What are spillover events?
Raina Plowright: Spillover is the transmission of a pathogen, like a virus or bacteria, between species. We call it zoonotic spillover if the pathogen is going from a non-human animal to a human. The disease that results from that is called a zoonotic disease.
Almost every pandemic in the last hundred years has started with a zoonotic spillover, but not all spillovers start pandemics. It’s like sparks and forest fires. All fires start with a spark, but most sparks do not cause forest fires. Spillovers are happening everywhere, all the time. When we walk outside and breathe the air, we’re breathing in viruses and bacteria from birds and other animals, from trees, and from the soil, but we rarely get sick. But when they do actually make us sick, they can be really serious.
How are spillovers related to pandemics?
RP: To have a pandemic, the zoonotic pathogen has to be able to transmit effectively from human to human. And that requires a virus first to get into our body and to get through our immune system and to the right place in our body where it can spread. Not only does the virus have to be able to spread among humans, but those humans have to be sufficiently connected to be able to spread the virus around.
For example, when Ebola emerged in West Africa in 2014 in a very well-connected, dense city, it spread efficiently and caused around 28,000 cases and 11,000 deaths in West Africa. In contrast, many avian influenza viruses haven’t spread amongst humans because they haven’t been able to infect humans’ upper respiratory tract. SARS-CoV-2 virus, on the other hand, infected the upper respiratory tract very well and was able to then be coughed into the air. It then survived in the air long enough to be spread to other humans.
Are there spillovers that don’t spread among humans?
RP: One common spillover pathogen is the rabies virus, which is transmitted when a rabid animal bites a human. About 60,000 people die of rabies every year, mostly in Asia and Africa. Lyme disease is another example of a zoonosis; when a tick bites an infected animal and then that tick bites a human, we can get Lyme disease from that animal. There are many, many common zoonoses, but most of them don’t spread when they get into humans. The human is a dead-end host.
What is the One Health approach to looking at spillover events?
Emily Gurley: One Health is the idea that human disease is often connected to what’s happening in our environment and in other species.
Once a spillover infection is identified, you need to look beyond the singular patient to try to understand where the infection came from, how that spillover occurred, and what the chain of events was. What other species were infected? What was the reservoir host, or animal that harbors the infection? What were the specific exposures that led to that infection?
How are those investigations conducted?
EG: The best way to do this is through a multidisciplinary approach where epidemiologists investigate human cases, clinicians investigate the disease, anthropologists and social scientists investigate any community interactions that may have occurred, and environmental scientists and ecologists investigate what’s happening more broadly in the environment and with other possible reservoir species.
One Health approaches are useful for many different kinds of outbreaks, not just zoonotic spillovers. But zoonotic spillovers are a special case where really that is the only way to understand how they occurred. It’s really in our best interest to understand how they happen, and that’s what a One Health approach can give us.
Who coordinates the different experts in a One Health approach?
EG: Any kind of outbreak investigation is led typically by public health because once a human has been infected, there is a responsibility on behalf of public health and governments to take action to investigate.
Public health authorities often have laboratories and clinicians on the team, and they may also have veterinarians that can help with investigations. They may not always have social or environmental science expertise so, often, to accomplish a One Health investigation, collaborations with academia or other partners are necessary.
That’s one reason One Health responses are more difficult to do. It’s harder to bring more people together, but if we’re able to overcome those barriers, we really have a lot to gain.
Can you offer an example of using a One Health approach to prevent spillover events?
RP: I work on a virus called Hendra virus, which is transmitted from fruit bats to horses in Australia. It has a very high fatality rate; about 80% of horses die. Humans who get the virus from sick horses have about a 60% fatality rate. I was investigating an outbreak where a horse had died, potentially exposing a human family. It had been raining really hard, and the horse had been moved from a muddy paddock into a lemon orchard, where a population of fruit bats had just moved from their typical forest habitat to eat the lemons.
We knew, as ecologists and veterinarians, that these bats, which tend to eat nectar rather than fruit, would eat lemons only if they were starving. In tracking the bats, we found that they were shedding a lot of Hendra virus because they were in a food shortage. We talked with community members who care for injured bats, and they reported signs of starvation in bats around that time. Looking over the data, we saw these spikes every few years preceded by El Niño events that were drying up the nectar on the trees, destroying the bats’ winter habitat, causing food shortages, and pushing them into farmlands. Bats were starving and shedding the virus, causing spillovers.
In 2000, we predicted a big cluster of Hendra virus spillovers based on El Niño patterns, but there were none. A remnant patch of flowers had bloomed that year, drawing bats back to their winter habitat. Data over 25 years showed that every time winter forest flowered, there were no spillover events. That gave us this whole new way of preventing spillovers. We can move horses away from trees where bats feed and vaccinate horses, which is quite effective. But we can also replant the winter habitat that has led to bats moving into the farmland in the first place. If we weren’t investigating with a One Health approach, we wouldn’t have learned those really important factors.
Tell us about the new Coursera course on One Health.
EG: I had the privilege of working with a One Health outbreak investigation team in Bangladesh for many years. We applied the approach to every outbreak where it made sense; we investigated anthrax, Nipah virus, and avian influenza events and even non-infectious causes like pesticide poisoning. All of these benefited from the One Health approach.
Our team put together a course that is designed for anyone who works on or is interested in outbreak investigation. It sets out the vision for what a One Health approach is with seven different modules from the perspective of each of the core disciplines that typically contribute to spillover investigations: clinical medicine, microbiology, epidemiology, social science, anthropology, ecology, and entomology. Students will learn what each perspective brings to an outbreak investigation and, importantly, how they work together to bring evidence from each perspective to build out our knowledge of a spillover event.
Why is a course like this important to public health?
EG: There’s a lot of discussion now about how to prevent pandemics. There’s a lot of emphasis on vaccines and responding quickly to outbreaks, which is great. But if we can work on getting further upstream to focus on prevention, I think there’s a lot of potential there.
Most global conversations about spillovers and how to prevent them are happening at a high level, focused on how government can respond. This course is designed to be the bottom-up support for those global frameworks so that we can train a workforce on how to do this and inspire them to try it out.
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