, but there are no guarantees that a more `virulent' COVID couldn't emerge down the road.
Illustrating this possibility, we have a new study from Penn State University that finds that the myxoma virus - imported into Australia in the 1950s to control their invasive rabbit population - had lost nearly half of its lethality over time, only to regain it in recent years.
First a link and some excerpts from the Penn State press release, followed by a link from the study. I'll have a brief postscript after the break.
Rabbit virus has evolved to become more deadly, new research finds
Findings underscore importance of monitoring human viruses, like SARS-CoV-2, monkeypox and polio, for increased virulence
OCTOBER 6, 2022By Sara LaJeunesse
UNIVERSITY PARK, Pa. — 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 Andrew Read, director of the Huck Institutes of the Life Sciences at Penn State. “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% 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 and 2015 in the laboratory to determine their virulence. The team determined that the viruses fell into three lineages: a, b and c.
(Continue . . . )
The full study is published in the Journal of Virology, at the link below:
Divergent Evolutionary Pathways of Myxoma Virus in Australia: Virulence Phenotypes in Susceptible and Partially Resistant Rabbits Indicate Possible Selection for Transmissibility
Authors: Peter J. Kerr, Isabella M. Cattadori, Derek Sim, June Liu, Edward C. Holmes https://orcid.org/0000-0001-9596-3552, Andrew F. Read
If viruses were stable and always left behind durable immunity in the survivors, then the ‘law of declining virulence’ might hold true. But neither are true.
Viruses are constantly evolving, and sometimes a single amino-acid change is all it takes to make a virus deadlier, or more transmissible (see A Single Mutation in Chikungunya Virus Affects Vector Specificity and Epidemic Potential).
Conventional wisdom has its place - particularly during the opening weeks and months of a pandemic when hard data is scarce or contradictory - but it needs to be taken with a very large grain of salt, else it can lead us down a very dangerous path.
