Evolution favours new diseases of 'intermediate' severity

November 11, 2020

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

Scientists tested the theory that pathogens (disease-causing organisms) that inflict intermediate levels of harm on their host are the most evolutionarily successful.

The study, by the University of Exeter, Arizona State University and Auburn University, found that natural selection favours pathogens of intermediate virulence (how much harm a pathogen causes) at the point the disease emerges in a new host species.

This occurs because virulence and transmission are linked, with virulence arising because pathogens need to exploit hosts to persist, replicate and transmit.

While too-low virulence will be detrimental for pathogens if they cannot transmit, virulence that is too high will also be a disadvantage if infection kills hosts so fast 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 held that new diseases evolved to become harmless," said Dr Camille Bonneaud, of the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall.

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

"Our study focussed 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."

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

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

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

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

"This tells us that transmission is not always a numbers game and that we cannot use pathogen numbers as a proxy for their success."
-end-
The paper, published in the journal Evolution Letters, is entitled: "Experimental evidence for stabilising selection on virulence in a bacterial pathogen."

University of Exeter

Related Pathogens Articles from Brightsurf:

Pathogens in the mouth induce oral cancer
Pathogens found in tissues that surround the teeth contribute to a highly aggressive type of oral cancer, according to a study published 1st October in the open-access journal PLOS Pathogens by Yvonne Kapila of the University of California, San Francisco, and colleagues.

A titanate nanowire mask that can eliminate pathogens
Researchers in Lásló Forró's lab at EPFL, Switzerland, are working on a membrane made of titanium oxide nanowires, similar in appearance to filter paper but with antibacterial and antiviral properties.

Plastics, pathogens and baby formula: What's in your shellfish?
The first landmark study using next-generation technology to comprehensively examine contaminants in oysters in Myanmar reveals alarming findings: the widespread presence of human bacterial pathogens and human-derived microdebris materials, including plastics, kerosene, paint, talc and milk supplement powders.

The Parkinson's disease gut has an overabundance of opportunistic pathogens
In 2003, Heiko Braak proposed that Parkinson's disease is caused by a pathogen in the gut that could pass through the intestinal mucosal barrier and spread to the brain through the nervous system.

Crop pathogens 'remarkably adaptable'
Pathogens that attack agricultural crops show remarkable adaptability to new climates and new plant hosts, new research shows.

Inexpensive, portable detector identifies pathogens in minutes
Most viral test kits rely on labor- and time-intensive laboratory preparation and analysis techniques; for example, tests for the novel coronavirus can take days to detect the virus from nasal swabs.

Outsmarting pathogens
A new influenza strain appears each flu season, rendering past vaccines ineffective.

Autonomous microtrap for pathogens
Antibiotics are more efficient when they can act on their target directly at the site of infestation, without dilution.

Acidic environment could boost power of harmful pathogens
New findings published in PLOS Pathogens suggest lower pH in the digestive tract may make some bacterial pathogens even more dangerous.

Protozoans and pathogens make for an infectious mix
The new observation that strains of V. cholerae can be expelled into the environment after being ingested by protozoa, and that these bacteria are then primed for colonisation and infection in humans, could help explain why cholera is so persistent in aquatic environments.

Read More: Pathogens News and Pathogens Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.