Microbes must balance reproducing quickly in hosts and keeping hosts healthy enough to remain social
One challenge of studying the ecology and evolution of infectious disease is that the times when the topics I’m most passionate about capture the public attention are often times that are also difficult. The past few weeks, this sentiment has become particularly unmistakable. In some ways, it has been exciting to see such interest in microbiology and ecology (because that’s what the spread of a disease is — population growth and ecology!) on social media, in the news, and among my friends and relatives.
For example, how many of you nonmicrobiologists now know (or want to know!) that coronavirus is an enveloped virus, which means that alcohol-based hand sanitizers are effective against it? Or that hand washing removes microbes even if it doesn’t kill them directly? And who knew how many great animations there were online of exponential population growth? At the same time, I share the same feelings of uncertainty, frustration and, yes, a little fear, as others watching this disease and our efforts to control it disrupt so many lives.
One question that caught my attention amid the overwhelming amount of commentary that is filling the news and social media is the question of whether the disease will get worse. I don’t mean, “Will more people get infected?” (the answer to that is certainly, yes), but rather, “Will the virus change and start to make people sicker?” This question can lead the imagination to some pretty terrifying places, but it is possible to use what we know about how selection shapes disease virulence to reign in these speculations and consider some realistic possibilities.
We know the case fatality rate (a measure of how deadly the virus is) appears to vary greatly in different parts of the world, different parts of a country and even … within a single location.
Virulence is a measure of how much damage a pathogen does to a host. Virulence is shaped by many different things, from pathogen traits like how quickly the pathogen replicates in the host and what kinds of adaptations it has to increase its own survival and transmission, to host traits like the age and health status of the host when they become infected (we’re seeing a good example of how host traits shape disease progression in COVID-19). Some pathogen traits affect how readily the pathogen reproduces, both within a host and in spreading from one host to another, and traits that impact reproduction are shaped by natural selection. Natural selection is nothing more than the fact that some traits cause their owners (e.g., viruses) to leave behind more offspring than others. A strain of virus with traits that cause it to infect more new hosts than a competing strain will become more common, and if those traits also affect how sick it makes the host, the average virulence of the virus will change accordingly.
Like many aspects of the biology of this coronavirus, there is still a lot that we do not know about its average virulence and whether that average is changing. We certainly know that it makes some people much sicker than others, but a lot of this variation can likely be explained by differences in patient ages, underlying health issues and (potentially), genetics. We also know that the case fatality rate (a measure of how deadly the virus is) appears to vary greatly in different parts of the world, different parts of a country and even different times within a single location. Case fatality rates can be tricky to interpret, especially in the middle of an ongoing epidemic, because they require you to know not only how many people have died from the disease, but also how many people have been infected. In many areas, testing has not been widespread enough to really know how many people have been infected (especially those who may have mild to no symptoms), so some of the apparent variation in fatality rates may arise from incomplete information. Fatality rates are also strongly tied to the kind and quality of health care available, which varies in different parts of the world.
Clues in trends
What we do know are some general trends that we have seen in other diseases. Transmission and virulence are often related because the faster a pathogen reproduces in its host, the more of that pathogen that can be produced and released, but rapid reproduction also does more damage to the host. If the pathogen relies on the host moving around and interacting with other potential hosts to increase its transmission, the microbe must strike a balance between reproducing quickly in the host and keeping the host healthy enough that they remain social. This tends to limit the virulence of pathogens that are spread by contact (which includes coughing, sneezing and touching door handles) like coronavirus. It also means that the fewer contacts an infected person has with others, the longer the pathogen must keep its host healthy to be transmitted, which may select for lower virulence.
Encouragingly, this suggests that reducing transmission, especially from the sickest patients, is probably one of the best things we can do to ensure that this microbe doesn’t become more deadly, and that is already something we are investing in heavily in many areas to protect the most vulnerable among us. Making sure that hospitals have the necessary personal protective equipment to keep the virus from spreading from sick patients is also essential, as is self-quarantining. So, as you’re continuing to think about the benefits of physical distancing (and looking forward to a time when things can get back to “normal”), consider virulence reduction as just one more potential benefit of the important work we are currently all part of.
If you’re interested in learning more about virulence evolution, I would highly recommend papers, books and TED talks by Dr. Paul Ewald, who authored the books “Evolution of Infectious Disease” and “Plague Time: The New Germ Theory of Disease.”
Dr. Allison Neal is an assistant professor of biology at Norwich University and co-director of the Vermont Science Fair. She studies parasites, especially how they interact with each other and their hosts.
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