Smithsonian National Museum of Natural History

Healthy Bats, Healthy Horses, Healthy Humans

Healthy Bats, Healthy Horses, Healthy Humans

Aired March 8, 2023

Ashley Peery:  I’m Ashley Peery, I’m an educator at the Smithsonian’s National Museum of Natural History. I’m a Caucasian woman. In my background, you are looking at a wall in my basement, there are some frame images and a large pink flower. On your screen is an image of some black flying foxes, as well as the date, time, and title of today’s program. You are here for "Healthy Bats, Healthy Horses, Healthy Humans." And welcome again to this month’s edition of the Shared Planet, Shared Health series. This is a monthly webinar, where we explore the connections between the health of humans, the health of animals, and the health of the environment, or as veterinarian Billy Karesh would say: "It’s all one health." So whether this is your first time joining us or you have attended a Natural History webinar before, welcome, I’m so glad you’re here.

Before we get started, I do need to go through a couple of housekeeping notes. So first, this discussion offers close captioning. You can turn those on or off using the CC button which is at the bottom of your Zoom interface. Second is a reminder about questions. We’re going to do questions after the presentation. You can put those into the Q&A box whenever you think of them. The time during the webinar for questions goes really quickly, so get your questions in early. Also, our team is going to be sharing some relevant links with you during the program, so keep an eye there. You’ll see a few things pop up as the webinar proceeds. And a note about format. Our presentation today is going to be from Dr. Alison Peel. She’ll give her presentation for about the first half of the webinar, and then once she concludes I’ll join her on screen to take your questions. So those are all the notes. Let’s go ahead and introduce today’s speaker. So Dr. Alison Peel is a Wildlife Disease Ecologist and Veterinarian with and interest in the dynamics and drivers of infectious disease. Her particular focus is on how human environmental impacts and climate cycles drive new viruses to emerge from bats. At present, her research focuses on the ecological drivers of Hendra virus and other bat paramyxoviruses in the Australian flying foxes. She’s passionate about understanding the processes underlying these complex systems and identifying sustainable solutions that harness the natural ecosystem processes to both prevent viral emergence and solve the environmental public health crisis we’ve created. So welcome, Alison. 

Alison Peel: Thank you very much. 

Ashley Peery: There you are.

Alison Peel: Let me just share my screen.

Ashley Peery: It looks good. I’m going to go ahead and go camera off. Take it away, Alison.

Alison Peel: Fabulous. Thanks so much, Ashley,  for the opportunity to speak today, and thank you to all of you for joining online. Okay. So here in Australia it is customary to start by paying respect to the Indigenous people who have been custodians of this land for more than 80,000 years. So here in Meanjin, Brisbane, shown on the map here, I’m on the land of the Jagera and the Turrbal people, and I acknowledge and pay my respects to them, to their connection to the land and water, and to their laws and customs.  So it’s 3:30 in the morning here, so the sun it’s yet to arise here in Brisbane. The possums are still scuttling around in the trees in my backyard. And the bats are still out feeding in nearby euclyps. And in a few hours the birds will start to sing. So with that in mind, I’d also like to pay my deep respects to the traditional elders past and present, and thank them for taking care of this land and its wildlife, which I’m fortunate enough to share with them now.

I thought of also sharing this map to provide some context to those of you who may not have traveled to Australia and don’t know its scale.

Much of my work is focused on the area that extends about 200 miles north and south of Brisbane, shown here in the box. But the bat species and viral species that I study extend from one tip of the east coast to the other. So for further context, this is the size of Australia relative to the USA, so we're thinking about processes here that occur over a huge geographical scale, and as you’d expect to find differences in climate, in ecology from Florida to Canada, so too we see differences across this scale in Australia. So much of my talk is really going to focus on what we know from this box in the middle of the east coast of Australia, but the general principles can be applied much broader than that.

Okay, I’d also like to acknowledge that the core of the webinar that we present here today comes from a study that my collaborators and I published in Nature Journal last year, and that was led by Dr. Peggy Eby and Professor Raina Plowright, but also acknowledge the huge efforts from all my colleagues as for their contributions to this work and I encourage you to take a look at the study in full, and I think some of the links that … that I’m referring to in these slides are going to be place in the chat, if you’re interested in following up.

Ok, so let’s get into it. Today I’m going to be talking about the process of zoonotic spillover, where spillover means that … transmission of infections from one species to another, so cross-species transmission, and zoonotic means that the spillover occurs from non-human vertebrates, so other animals, to people. We have various examples of viruses that can spill over from some bat species to people, and this is often true, what we call a bridging host. So for SARS Coronavirus 2, which has caused the current pandemic, we’re still not sure what that bridging host might have been, but for the … sorry … but for the original SARS, civets have been implicated as that bridging host. And for MERS, transmissions from, to humans is by camels. For Ebola virus, great apes may play a role. For Nipah virus, transmission has occurred by [Indistinct] pigs and also directly from bats to people. And the one that we’re going to be focusing on today, and will come back to in more detail in a moment, is Hendra virus.

Okay, so first, though, the outcomes of this zoonotic spillover event can vary widely, so sometimes it can result in just a single infection in humans or a local, small local epidemic, and rarely does it extend to this sort of global pandemic that we're still experiencing now. In such a case, study into the event that caused this pandemic, can be delayed and dwarfed by the immediate need of resources to fight on the public health response in people, and the opportunity to understand that initial spillover event is lost. 

