Evolution favors new diseases of “intermediate” severity

New epidemic diseases have an evolutionary advantage if they are of “intermediate” severity, the research shows.

Scientists tested the theory that pathogens (pathogenic organisms) that inflict intermediate levels of damage on their host have the most evolutionary success.

The study, led by the University of Exeter, Arizona State University and Auburn University, found that natural selection favors intermediate virulence pathogens (how much harm a pathogen does) as the disease emerges in a new species. guest.

This occurs because virulence and transmission are linked, with virulence manifesting as pathogens have to exploit hosts to persist, replicate and transmit.

While too low virulence will be detrimental to pathogens if they cannot transmit, too high virulence will also be a disadvantage if the infection kills the hosts so quickly that the pathogen does not have time to transmit.

Over time, pathogens that show intermediate levels of virulence should therefore have an evolutionary advantage.

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

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

“Our study focused on the ‘compromise of virulence transmission’ hypothesis, which allows us to make predictions about the evolution of pathogens.

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

In the study, domestic finches from populations that had never encountered the disease were exposed to one of several variants, simulating the conditions at the outbreak of the epidemic.

“We found that the variants that were more virulent passed on faster, but that the variants of intermediate virulence were the most evolutionarily effective,” said Dr. Bonneaud.

“Our results therefore provide support for using the virulence transmission compromise hypothesis as a framework for understanding and predicting the evolution of emerging pathogens.”

Contrary to commonly held beliefs, however, variants of the pathogen that replicated faster during infection and reached higher densities did not transmit any better or faster than those that reached lower densities.

“This tells us that transmission isn’t always a numbers game and that we can’t use pathogen numbers as a proxy for their success.”

The document, published in the journal Evolution letters, is titled: “Experimental Evidence to Stabilize Selection on Virulence in a Bacterial Pathogen”.