The viral spillovers that created the coronavirus are only worsening

The novel coronavirus is the latest in a long list of pathogens that have jumped from animals to human beings, triggering pandemics that have killed hundreds of millions.

Why it matters: COVID-19 underscores the desperate need to better understand and control the intersection of animal and human health. Preventing future pandemics will come down in part to better policing the border zones between animal health and human health.

The 21st century has already experienced four major spillovers: SARS (horseshoe bats via civet cats), H1N1 flu (pig), MERS (bats via camel), and COVID-19 (bats via an intermediate).

The big picture: Nearly 1.7 million as yet undiscovered viruses are believed to exist in wildlife, and Thomas Gillespie, a disease ecologist at Emory University, notes that we still lack data for almost 90% of zoonotic viruses in wild mammal species.

• Despite the clear biological connections between animals and humans, animal health receives perhaps $1 for every $50 that goes to human health, estimates Gregory Gray, an epidemiologist at Duke University.
• Experts are urging more funding to characterize those pathogens and track wet market workers and others who are likely to be the first people infected in a spillover.
• The government funding for PREDICT, a program that was meant to do just that, was initially not going to be extended by the Trump Administration last year, before an extension was granted last month. The U.S. Agency for International Development is also launching a similar program called STOP Spillover.

"If we could get hold of emerging viruses before they fully adapt to humans, it would help us better understand it and develop better treatments. That might help us avoid the next viral crossover."

What's happening: Many experts are urging wet markets, where live animals and humans may be in close contact, be closed.

• White House coronavirus task force member Anthony Fauci told Fox last month that it "boggles my mind" they haven't been shut down in China and other countries. It's worth remembering though that the U.S. itself has live-animal markets, including dozens in New York City.
• But while China announced in February what it called a "permanent ban" on wildlife trade and consumption — which also threatens endangered species — some experts worry that could drive the markets underground. "A better idea is to move them out of cities to more rural areas," says Duke's Gray.

How it works: The most widely accepted theory of the origins of COVID-19 is a textbook example of how "zoonotic spillovers" occur.

• From a bat — which often feature as the reservoir species for zoonoses, in part because there are simply so many of them — the novel coronavirus probably jumped to an intermediate species more likely to come in contact with humans.
• One candidate is the pangolin, a scaly anteater sometimes eaten or used for medicine in China, and one of countless wild species sold live in the country's wet markets. Such wet markets have emerged as what Chris Walzer of the Wildlife Conservation Society calls "the biggest risk factor" for spillovers, as highly stressed wild animals come into close contact with human beings through handling, butchering, and consumption.
• Based on genetic analyses of the novel coronavirus, it likely took only a single spillover event from an infected animal to a human being to kick off a pandemic that has already killed more than 250,000 people around the world.

Background: By one count 70% of emerging diseases can be traced back to wildlife, and since 1980 the number of outbreaks per year has more than tripled.

• Some of that is likely due to better surveillance picking up small outbreaks that might have been missed in the past. But human population increase and closer contact between people and potentially infected wildlife means "viral spillover is increasing," says Emory's Gillespie.
• A study published last month found that deforestation in western Uganda that had left only patches of intact forest increased the likelihood that pathogens would jump from animals to human beings. One lesson is that "instituting buffer zones between wild animal and human habitats could decrease human-animal contact events," says Laura Bloomfield, a doctoral candidate at Stanford University and the study's lead author.
• The growing industrialization of meat production around the world plays a role as well, as pathogens pass from wild animals into packed livestock farms, where the viruses can be amplified as they burn through domestic animals.

The bottom line: Humans and animals share this planet, and increasingly they share deadly pathogens as well. If we don't fully recognize that shared threat, COVID-19 won't be the last zoonotic pandemic.

Go deeper: Coronavirus is tied to climate and biodiversity crises