So one strength of studying Hendra virus is that spillovers occur regularly and it doesn't cause on-going human transmission. 

So we can study the process of spillover without being consumed by the public health process. So let me take you back to Brisbane in 1994, into the suburb of Hendra, a Hendra zone, Hendra in particular. 

So this is the Brisbane CBD and the suburb of Hendra, which is nestled between the CBD and the bay of the ocean. 

Hendra is home to a race track and some stables and so it was not unusual at the time to see horses being walked through the street, on their way to the track, including by this man here who is Vic Rail, who was a prominent trainer at the time. 

So one day in September kicked off a series of events that have gone down in history, a sick horse have been brought into the stable from a paddock in a nearby suburb with a fever and reluctance to walk, and a vet, you know, attended that horse, but by early the next morning she was dead. 

 Two weeks later, veterinarian Peter Reid was called back because there were more than a dozen sick horses in the stables exhibiting horrific and distressing symptoms that proceeded towards a rapid death. 

Soon after that, trainer Vic Rail died, and a stable hand was unwell, and yet no one knew what the cause was.  

In the end of this outbreak about 20 horses had become infected or died, and died or were euthanized, and this story is really, it's fascinating, it’s told in a lot of fascinating details in this ABC, Australian ABC Podcast, and this story here called, "The deadly disease outbreak in sleepy suburbia." 

I found it really fascinating to listen to in terms of understanding disease investigations and outbreaks ... and I thoroughly recommend it if you're interested in hearing more. 

Okay, so this happened almost 30 years ago now. What do we know about Hendra virus now here in 2023? 

So we now know that Hendra virus is naturally circulating in flying fox populations, so flying foxes are a type of fruit bat and you'll find that we tend to use the terms bat and flying fox interchangeably, that's just a general term for a flying fox. 

We know now that Hendra virus can be transmitted from flying foxes to horses via their urine, and then it goes from horses to people. 

So we can refer to the flying foxes as reservoir host of Hendra because the natural circulation of Hendra within their population acts as a reservoir of infection for horses. 

There's no evidence of direct transmission from flying foxes to people and it's always been via horses, and it doesn't tend to make the bat sick but it's really highly fatal in horses and people. 

So since 1994, there've been about 66 spillover events, and in the south tropics, this area here, which is the same area I show you in the … the box on one of my earlier slides. 

We see that spillover from bats to horses tend to show a winter seasonality that's here in June, July, August, in this region, and that also, there's a lot of interannual variation with some years showing particularly elevated risk. 

For example this year, in 2011, we saw a very large number of spillovers within a short space of time.  

So while it doesn't tend to cause outbreaks with ongoing transmission in horses or people here in Australia, because these spillovers occur annually into species that we're in close contact with, that we're monitoring closely, the symptoms are mostly ... closely monitored, and because we have extensive data sets and long term data sets on bat ecology and on natural habitats and climate. 

This together gives us the data to understand the process that's driving that spillover event, and to get to the point where we might be able to predict spillovers. So all these reasons, for us it sort of indicates that there's a really good model system for understanding the drivers of spillover. 

Many of you may be aware that several studies have found correlations between spillovers of viruses from wildlife to people and land-use change, so that land-use change has offered habitat clearing, conversion to agricultural or urban centers. 

So these figures here show ... are figures from a range of different studies that have shown, in detecting hotspots where sort of … high risk of spillover is predicted, and generally these studies detect associations between these factors when spatial values  of the land-use, the known  distribution of the natural reservoir host and the occurrence of spillover are overlaid, along with environmental factors.  

So these studies are helpful in, helping us hypothesize on the underlying mechanisms that drive these correlations, but it's very rare to have the data to investigate those mechanisms in any detail.

So this is where our fabulous conceptual model to ... consider the underlying mechanisms of the spillover come in, and this is from a paper that was led by Raina Plowright in Nature Reviews Microbiology.

And it's a breakdown of the spillover process that allows us to see that for a pathogen to pass from one species to another, a number of barriers must be crossed.

So firstly the reservoir host must be present at a sufficient density, in a given space and time to support virus transmission within its populations and release or shed the virus into the environment at a sufficient load that could have infected your host. But first the virus must survive in the environment for long enough to come in contact with that new host, and the new host must be exposed, the virus must … breach the structural or immune barriers to infection in order to initiate that infection.

And what we also know is that many of these barriers are dynamic in space and time, so if you don't have the data and the inside of the key layers, then spillover is going to appear very random and unpredictable.

But we think that if we understand, you know, the process that occur across each of these layers in space and time, we can begin to predict when spillover is most likely to occur. 

So our team are focusing on generating key pieces of data from each layer and using models to pull it all that together in a framework. 

So this brings me back to our study that we published last year, led by Peggy Eby, where we collected empirical data. So raw data on flying foxes, their environment, and Hendra virus spillover, for 25 years, from 1996 through to 2020. 

So this included data on things like the locations and occupancy and characteristics of flying fox roosts, where they sleep and gather at night, oh, sorry, during the day. Their foraging areas, where they go out to feed from around that roost. 

Things like the climate, the food availability for them over time, measures of their health, the overall, you know, habitat availability, and of course also the presence of Hendra virus spillovers. 

So let's start by considering what we can learn by looking at the amazing ecology of flying foxes. 

