Charting the Spread of Disease Ecology

Image Credit: Davian Ho, Maya Peters Kostman, and Philippa Steinberg for the Innovative Genomics Institute, CC BY-NC-SA 4.0, Image Cropped

There’s a certain poetry to the popularity of disease ecology. Once a quirky biological sub-field, the study of diseases in an ecological context had spread steadily in popularity over the last two decades. Then COVID hit, and much like the disease itself, disease ecology rocketed into the forefront of natural sciences.

This wasn’t just contained to university and hospital corridors. Before COVID, how often did you hear words like “transmission”, “virulence” and “pathogen”? While disease ecology is the crux of my professional life now, there’s little chance I would have been able to make a career of it twenty years ago.

To get some perspective, I decided to talk to people who have been there for the surge in relevance disease ecology has experienced in that time. I was recently in Kruger National Park, South Africa for the 4th International Congress on Parasites of Wildlife, and had the pleasure of sitting down with two prominent disease ecologists, Dr. Sandra Telfer and Dr. Vanessa Ezenwa, in separate meetings to talk about how the field has changed over the course of their careers.

Sandra Telfer (ST): I didn’t start out in disease ecology, my PhD was on the dispersal of animals instead. I sort of fell into disease ecology through my first postdoctoral position. At that point we were still really focused on whether these parasites or pathogens in wild animals even had an effect on the host. Such a simple question at that stage – does a parasite have any effect on survival or reproduction?

Vanessa Ezenwa (VE): I didn’t go to graduate school to be a disease ecologist. Back then, people studied infectious diseases in an ecology context, but that word, ‘disease ecology’, is a mainstream thing now. People were doing disease ecology but there wasn’t a specific conference on the topic, there wasn’t this buzz, and this growing mass of people around it. It’s so exciting. There are so many early career researchers who are attracted to disease ecology and evolution now, it’s this ever-expanding field which is really exciting.

So, neither disease ecologist started out studying parasites, but they both ended up there anyway. To me, this really highlights how parasites are everywhere and you can’t escape their influence (even if you want to!).

Technological Breakthroughs

Tech breakthroughs can often shape the entire direction of a discipline. Sandra has been lucky enough to work with some of the newest genomic technology.

Dr. Sandra Telfer, Senior Research Fellow at the School of Biological Sciences, the University of Aberdeen.

ST: It’s kind of insane compared to what we had before. I work with people who do fancy genetic stuff and it just totally transforms your ability to look at a whole range of questions. From doing sequencing of multiple individuals in a population to determine which hosts are infected, to doing whole genome sequences of a parasite like Bartonella, none of it would have been possible at all not that long ago. It opens up whole new possibilities. We can look for genes that are responsible for certain things like host specificity. We can understand how a single parasite species that we see in one host species is the same as they are in another host species. I work with the genes that are associated with host specificity, and in my system in Madagascar, we can now look at genealogy and genetics of parasites, pathogens, viruses, across an entire landscape. It’s become so much cheaper.

Me: That advance in technology, does it allow you to ask new questions? Or are they giving more nuance to the questions you were already asking?

ST: I guess they’re more nuanced questions. We understand the system much better. Obviously we’ve been able to use population genetics to understand things like host movements for a long time, but now we’re able to do it at higher resolution. It’s one thing saying we’ve got genetic differentiation at a scale of a few or ten kilometres, but if you’re able to understand movements and variation in hosts and parasites at an even more local scale, it allows us to understand the system in a whole new way.

While Sandra’s excitement has been sparked by genomic advancements, Vanessa has had her interest piqued recently by community ecology, and the interactions not just between a parasite and its host, but between different parasites within a host.

