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If pathogens wish to spread, why make their hosts sick?

Scientists have long pondered why pathogens often cause illness in their hosts. Although, at first sight, making a host sick or even killing it may not seem a good way for pathogens to spread, a team of researchers led by Virginia Tech has now found that, in certain cases, making hosts ill is in fact crucial for increasing the infectiousness of some diseases. 

“For a pathogen, ‘spreading’ is their key form of reproduction. And when we think about why pathogens make their hosts sick, it’s long been a mystery, because making a host sick or making your host die is superficially not a good way for a pathogen to be able to spread. A very sick host will stay home and not interact as much as others, which means less spread potential for a pathogen,” said study lead author Dana Hawley, a professor of Biology at Virginia Tech.

Yet, “making your hosts feel ill can be important for getting some of the copies of yourself out of the host you are infecting and into another. So there is a trade-off for the pathogen. Making your host feel sick means that host may not interact with as many other hosts as they normally would – this is bad for the pathogen – but when they do have interactions, a very sick host that is coughing or has swollen eyes is going to be much more likely to spread whatever pathogen it has in its body. This is good for the pathogen.”

To better understand this phenomenon, the scientists used finches – a species of songbirds whose populations are affected by a bacterial pink-eye disease (conjunctivitis) – as a case study. They divided the birds into three groups – not ill, mildly ill, and strongly ill – and, after applying a UV fluorescent powder coating around their eyes, housed each of them with four healthy flockmates. The fluorescent coating allowed the researchers to track how much powder was spread to flockmates from birds with different symptoms.

Although the finches experiencing the strongest conjunctivitis symptoms were far less likely to eat, they nonetheless spread the powder at a higher rate than the mildly ill birds who spent more time feeding. “In our study system, the benefits of making your host sicker by increasing eye swelling outweighed the cost of making the finches feed and interact less,” Hawley explained. “So overall, this pathogen is going to likely evolve to cause more harm to birds in nature so that it can spread at a higher rate, but up to some limit, because if the pathogen kills a bird immediately, the pathogen doesn’t have a chance to spread at all.”

These findings could have important implications for managing human-infecting pathogens too. “This goes back to the idea that everyone hoped that COVID would evolve to become milder over time. Our study shows that the pressures on pathogens are complicated. On the one hand, being mild is good for pathogens if it keeps your host out and about and in others’ company – good for spread, but on the other hand, being mild may mean that none of the pathogen makes it out of the host and into another because your host isn’t coughing or depositing as much pathogen onto hands or other surfaces. So pathogens are in many cases going to be favored to make us sick,” Hawley concluded.

The study is published in the journal Royal Society Open Science.

By Andrei Ionescu, Staff Writer

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