So flying foxes feed on pollen, nectar and fruit of our native forest. 

So like birds and insects, they pollinate and disperse seeds into the forest. 

This is a crucial ecosystem service that they provide, but they do this over much larger distances typically than birds and insects, and they're able to connect increasingly fragmented pockets of forest. 

This is a map here zoomed in, so again, around that Brisbane region, with the circles showing flying fox roosts, and their size relative to the number of individuals in that roost across each month. 

And the plot here shows how flying foxes come and go across the landscape as they move nomadically to track short lived pulses of flowering in eucalyptus species.

So flying foxes feed on a large number of diet species, and again, this is the map of the east or southeast coast of Australia, and during summer, there's generally plenty of choice, over this wide geographical area.

And we can see across all the feeding habitats in this area shown in the red shading of this map. 

But during winter very few tree species provide food for flying foxes, and their distribution is also very limited, so you can barely even see the red shaded areas of reliable winter habitat on this map. It's restricted to just four main diet species. 

Yet when there is a good winter flowering an abundant flowering of these species in the remnant forests that do remain, a high proportion of the flying fox population can become concentrated in small areas, like it's shown here in July, 1998, with occupied roosts here, are shown by the filled circles and the size of it indicating how large those roosts are.

And 92 percent of the grey-headed flying fox population from across Australia come and gather in these regions around Brisbane to feed. 

So these limited winter food resources are particularly precious, if we lose them it's not really hard to imagine how it can have severe impact on flying fox behaviour and survival. 

So incidentally we do have an insight into what happens when we lose food resources. Since natural climate cycles intermittently result in a complete failure of flowering and starvation, avoidance behaviors in flying foxes. 

So this is just a news piece from one of the more recent food shortages that we had, from 2019, with flying foxes appearing in odd places, because they are running out of food. 

So this schematic here shows the total number of roosts at any given time, at the white axis there, vertically. And the size of the circle indicates the number of bats within each roost. 

So flying foxes respond to these acute food shortages by fissioning or breaking up into, from a few very large nomadic roosts, into many small populations    as as they expand their distribution in search of food resources and to avoid dying of starvation. 

So eventually the flowering naturally resumes and they will return to the nomadic movements. 

And these natural food shortages have been documented intermittently for over a hundred years so coming and going, so it's a natural process. 

So we identify and quantify the presence of the food shortages, which are otherwise difficult to assess over such large areas. 

Firstly, since bees use many of the same nectar sources, we use data from apiarists, who identified nine food shortages over a study period. And we found that these food shortages found by the apiarists, which is shown here by these orange bars across this time scale, that they had a clear fitness cost on flying foxes. 

So this is a plot of the intake of flying foxes into wildlife rehabilitators and we can see peaks in intake indicating sort of, bats that are coming into care through [indistinct], for starvation ... 

They're associated with those food shortage events.  

We also see massive reproduction losses associated with food shortage events, where in normal years the reproductive output so the proportion of females that are successfully rearing their young through to point of weaning is very high, it's up in the 90s, but associated with food shortage, that proportion that successfully rear young, drops quite dramatically. 

So it’s obviously having a significant effect on those animals. 

We also found strong evidence that food shortages were climate-driven, so all strong El Niño events were followed by a food shortage in the following year. 

We found that we can very loosely divide the data into two periods, so in the first period we saw populations fissioned or break up during food shortages, they returned to the baseline state after, and so this base line, number of roosts, is very stable over time. 

During this time the roosting and foraging behaviors were stable, and no Hendra virus spillovers were detected. 

More recently during what we roughly call a period of rapid change, we saw the baseline number of roosts sort of increase threefold and this is often in a step-wise manner associated with those food shortage events. 

So we think that flying foxes are no longer returning to that baseline nomadic condition after food shortages to the same extent as previously, and some of the roost fissioning due to food shortage is persisting.  

So also in this period we saw 40 Hendra virus spillovers in a study area, with about three quarters of them occurring in annual clusters of three or more. 

If we now look at these two periods and how they look on these maps here, so we've got a map, again, around this Brisbane area from 1998 and 2019, where the roosts are shown by these small circles and they’re ... filled in with yellow, if those roosts are occupied during winter time. 

The broader circles border around them indicate the foraging area, or sort of the area where the bats in those roosts used to feed. 

So we can see from this one time period to the next that there are now many more roosts, they are much more tightly packed together and a large proportion of them are occupied in winter.

Particularly in areas that did not previously support overwintering, it didn't provide food during winter.

The foraging ranges have contracted particularly in those urban areas as the roosts become more densely packed, but they also expanded into new areas, so if we compare this sort of space here where we previously did not see any foraging area encompassed, and that sort of being filled in as these new roosts form. 

And these new areas often overlap with regional and agricultural landscapes. 

So we understand or we interpret that this fissioning or breaking up reduces the energetic cost of the foraging and allows the bats to protect themselves from the effects of nutritional and energetic stress. 

It's the adaptive response.

To better understand the mechanisms associated with this rapid change of behavior, we next looked at the loss of winter foraging habitat across far South East Queensland.

So this is a map here that shows that prior to European settlement, winter habitat was really extensive in this region, shown by the green shading. 

More than 70 percent of the forest that provided winter habitat was cleared before 1996, and then clearing continued at a constant rate through to 2018    where we have the data here. 

