Before an Animal Disease Becomes a Human Epidemic

Most animal-health research focuses on infections that have already made the leap to people. A veterinary scientist explains why that may be bad for public health.

In 1920, a shipment of cattle on its way from India to Brazil made a pitstop at the port of Antwerp, where it deposited a surprise—and unwelcome—gift: rinderpest, a viral cattle disease with a death rate close to 100 percent.

The Belgian outbreak was a catalyst for the formation of Office International des Epizooties (OIE), an international body that established itself four years later to regulate animal health in international trade. Rinderpest was formally eradicated in 2011, but the OIE—which kept its original acronym through a name change to the World Organization for Animal Health—has since expanded the scope of its monitoring. Today, the group keeps a running list of “notifiable” diseases, requiring its member states to report all animal cases of infections both familiar (anthrax, West Nile) and obscure (goat pox, rabbit hemorrhagic disease).

The official list, updated each year, currently tops out at more than 100—but a small minority of those diseases receive the lion’s share of research funding, according to new research from the University of Sydney. In a study published last week in the Proceedings of the National Academy of Sciences, the researchers compiled around 16,000 studies published between 1912 and 2013 that mentioned wildlife, domesticated animals or livestock, and one of 118 OIE-flagged diseases. Just 10, they found, accounted for roughly half of all published research. And of those 10, nearly all were zoonoses, or diseases that had been transmitted from animals to humans—an approach that the authors argue amounts to too little, too late.

A more effective use of time and money, they say, would be to focus on wildlife diseases before they spread to livestock, and livestock diseases before they spread to people. “Infectious diseases at the wildlife–livestock interface threaten the health and well-being of wildlife, livestock, and human populations, and contribute to significant economic losses to each sector,” they write. “Surveillance and research strategies that target specific wildlife–livestock interfaces may yield the greatest return in investment.”

I spoke with Siobhan Mor, one of the study’s co-authors and a professor of veterinary science at the University of Sydney, about the animal diseases that get attention, the ones that don’t, and the public-health case for focusing on pathogens that haven’t yet made the leap to humans. Below is a lightly edited and condensed transcript of our conversation.

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Cari Romm: Can you walk me through your study? What was the goal of looking through the past hundred years’ worth of research?

Siobhan Mor: A lot of research before this has set out to look at diseases that affect humans that come from animals, so emerging infectious diseases that spill over from animals to people. Our study was a little different in that we set out to look at diseases that are shared between wildlife and livestock. Oftentimes when there’s a disease that comes from animals into people, it first spreads from wildlife into domestic animals, which represent an intermediate host for the diseases, and then spreads from livestock into people.

So our aim was to try to identify the diseases are that are occurring or that are being shared between wildlife and livestock, what types of animals are that are involved in those types of transmission events, and then also how they spread geographically and how things might have changed over time. It says a lot about where we’ve put our research investments. We’ve focused very much on human-health concerns, and where that becomes potentially problematic is it means we might not have the knowledge base around what kinds of diseases are in animals, and what kinds of diseases have yet to actually spread to humans.

Wildlife health for a long time has been fairly neglected. Wild animals were just wild animals, and we didn’t investigate their health problems to the extent that we do with livestock. But in recent decades, many of the diseases that spread to humans have origins at this interface between wildlife and livestock, and the interactions between those animals leads to spillover into humans.

Romm: What are some of the diseases that have jumped from wildlife to livestock to humans?

Mor: An example there might be Middle East Respiratory Syndrome, or MERS. More recently there’s been transmission in hospitals settings and health-care settings. But the research now suggests that camels as much as 20 years ago were experiencing that infection, but it wasn’t something we were looking for at the time. SARS would be another example. The wildlife in that case was bats. And in Australia, we would use the example of the Hendra virus, which goes from bats into horses and horses into people.

Romm: What are the animal diseases that have gotten the most attention?

Mor: In the top 10, the first one was avian influenza. It was quite an outlier. Avian influenza spreads potentially from domestic birds to people, and wild birds are sometimes implicated in the disseminations of the virus. But research also shows that trading routes for domestic birds can contribute to the spread of the infection, so wild birds aren’t always the transmission. And then for the other [frequently studied] diseases, salmonella was one. Bovine tuberculosis and rabies were also diseases that were ranked quite highly in terms of research.

Romm: In your opinion, are there any animal diseases in particular that should be receiving more attention than they currently are?

Mor: It’s a tricky question, because there are some diseases like rabies, for instance, that have received a fair amount of attention from the research point of view, and yet we haven’t been able to successfully eradicate it. We talk about having the technology to eradicate rabies, but we haven’t been able to do that—the investment has been on the research side of stuff, but it hasn’t necessarily led to tangible outcomes on the disease-control side.

