The phrase “You cannot make an omelet without breaking eggs” is often attributed to Robert Louis Stevenson, the author of “Treasure Island”. While it may have become a cliche for humans, it holds true for pathogens in their quest for survival.
As with any living organism, pathogens strive to thrive, and their ultimate goal is often to spread from host to host.
Pathogens, from minor illnesses like pink eye and the common cold to more severe diseases like COVID-19, use the hosts they infect to spread. These hosts can be thought of as eggs, with the pathogens seeking to make them ill in order to facilitate spread, whether through swollen red eyes, coughing, sneezing, or passing through bodily fluids, as explained by Virginia Tech biologist Dana Hawley.
“For a pathogen, ‘spreading’ is their key form of reproduction,” remarks professor Hawley.
“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.”
However, there is a trade-off for the pathogen, according to Hawley. While causing illness in their host can aid in the spread of the pathogen, it also means that the host may not come into contact with as many other potential hosts. Thus, while a sick host may be less likely to interact with others, when they do, they are more likely to spread the pathogen through coughing or other symptoms, making it a benefit to the pathogen’s survival.
Using finches, a type of songbirds, which are commonly affected by pink-eye disease in nature, Hawley and his team at Virginia Tech and the University of Memphis in Tennessee demonstrated how easily these pathogens can spread. They conducted the experiment without using the actual pink eye pathogen, instead using UV fluorescent powder coatings. The birds were divided into three groups: healthy, mildly ill, and severely ill, all with conjunctivitis, and each bird was housed with four healthy flockmates. The researchers applied a powder coating around the outer eye of each bird, but not inside the eye, and tracked how much powder was spread to the flockmates from birds that were severely ill, mildly ill, or healthy with pink eye.
“We weren’t actually tracing the spread of conjunctivitis. We were tracing the spread of powder as a model for the likely spread of conjunctivitis,” comments Hawley.
Birds were housed in large-flight cages throughout the investigation, sharing food and possibly distributing powder to cage mates. According to the research, feeder surfaces are a major route for the powder.
Hawley said that there were a few surprises along the road. The finches who had the most severe conjunctivitis symptoms were far less willing to eat, yet they nevertheless spread the powder more quickly than the more active, somewhat unwell birds.
“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 adds. “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.”
The Human element
What does all of this imply for people and the transmission of a common cold or COVID-19 at a doctor’s office or a movie theater, or the development of conjunctivitis in a nursery school or daycare?
Hawley says that “stay home if you’re sick” is still more important than ever. This study shows that having symptoms makes it much more likely that anything you have will spread. And wear a face mask — and not just for the potential spread of COVID.
“Wearing masks when you are coughing from any illness can likely go a long way in preventing disease spread,” Hawley adds. “For pink eye, keeping kids isolated is going to be key because young children are just not going to be able to wash their hands or avoid touching each other — speaking from experience as a parent.”
Furthermore, evolution is important. Once again, diseases are living things that are subject to natural laws like evolution. This reinforces the notion that everyone had thought COVID would gradually get milder, according to Hawley.
“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.”
In other words, common sense behaviors may save people from becoming the metaphorical “eggs” mentioned by Stevenson.
The findings of the study were published today in the journal Royal Society Open Science.
Image Credit: Getty