VE: Now more than ever there’s this realization that even parasites are never in a host in isolation. We’ve only scratched the surface, while we might look at a few small subsets of parasites together as co-infections, but there’s entire communities of parasites, of other symbionts. How does this huge community of organisms that live together inside a host influence one another? How are they influencing the ecology and evolution of the hosts themselves? We can now try and understand everything that’s inside a host. The viruses, the bacteria, the archaea. If we can understand the networks of interactions, we can better understand any given host-pathogen interaction. A good example of that is COVID. In the midst of COVID there were all these other respiratory and non-respiratory infections that COVID may be interacting with. How all of these things interact with one another might affect your chances of survival, the efficacy of your vaccine.

The COVID Factor

Dr. Vanessa Ezenwa, Professor of Ecology and Evolutionary Biology, Yale University.

After almost three years in a pandemic, it’s now blindingly obvious how important an understanding of diseases is not just in ecology, but in wider society. Vanessa had some thoughts on how she’s seen COVID shape the world around her.

VE: COVID has inspired many more people to study disease ecology. [Recently] I taught an undergrad Disease Ecology and Evolution class, and half of those undergrads weren’t from either of our biology programs. We had students from such a diverse range of programs, even from literature! And I think it was inspired by COVID. The number of students that saw this class and what it was about and thought “I want to take this course and learn more about infectious diseases” exploded just because of what we’ve all gone through in the last few years. It’s motivating a lot of undergrads to explore this field. And whether they go on to become disease ecologists or not is irrelevant, they’re at least learning about infectious diseases, and parasites in an ecological and evolutionary context.

Maybe that’s a silver lining of COVID, this kind of awareness of infectious diseases as organisms. It’s not just “they infect us, they kill us and we need some kind of control”. Of course that’s important, but we also need to understand just their basic ecology and evolution.

The Next Big Question

Finally, I asked them where they think the field is going next. What’s missing? What’s the next big question?

ST: Always when you’re doing fieldwork, new gadgets are great. In understanding disease dynamics in wild systems, the movement of individuals are just really important. Radio tracking in the past now seems so limited, there’s much more sophisticated loggers now. Some brands even have temperature sensors on them, so we could attach them to rodents and other burrowers, and find out more about microclimates within a burrow! From the more applied side of what I do, then building really good strong collaborations between different disciplines is crucial. We’re always trying to do it, I don’t know how good we are at doing it. Not only between disciplines, between different countries as well.The capacity building aspect, I think most of us could be much stronger at that. You can’t be an expert in everything so you need to be able to work with other experts.

Gerbillus species gerbil. New and advanced radio trackers could let us understand not only how these little animals move around, but what their burrows are like!

VE: The level of activity since COVID is huge. And on that note, we have to understand how to predict where new diseases will emerge. Which species, which regions? And how can we use the tons and tons of data that’s already available to us, to answer those questions? You might imagine, can we just go and do surveillance everywhere. No. We don’t have enough money, we don’t have enough resources. It’s not possible. And now new groups of researchers are out there using all of these new approaches, whether it’s going and getting genomic data from global surveys, or using new computational techniques and machine learning to predict which species might be associated with the type of pathogens that are emerging threats.

That area, predicting pandemics, is hard, but it’s necessary. And it’s not 100% surprising that there had been papers talking about coronaviruses being a threat going back to early last decade. We can narrow the effort to where the surveillance of these threats has to happen, using these global genomic databases. Because people are sampling broadly, not broad enough, but enough to give us a huge head start collecting data. We can even do lab experiments to recreate the conditions in places where we think emergence is likely. If we’re worried about the wet markets, can we recreate that? Can we try to understand the process by which emergence might happen in the conditions that are present in a wet market.

And there you have it, the field of disease ecology has grown so much, enough that it’s almost completely different than it was at the beginning of Sandra and Vanessa’s careers! There are still so many questions left to answer, and so much left to learn. We stand on the shoulders of giants like Sandra and Vanessa, and going forward I can’t wait to see what both they and the next generation of disease ecologists can teach us about disease ecology.

Dr. Adam Hasik is an evolutionary ecologist and Zuckerman Postdoctoral Fellow interested in the ecological and evolutionary dynamics of host-parasite interactions who hopes to have even half the career that these two have had. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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