So within those initial six years of stability when no spillover occurred, four years had large aggregations of nomadic bats with mass pulses of flowering in winter habitats. 

So this is shown in the plot down here, which is the maximum roost size, we see four years in a row in winter time, really large populations of bats    congregating in this area. 

By 2018 though we had lost nearly a third of the habitat that have been present in 1996, had been cleared. The number of roosts had increased fivefold and most of those new roosts have been in urban areas. 

And by 2020, when we looked at these periods, we had only one of those really large nomadic aggregations.     

So in this particular area we have lost almost all of that sort of nomadic aggregation. 

So how does this relate to spillover? 

This here is a simplex plot where each small dot is a flying fox roost at a given point and time, and its placement depends on the proportion of its foraging area around the roost which contains buildings, on this axis, so down in this corner here it's really highly urbanized environments, or the proportion of forest along this axis, so point out in this corner in, you know, forested, in densely forested land. 

And then finally this axis here shows the proportion of sort of agriculture or croplands. 

So up in this corner here would be open cropping land or pastures. 

So although we saw that a lot of the ... most of the new bat roosts were forming in these urban areas, most of the spillovers were associated with these newly formed roosts in agricultural areas. 

And particularly in mosaic agricultural landscapes.

So presumably a large part of this is because horses were present at high densities in agricultural areas than in urban areas, that makes sense. 

But there may also be a component of higher Hendra virus shedding in those areas as well. 

So this is another study that was led by Daniel Becker at the University of Oklahoma. 

And what we found was that Hendra virus in newly formed roosts, so this plot shows the proportion of samples tested that were positive for Hendra virus in these newly formed roosts, in the blue, peaked over winter and were higher than in the more historic roosts. 

And we found also that the Hendra virus shedding rate was even higher in those years after food shortage, shown here.  

So from all of these observations, we formed some ideas or hypotheses about the connections between these drastic changes in flying fox populations    that were being observed and the emergence in ongoing spillover of Hendra virus. 

And my wonderful collaborator, Andy Hoegh, tested them using a Bayesian Network Model. 

So our model supported that there's a climatic threshold that exists which if it's passed, it triggers that acute food shortage for flying foxes in the following year. 

This food shortage in turn triggers that adaptive behavioral response in flying foxes which forces them to fission, break up their roosts and spread across the landscape looking for food. 

These roosts are now forming and persisting in agricultural areas and these factors together culminate in an increased risk of spillover in the following winter. 

So the associations that we detected here and that are strongly supported by the model, give us a two years advance warning of that increase spillover risk. One year between that climate threshold being crossed and the food shortage, and then another year between the food shortage and that increased risk of spillover. 

But this increased risk can be stopped stop in its tracks if there is a highly productive winter flowering pulse, the spillover risk drops to near zero. 

This flowering pulse trumps any of these ... the increased risk from any of these other processes, by providing high quality food resources to presumably to support flying fox health and encourage flying fox foraging far from people and horses. 

If we look at this in a little bit more detail, this is looking at the overlap between single spillover events and flowering pulses at a relatively fine temporal scale. 

This plot here shows, each row is a year where we have data on the flowering and large aggregations of flying foxes. 

The dark gray bars represent years where ... or periods of time where there were large aggregations of flying foxes, because of large productive winter flowering in the area. 

Whereas the light gray indicates when there were no large aggregations of, you know, more than a hundred thousand individuals, again, because of a lack of that sort of strong winter flowering.

The white here just shows whenever we don't have any data.

So the first thing to notice here is that in this earlier period of stability, the flowering across the entire winter was more ... persistently predictable, in all the years that we have data, and so that was, yeah, sort of a more reliable resource for the flying foxes, compared to these lower years where we see entire winters where there is no food available or these sort of patchy arrangements where … part of the winter is available … mass flowering and in other parts there are not. 

When we add the spillovers and look at the time year when the spillovers occurred, we can see that no spillovers occurred when there were periods of highly productive winter flowering. 

So I'll say that again. When there was adequate natural flowering to support large aggregations of flying foxes, no spillovers were detected. 

All the spillovers occurred in this light gray areas when there's no productive flowering. 

So another thing to note. So now is when we see the climate driven food shortages occurring. 

The good winter flowering is rare, meaning that that protective capability is also rare. 

So food shortage is now more likely to be followed by this total patchy absence of winter flowering, which can result in increased risk of Hendra virus spillover risk. So, we tested our models with some simulated data sets as well, and our models show that a cluster spillover will occur following food shortage years without a flowering pulse. 

So, if we have food shortage and then no winter flowering pulse in the following year, we see about an 80 percent likelihood of there being a cluster of Hendra virus spillovers in the following year. Conversely, if that food shortage is followed by a good winter flowering pulse, then there's about an 80 percent likelihood in the other direction, that there will be no cluster of spillovers. 

When we look at this year-on-year, we see that our predictions match the observations really well. So, these black circles here indicate where we did observe clusters of Hendra virus spillovers, and that was predicted by our model. 

So, there's an interesting case in 2020 where we used our understanding of the climatic and food shortage signals to initially predict that it was going to be a year of high Hendra virus spillover risk, and it led us to publish a warning to horse owners and veterinarians before we fully understood how powerful the effect of the winter flowering is. 

