The expansion, intensification, and advancement of agriculture—particularly in developing countries, and especially in the tropics—brings greater food security and economic development to millions. But as populations swell and agriculture’s footprint sprawls, there are some real, and serious, public health consequences. When environments collide—when livestock graze on recently deforested land, or when bushmeat is sold at wet markets in crowded cities—when livestock, wildlife, and humans all collide—diseases can jump from their wild hosts into an accidental host, a process called “spillover.” These viruses that spillover from animals to humans, called zoonotic diseases, are responsible for 2.4 billion illnesses and 2.2 million deaths annually—making agriculture’s role in spreading these diseases no small issue.
Two major ways for these wild viruses to come into contact with humans and livestock are through deforestation, and increased proximity of humans, wildlife, and livestock. Clearing forests for more farmland, and doing more with what land farmers do have, are natural responses—and often necessary—to meet the food demands of the world’s growing population. But these actions sometimes have unintended consequences: deforestation drives animals out of their habitats and into contact with livestock, and an increased density of humans and animals makes it easier, and more likely, for diseases to spread.
Bats are one of the best examples of how spillovers happen. Bats, especially the large fruit-eating bats of the tropics, like the flying fox, are suspected to harbor a number of the most famous, and serious zoonotic diseases, from Hendra and Nipah, to Ebola. Bats are able to live with these viruses unharmed because their unique, robust immune systems keep these viruses in check. But when deforestation, drought, or general human encroachment places them, livestock, and humans in close proximity, there’s rare chance for these viruses to jump from bats to humans and livestock.
In 1999 in Malaysia, a massive drought forced bats out of the forests and into orchards in search of food. A virus—Nipah—was able to jump from bats to swine to humans. One third of the outbreak’s human cases died, and over a million pigs were euthanized to control the disease, devastating Malaysia’s pork industry. Since then, smaller outbreaks have sprung up across Bangladesh and into India, and related viruses, like Hendra, have been identified in places like Australia, where both horses and humans have been sickened. Hendra is another flying fox-associated virus, one that kills three-quarters of horses and 60 percent of humans it infects; it’s no wonder these off-chance spillover events grab headlines and pique public worry. But these off-chance spillover events aren’t the ones that keep public health scientists up at night.
Influenza—including the common human flu—is a family of related viruses that can infect hosts ranging from whales and seals, to birds, swine, and humans. It’s a flexible virus, and if someone, or something, is infected with multiple strains at once, those strains can reassemble inside of that person or animal, and create a whole new strain of flu. This is what happened in 2009, when a H1N1 influenza virus—made of three strains of swine flu, one bird flu, and one human flu—raised fears of a global pandemic. And it’s what happened in 1918, when the Spanish flu—another H1N1 human-swine flu hybrid—infected a third of the world’s people and killed nearly 3 percent of the global population.
A virus like that, a hybrid influenza virus, or a fast-spreading respiratory virus with spillover origins, like SARS—these are the viruses that keep global pandemic watchers up at night. These are the kind of human-animal virus crossovers that kill large numbers, and these are the kind of viruses that can thrive when livestock and humans live in close proximity, at high density, and with low biosecurity. Even in the United States, scientists are constantly on the lookout for virus transfer from migrating birds to poultry; but in other parts of the world, in developing and tropical countries, in those countries facing enormous pressure to use land resources to their maximum capacity—this is where scientists worry most. It’s no secret that many of the most worrying cross-over diseases, the most human-bird flu crossovers, the scariest novel spillovers like SARS, come from those agricultural areas with the highest density of animals, livestock, and people. South and Southeast Asia; megacities like Hong Kong and Singapore. These have been ground zero for many outbreaks, from SARS to Japanese Encephalitis, and are a warning to the rest of our rapidly peopling world: this can happen to you, too.
As agriculture expands and intensifies, it means that there’s less room—economically, and specially—for everyone born into rural agricultural communities to stay put where they were born. For work, for opportunities, they often have to move to major cities in their own countries or abroad. For them, the chance for a taste of home becomes a rarity: a delicacy. This demand has created a startlingly large trade in bushmeat: the meat of wild animals captured in the forest and elsewhere. Black-market supply chains for the meat of antelopes, monkeys, even bats, run from the heart of Africa and Asia’s remaining forests, right into the bustling centers of cities like Lagos, and beyond: even Paris, London, and New York. It would be misleading to say that bushmeat poses a larger risk of pandemic than swine and avian flu. And it is unlikely to spark the kind of livestock-to-human outbreak seen in the deforested tropics. But the idea that a smuggled bat—the suspected source of viruses like Ebola—could be shipped in secret to the heart of New York; it’s the real-life set-up necessary for the next great outbreak.
The world’s food supply has never been safer. And infectious disease control never more airtight. But it’s not perfect. And although zoonotic spillover may be a conversation usually left to the public health community, it’s important for us in agriculture to understand our role in this. Outbreaks and diseases you hear about on the news impacting the developing world aren’t just a problem for people over there. Migrating birds carry bird flu across continents. Bushmeat is already somewhere in your city, be it Chicago, London, or Dar es Salaam. And you only have to remember the global panic West Africa’s 2014 Ebola outbreak caused to know how interconnected we are on this. Agriculture is the foundation of economic development, and a key economic lever to lift the bottom billion out of poverty; it’s what feeds the world. But it’s not without cost. And it’s not without risk. That’s why it’s so important to understand the interconnectedness of agriculture and public health, and to better see how we in agriculture are part of an even bigger picture than feeding the world.