One other thing we said [in the study] is that there’s been fairly large investment around diseases that involve poultry and cattle, and relatively less investment in research on pigs, diseases that spread to pigs from wildlife and vice versa. But pigs are an important species because they are very rapidly growing as an industry, globally. There’s a lot of investment in pork production around the world. So we make the case that it might be important to invest more strongly in the future in that particular interface, and to understand what particular risks might come from pig production in different countries.

Romm: You mentioned eradicating rabies—how would a disease that originates in wildlife be eradicated?

Mor: We know that the dog vaccine is highly effective, and we know that it’s very cheap and affordable and quite easy to roll out. The bigger struggle is … Oftentimes in developing countries, which is where the burden of this falls, in Africa and parts of Asia, there’s not good data on how common rabies is. And because the recorded cases are likely to be an underestimate of what is truly happening, it means that the evidence base isn’t there for governments to make decisions about how important the disease is.

But the reality is, we need to realign our goals. It’s not about eradicating the animals that have the disease; it’s about managing our behaviors and our interactions with those animals. A lot of human diseases that originated in wildlife have actually stemmed from human activity. We’ve cleared [much of] the earth’s surface in order to make way for our domestic animals and urban environments, and to produce our food supplies. So that obviously had a massive impact on wild species that have lost their habitats, and that means two things. It means domestic animals are in closer contact with the wildlife species, because they’re effectively moving into the wild habitats. But also, for displaced wildlife, it means that they’re in search of other areas to find food and shelter. It means that wildlife are now potentially in urban environments as well.

That’s one of the reasons why the Hendra virus in Australia spread from bats to horses to humans: Bats used to live in forests where there are no horses, but we cut down those forests, and now the bats take refuge in other areas that bring them into closer contact with horses. So a lot of it’s not about necessarily controlling a disease in wildlife. It’s about being more conscientious about our impact on wildlife and when we’re encroaching on their habitats, and modifying our behaviors around them.

I know when SARS disappeared from the human population that some scientists did call it eradication, when in reality, viruses that are relatives of the SARS virus were still present in bats. A lot of it relates to semantics in some ways. The whole reason why pathogens are able to jump species  relates to their evolutionary history, and they might evolve to acquire the ability to spill over into these new hosts. So whether or not you can say it’s been eradicated is debatable. Some people say, yes, that virus with that exact makeup has been eradicated. But on the other hand, the risks still exist, in my opinion, because you still have viruses that are potentially the precursor to another type of infection.

Romm: So when people talk about eradicating diseases, like with rabies, they're really talking about putting up a barrier between domesticated animals and humans?

Mor: Or between the wildlife and the livestock—decreasing contact between wildlife and livestock, and also reconsidering some of the ways in which we’re raising livestock. We’re now raising livestock in very densely populated conditions—feed lots, for instance—and this is why pigs are so important, because in many system they’re raised on contact with a large number of other pigs. Poultry is similar. In some ways it’s sort of the animal equivalent of a slum, and we all recognize on the human side that when you put lots of people together, that sort of condition is very conducive to transmission of infection. So when we put animals in that condition, that’s exactly what happens. The part about strategy, in addition to preventing contact between wildlife and domestic animal species, is also trying to determine how to better raise livestock in conditions that don’t favor disease transmission.

Romm: Among the zoonotic diseases you studied, what determines which ones get more attention than others?

Mor: One thing we found in our research was how much the perception of a problem was really driving the research. So in our paper we give two examples, one being avian influenza and one being bovine tuberculosis. In both cases, we have the perception that they have a large potential to cause harm to humans, and therefore there’s a lot of research investment.

But if we look back and think about the actual impact  to animal health, production, and welfare, those two diseases are not quite so important. On the avian influenza side, there’s a lot of investment around wild-bird surveillance, even in very resource-poor countries that don’t have the best surveillance systems for a whole range of diseases that actually have more impact from an animal-health point of view, as well as from a human-health point of view. And yet the investment was on this perception of risk. So we need better ways in the future to identify and prioritize diseases based on actual risk to animal health and public health.

Romm: How would you identify those diseases? Is it possible to predict ahead of time which animal diseases will make the leap to humans?

Mor: At the moment, we’re not quite in a position to be able to do that. There are researchers that are looking in wildlife to determine what’s out there, and trying to understand what families of viruses are more likely than others, based on certain characteristics and historical examples, to be candidates for spillover. I keep using viruses as an example, but the same can be said of bacteria and to a certain extent parasites as well.

Our best bet at the moment is just trying to understand are the particular mutations that allow viruses to change. It’s through those changes that they can propagate readily in humans. For a lot of the diseases that make headlines, the spillover from animals into humans could just be a singular event, and it’s when that pathogen has acquired the ability to transmit from human to human that it becomes a successful human pathogen. The origin of the disease in animals, once it becomes a human infection, is in some ways irrelevant. That’s the case with Ebola in west Africa—our understanding is that it came most likely from direct human contact with a bat, but once it was in people it largely becomes a health-care-transmitted infection and the animals themselves are pretty irrelevant to the picture.