So, in 2020, there was one Hendra virus spillover in late May at the beginning of winter, and we thought, "right, here we go." But then there was this large, a mass winter flowering event that led to the formation of a nomadic roost of more than 250 000 bats, that formed over a couple of weeks, and then we saw no further spillovers as that lasted for the remainder of the winter. 

So, while we currently have limited, well, we have the ability to predict the years of high risk of Hendra virus spillover, but more limited ability to predict the Euclid flowering in advance. 

But these associations, I think, offer us hope that sustainable solutions that are out there to prevent that viral spillover. 

In this case, it could be as simple as planting trees in carefully chosen locations, and then it can provide win-win sort of situations for both human and animal health in the environment. 

So, Peggy Eby is currently working with various habitat restoration efforts now, and we have some recent indications that trees can come into flowering within about 10 years of planting, so these solutions are achievable, and we should start now. 

And this doesn't just apply to Hendra virus, either. 

This is a figure from a recent report from the One Health High-Level Expert Panel, OHHLEP, that reports to the FAO, World Health Organization, World Organization for Animal Health, and UNEP, and they argue that we must shift our infectious disease control paradigm from largely reactive where actions like development of vaccine and therapeutics are taken to limit spread within human populations. 

Of course, you know, all those actions are really important, and we need to continue them, but we need to shift to also being more proactive, where we are taking actions to reduce the risk of spillover in the first place by better understanding infection dynamics in the natural hosts in the environment, by addressing those human activities that are increasing spillover risk and by undertaking surveillance to detect spillovers early before they spread. 

So, we need to focus more of our attention in that area. 

So, in short, we've long known that human health depends on a healthy environment, and we think that this is a really clear example of that. 

I think we can be confident that SARS-Coronavirus-2 won't be the last virus to jump species and upend the world, and so we need, well, we need to continue to plan ways to better respond to that next pandemic.

We can also take our chance to prevent it, and that's by restoring and protecting the natural barriers, which for so long have kept us safe. 

So, with that, I will thank you. Thank you for listening. Thank you to all my collaborators, particularly those in our BatOneHealth group, which is a large consortium of researchers across multiple continents. I've got the website listed here. I encourage you to go and take a look at all the fabulous work that our team is involved in. Follow us on Twitter if you are into that. And I'll thank my funding sources, and thank you again for the opportunity to speak today.

Ashley Peery: Thank you so much, Alison. That’s so fascinating. We do have some questions coming in to the Q&A box, and this is also a verbal cue to those of you that haven’t gotten your questions in, go ahead and drop those in. Alison, you’re ready to start answering questions from the audience? 

Alison Peel: Sure, absolutely. I’ll take a sip of my coffee and you can fire away.

Ashley Peery: Yeah, get some caffeine on your brain. So, our first question, I love this, it’s from Felix, age six, who is a future chiropterologist, a future bat expert. And Felix wants to know, "Do you think Hendra virus will come to America?"

Alison Peel: Great question, Felix. And I’m glad to hear that you love bats, and you see them in your future. They’re absolutely fascinating creatures. So, we know that, like all animals, you know, bats have sort of viruses and bacteria that they coexist with and don’t make them sick. And we tend to find that certain viruses tend to associate with certain bat hosts. And so we find Hendra virus here in Australia and in the flying foxes that we have here. And then in related flying foxes, but different flying fox species across Southeast Asia, we see another related virus called Nipah virus, which is very similar. So, we don’t think that Hendra virus exists in those bat populations that have Nipah virus because it’s a very, very similar virus. And, conversely, we don’t think we have Nipah virus here in Australia in our bats. And so these bats have coevolved with these viruses over long periods of time, and each of them have got their own little niche viruses. So, in America, you don’t have any of these large fruit bat species like flying foxes, so, it’s exceptionally unlikely that Hendra virus would show up on your shores there. 

Ashley Peery: Thanks for that. So, our next question, it’s another question about the bat. Alice asks, "Do flying foxes have natural predators?"

Alison Peel: Yes, often it goes unobserved, but one of the reasons, potentially, that they roost together in large roofs can be protection from predators as well. We think that they share information within those roofs somehow, although it’s a mystery exactly how that’s done. But, yeah, sometimes a range of different animal species will congregate together as protection from predators. So, in Australia, probably the two main types of predators that we see for flying foxes are raptors, sort of owls, and eagles, and falcons and things like that. And snakes, pythons in particular. I’ve seen some pythons within a roost, and in the middle of … Yeah.

Ashley Peery: Wow.

Alison Peel: Predating on a flying fox, I guess.

Ashley Peery: I’m just imagining what size those pythons must be, because these bats are large. 

Alison Peel: Yes. That’s another one of the … I mean, Brisbane has a lot of wildlife, you know, we have a lot of wildlife in our backyard, even though we’re quite close to the city. We’ve had some, you know, three-meter pythons in our backyard, so that could take a flying fox easily.

Ashley Peery: So, moving on from that, we have an interesting question from Curtis. Curtis says, "Is there a better word that we can devise than spillover? This is a complex series of steps necessary before a pathogen can move from circulating in animals to circulating in humans. And spillover makes it sound like an ‘oops’ that occurs with the cup of tea when you reach for the papers in the inbox."

Alison Peel: Yeah, that’s a good question. It’s, I mean, because it is so complex and there’s so many steps, it’s often hard to encapsulate in a single word. And I gave a talk once to a South American audience that was being translated, and it turns out there is no direct translation. Well, I was told that there was no direct translation in Spanish to spillover, so they had a lot of trouble with that with the amount of times that I said it within the talk. So, I would, you know, yeah, I think probably the community would be open to ideas about other words, but it’s fairly entrenched now.

