Epidemic study finds ‘intermediate’ severity diseases are ‘most evolutionarily successful’

Researchers at the University of Exeter have found new epidemic diseases are most successful if they are of “intermediate” severity. Until now, the scientists said conventional wisdom held new diseases evolved to be harmless. But a study published on November 12 in the journal Evolution Letters has challenged the notion by studying more than 50 variants of a bacterial pathogen that infects birds.

The news comes amid the SARS-COV-2 coronavirus pandemic, which has infected more than 53 million people and killed nearly 1.3 million since last year.

Dr Camille Bonneaud, of the Centre for Ecology and Conservation on Exeter’s Penryn Campus in Cornwall, said: “For a long time, conventional wisdom held that new diseases evolved to become harmless.

“Although theoretical developments in evolutionary biology in the 1980s showed that this was not necessarily the case, such belief still holds firm, even today.”

The new study was led by Exeter University in partnership with Arizona State University in the US and Auburn University in Australia.

The researchers focused their study on pathogen virulence or how much harm a disease causes when it appears in a new host.

Pathogens with low virulence will grow at a slower pace and may take longer to jump from host to host.

Pathogens with high virulence, however, run the risk of killing off their host before they spread.

But pathogens of an intermediate virulence appear to have hit the middle-ground and are favoured by natural selection.

Dr Bonneaud said: “Our study focused on the ‘virulence-transmission trade-off’ hypothesis, which allows us to make predictions about pathogen evolution.

“Experimental evidence for this theory is rare, but we were able to test it by using more than 50 variants of the infectious bacterial pathogen Mycoplasma gallisepticum, which infects house finches.”

For the purpose of the study, the researchers picked out house finches from a population that has never been exposed to the bacteria.

The birds were then exposed to the different variants Mycoplasma gallisepticum to simulate the outbreak outbreak of an epidemic.

Dr Bonneaud said: “We found that variants that were more virulent transmitted faster, but that variants of intermediate virulence were the most evolutionarily successful.

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“Our results, therefore, provide support for using the virulence-transmission trade-off hypothesis as a framework for understanding and predicting emerging pathogen evolution.”

Contrary to popular belief, pathogens that multiplied at much faster rates and hit higher densities, did not transmit any better or faster than those that achieved lower densities.

Dr Bonneaud said: “This tells us that transmission is not always a numbers game and that we cannot use pathogen numbers as a proxy for their success.”

Epidemics like COVID-19 have proven exceptionally good at spreading among large populations.

According to the World Health Organization (WHO), the coronavirus spreads to new hosts whether or not they are showing symptoms.

And people who have developed a more severe case of COVID-19 may be more infectious for longer periods of time.

The WHO said: “Whether or not they have symptoms, infected people can be contagious and the virus can spread from them to other people.

“Laboratory data suggests that infected people appear to be most infectious just before they develop symptoms (namely two days before they develop symptoms) and early in their illness. People who develop severe disease can be infectious for longer.

“While someone who never develops symptoms can pass the virus to others, it is still not clear how frequently this occurs and more research is needed in this area.”

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