Ashley Peery: Understood. We have another question coming from, this one… Apologies if I get your name right … Or get your name wrong. Comes from Yesenia. She says, "Hello from Texas. Great presentation. What are humans doing to protect themselves against Hendra?" And then the second part is, "What recommendations would you give horse owners to protect their horses?"

Alison Peel: Great questions. Okay, so, in terms of what people are doing to protect themselves against Hendra. So, because it actually seems like it’s quite difficult to catch Hendra from a bat, so we have a very large number of flying fox carers or rehabilitators here in Australia. So, often, as I had at the end of the slide, we can get a lot of orphan pups in the birthing season. These flying foxes, that end up caught in fruit netting or barbed wire and things like that, need rehabilitation before they can be released into the wild. And so we have, you know, these carers that have very close sort of, you know, intimate contact with these bats as they’re being nurse back to health and released. Fortunately, we’ve not seen any incidences of Hendra virus being transmitted from these flying foxes to people. Of course, those wildlife carers should take general hygiene measures, washing their hands and things like that. But thankfully it seems a little bit difficult to catch directly for ourselves when we are working within a flying fox roost, obviously, we’re down a lot closer. We’re specifically picking up sort of urine and feces from a flying fox. So, we use the kinds of PPE, Personal Protective Equipment, that you would’ve seen people wearing during the COVID pandemic. So, masks and gloves and eye protection. Because while we’re collecting the samples, the bats are roosting above us, so we can get urinated and defecated on as well. So, yeah. General hygiene, but thankfully, you know, these bats are living around us and thankfully the risk of direct transmission seems to be low. In terms of the horse owners, that’s a really good question, because what happens with horses is that they appear to be what we call amplifying hosts. So, because they have no natural defenses to Hendra virus, when the virus does get into their system, their immune system can’t cope and the virus replicates and increases rapidly. And so when they get very sick, a lot of their excretions, from their saliva or from their nose, are an absolute chunk of blood full of virus. That’s why it’s much easier for humans to catch Hendra virus from a horse rather than from a bat. So, again, you know, horse owners need to take precautions, general precautions when working with their horses, calling their vet if they suspect any clinical signs in their horses that are consistent with Hendra virus, which can be fairly vague. But the most, one of the most effective things that they can do is that there is a vaccine available for horses for Hendra virus that was introduced in 2012. And since it was introduced, none of the Hendra virus spillovers have been in vaccinated horses. So, it seems to be highly effective.

Ashley Peery: That’s great news. And actually there’s a follow-up question related to the vaccine for horses. George wants to know if there’s any risk that horses who have been vaccinated will have breakthrough infections and then potentially spread the virus to humans, but the horse is perhaps asymptomatic.

Alison Peel: Yeah, I mean, I think, because Hendra virus is so fatal to study it in a laboratory, it requires these really sort of high containment environments, so it’s very hard to, you know, and also with, you know, having a horse in a laboratory, you know, there’s only a very few number of locations where that can be done. So, there’s not been any studies that I’m aware of that have demonstrated, I guess, you know, any horses being able to be infected who have been vaccinated. The vaccine appears to be really strongly protective. So, I don’t think we think that that’s a potential risk, I guess, compared to, I mean, an analogy for the Hendra virus vaccine would be like, you know, with measles or one of our sort of main childhood vaccinations where it’s just really, really strongly protective against that infection in the first place.

Ashley Peery: Great to hear. The next two questions come from Declan. The first one is another follow up on the vaccine question and then the second one is different. So, the first one, "Equivac, which is that vaccine, has been made available as a veterinary vaccine in Australia for some time. Is there any hesitancy of horse owners to use it?"

Alison Peel: There has been some hesitancy and, that, so it was introduced in 2012 and as I mentioned, 2011 was a year where there were a very large number of spillovers. And so, I think the vaccine had, you know, been in early stages of development for a little while, but that sort of really large number of spillovers created that sense of urgency, I guess, to develop that vaccine as we saw with, again, with the COVID vaccines that were developed. And so when the vaccine was initially released in 2012, it was done so on a provisional license. That meant that it had some restrictions on how it could be used, it had to be administered by a vet, the horses had to be microchipped or with a PIT tag, so they could be individually identified and things like that. And, at that time, it hadn’t been tested in pregnant mares or pregnant female horses. And so, associated with that and, you know, similar as we saw with the COVID vaccine, that there were people that were concerned that the vaccine had been rushed, that, you know, the fact that it wasn’t registered for pregnant mares meant that it was unsafe. And as a result of that, there was some reluctance, you know, in some proportions of the population to use the vaccine. It’s an ongoing situation where the vaccination rates are lower than ideal, but they certainly seem to be preventing infections to some extent.

Ashley Peery: Good. So, the second question from Declan, they’re drawing a … Or the question is kind of … Jumps over to Nipah virus, which you mentioned in your talk is related to Hendra virus. Declan says, "Spillover events of NiV to swine and people in Bangladesh/Malaysia is a significant public health concern, do you know how the different ecological environment in the more crowded urban centers/farms, or I guess versus farms, contribute to the spillover?"

Alison Peel: Yeah, great question. So, yes, you’re right. Nipah virus first emerged after Hendra virus, so we had a little bit of a template to work with there, but it emerged in Malaysia, and it was associated with deaths and illness in piggeries, and then sickness and deaths in people as well. It resulted in large scale culling of pigs in Malaysia to try and get the outbreak under control. Some studies after that event identified that what was happening was that flying foxes were coming and feeding on fruit trees that had been planted alongside those pig pens in the piggeries. So, again, we had this situation where flying foxes had, sorry, I’ll backtrack a second as well. And it also followed a period of time where there had been a lot of fires and drought within the region. So, there was a study that looked into whether those climate drivers or potentially smoke from all the wildfires had driven the spillover events. They found that there wasn’t, they didn’t find any evidence that those climate drivers were involved, but there may have been more complex series of events than were studied at the time and similar to what we’re seeing now with Hendra virus. And so, as a result of understanding, you know, that transmission pathway, I think now the management actions have been to ensure that there’s no fruit trees alongside piggery farms. It’s a really simple intervention that can prevent spillover. In Bangladesh, where we see spillover can happen directly from people to, sorry, from bats to people through drinking date palm sap. 

Actually, within BatOneHealth group, we have some researchers that sort of have a field study based in Bangladesh, so we’re trying to study the same processes in the same way as we do here in Australia. And so what we think in some ways, there obviously has been a large amount of land clearing of natural environments in Bangladesh. And we think that the Bangladesh bat populations are further along the timescale than we see here in Australia. So, there’s lots of very, very small populations of less than 100 bats or 100 bats within roosts. It’s scattered amongst sort of villages and towns within Bangladesh, and we think that those same processes might be occurring, but it’s definitely an active area of study for us.

Ashley Peery: I’m interested to see, and one of our attendees is as well … Omari, actually, you kind of answered her question as you were answering this one, but she asked, "Do you believe the same methodologies used to understand that Hendra transmission could be used to understand Nipah?" It sounds like the answer is you think so, and your team is working on it, but those answers are still coming.

Alison Peel: Yeah, absolutely. And even more broadly than that. So I think, you know, what we can do is, you know, in the Nipah example, we are using both our understanding of the Hendra virus, well, the spillover process and our understanding of flying fox ecology to look for those analogs in the flying fox Nipah virus system in Bangladesh. So, there’s a kind of, you know, it’s obviously a very different environment. The species are slightly different, but it’s a very close analogy. But we are also looking at ways that we can adapt our understanding more broadly. So, looking at spillover of viruses from cave-dwelling insectivorous bats in other parts of the world. And, so, for me, it’s about sort of thinking about what potential natural reservoir hosts are there? What resources do they need? Are those resources under pressure? Have they been cleared? Have they been disturbed? And how are the bats responding to those disturbances? And is that behavioral change that I think that we really highlight in our study that is critical and that can result in increases in spillover risk in ways that we don’t necessarily think of in advance. It’s not just those areas where people are encroaching on wildlife populations, sort of into pristine, you know, rainforests or anything like that, but that when we clear areas of natural habitat, that the animals will respond and shift somewhere else, and that risk could emerge somewhere where, you know, more distant to where the actual encroachment is currently happening. 

Ashley Peery: I’m looking forward to seeing that research as it comes out. So, our next question is from Susan. Susan says, "Who is working on the prevention methods and what success is there?" Susan, chime back in if I’m misinterpreting this. Are you asking who’s working on those ecological solutions? Is that what you mean by prevention methods? But I guess, Alison, do you have any success you can share related to prevention methods?

Alison Peel: Yeah, so, as I mentioned, Dr. Peggy Eby is working closely with those habitat restoration efforts that, you know, as you described it recently, is a happy coincidence that some of the species that we think that we should replant and restore for flying fox health and to prevent Hendra virus also happen to be habitat trees for koalas. So, there’s a lot, you know, koalas are obviously very charismatic. They attract a lot more attention and there’s a lot of funding available and efforts ongoing to restore habitats, for example, that have been, again, lost through clearing, but also lost through wildfires as well. And so, Peggy is working with those efforts to ensure that the tree species that are being replanted have that sort of multiple purpose. They can be habitat trees for koalas, but also provide that critical food resource for flying foxes and other species. So, yeah, absolutely that work is ongoing. Initially, when we started to propose a solution, we were thinking, you know, it might be 20, 30 years before these trees, you know, these newly planted trees actually providing food sources for flying fox, but we’ve had some really encouraging indications so far that it could be much shorter than that.

Ashley Peery: That’s so wonderful to hear. Susan did chime in with a clarification, "This ongoing work, does it require government cooperation or are regular people pretty willing to plant the trees, whether they’re helping the koalas and just accidentally helping the bats as well?"

Alison Peel: Yeah, it’s a combination of both. I mean, government corporation leadership certainly helps, and a number of the programs that I mentioned are restoration programs that are funded by various levels of government from councils to state government or federal funding. But it is also a lot of NGOs, non-government organizations, that are driven by volunteers or private landholders who have property in these regions who are keen to restore some of the natural habitats. So it’s really a mix of all of the above.

Ashley Peery: Excellent. So, our next question is from Haley. Haley asks, "Do you think that the surveillance systems currently in place are robust enough to properly monitor spillover events, as it seems they will become more and more commonplace?"

Alison Peel: Okay, so, I’ll answer this question in two parts. Firstly, I’m not sure if you were talking about for Hendra virus or spillover events more broadly. So, for Hendra virus, I think we have, you know, it can always be improved, but I think we have reasonably good surveillance efforts. Obviously, as I mentioned, horses are generally, they sort of fill this niche between being pets and livestock and so, but they, in other cases, there’s often quite close contact, so if they become sick, it’s expected to be noticed compared to some other species. And so I think, for sure, we are likely to miss some spillovers. And, in fact, just last year there was a new variant of Hendra virus that we’ve detected. We can see from the genetic studies that, you know, it’s not newly evolved or anything like that. It’s been around for a long period of time, but the PCR tests that we were using to detect Hendra virus did not pick up this particular variant because it was just slightly different enough from the original one that it wasn’t detected by the assay. And so, for sure, there are probably more of those like that and spillovers that we’ve missed in the past because of issues like testing like that. In terms of surveillance to spillover more broadly, globally, I think no, we’re drastically under searching, the surveillance efforts could be scaled up drastically and in a really targeted way. I think that there’s any number of different species that could be monitored over space and time, and those efforts are really time-consuming, you know, intensive and costly. So, we can’t sort of look at everything. But some of the key areas to look at will be where there’s been rapid environmental change, where there’s bridging hosts, particularly those that are held in high densities and contact with people, and then again, those that make it into live animal markets and things like that where that opportunity for that spillover from the bridging host to people can merge.

Ashley Peery: That seems logical. So, we have time for one final question. This question is from Karen. It’s kind of a two-parter. "Is Hendra virus always present in bats? Is it beneficial to them?" And then the follow-up. "Is there any research into treatment of bats themselves to prevent Hendra kind of at the source, so to speak?"

Alison Peel: Okay. So, yes, we think that Hendra is persistent within the flying fox population. So, in that sense, at the population level, it’s always present. But in terms of the virus actually being sort of excreted or shared into the environment, there tends to be a seasonality to that. And, as mentioned, particularly around wintertime, it can be detected at any time of year, but the highest proportion of individuals that are infected tend to be around mid-winter, in this part of the world anyway. But even at those sort of peaks in time in the middle of winter, it’s probably up to about 10 percent of bats that may be infected at any given time. And other times a year, it’s close to zero. We have not been able, other researchers have not been able to detect any negative effects of the infection on bats. As to whether it has any beneficial impacts, then yeah, that would be really exciting to know. But, yeah, not something that we are aware of at the moment. There’s lots of studies that are looking into the immune system of bats to try and understand how they’ve coevolved with these viruses over long periods of times and even right back from, you know, when they first emerged or developed the ability of flight. So, in terms of treatment, I think for the health of bats themselves, there’s no need. But in terms of, you know, vaccinating bats or trying to prevent Hendra virus circulating in bats, there are very few systems where vaccinating wild populations of animals is feasible. The delivery systems and things like that are just very difficult. So, there has been this… A newspaper came out yesterday actually that showed the feasibility of doing something like that to reduce rabies transmission within vampire bats, but that’s a very particular system that sort of allows a transmissible vaccine. So, yeah, no, we don’t think that that’s a good approach for Hendra virus and really targeting that spillover host, the bridging host level is where it’s at. 

Ashley Peery: Well, thank you so much, Alison. That is all the time we have today for questions. Please, join me in giving Alison a round of applause. That was just wonderful. So, I need to say, you know, thanks to Alison, thanks to the audience for tuning in, thanks for all your wonderful questions, and thank you also to our donors, our volunteers, and all of our partners who help us reach, educate, and empower millions of people around the world every day. These webinars would not be possible without what you do, so thanks again. There’s one more link that you’ll see in the Q&A that should pop up. This is a link to our survey, so please take the survey. It helps us to improve our webinars so that you get content that you want for our future webinars. We will be continuing this over the next couple of months. Keep an eye on your inboxes and an eye on our NMNH events page for our future “Shared planet, shared health.” So, thanks again. Have a wonderful day. We’ll see you next time. 

Alison Peel: Thanks very much.

Archived Webinar

This Zoom webinar aired March 8, 2023, as part of the "Shared Planet, Shared Health" series.


Hendra virus spreads from flying foxes, to horses, and sometimes from horses to humans. The ecology of this transmission route involves an intricate interplay between the environment, bats, horses, and humans that has fascinated researchers since the virus was first characterized in 1994. Our understanding on when and why Hendra outbreaks occur has become clearer, and provides insights into how preserving bat habitats can reduce infections in horses and humans. 

In this video, wildlife disease ecologist and veterinarian Dr. Alison Peel discusses the ecology of Hendra virus spillover and shares some intriguing new research with big implications for how caring for the environment could help prevent disease spillovers in our shared planet.

Moderator: Ashley Peery, educator for the exhibition Outbreak: Epidemics in a Connected World at the Smithsonian’s National Museum of Natural History

Webinar Series: Shared Planet, Shared Health

Our virtual "Shared Planet, Shared Health" programs spotlight leading research on the interconnection between humans, nonhuman animals, plants, and their shared environment. These programs feature a presentation by an expert followed by an audience Q&A session moderated by Ashley Peery, educator for the Outbreak: Epidemics in a Connected World exhibition at the Smithsonian’s National Museum of Natural History.

Resource Type
Videos and Webcasts
Life Science
Outbreak: Epidemics in a Connected World