New York City's crows were dying.

It was the summer of 1999, and Tracey McNamara, then the chief veterinary pathologist at the Bronx Zoo, was growing concerned. The crows were dying in scores—staggering, having seizures, keeling over. Soon, the mysterious illness came for the zoo's exotic birds. Three flamingos, a cormorant, an Asian pheasant—all dead within a few days of one another.

"Anything that dropped dead on our grounds got necropsied, and I pursued a diagnosis," says McNamara, now a professor at Western University. McNamara had a mystery: What was killing the birds? "I already knew that we were not dealing with anything known to veterinary medicine," she says. "It was something new. And then when I heard that people were dying of an unusual encephalitis, I'm like, 'Oh, there's a link.'" That September, several residents of New York City had contracted and died of a similar illness.

McNamara called the Centers for Disease Control and Prevention to warn them. They brushed her off. "I was told I was wrong and there was no possible connection between the two events," she says. She also went to the Agriculture Department, but the USDA did not have the lab capabilities to finalize a diagnosis. Everything was moving too slowly.

"The veterinarian cracked the case, and no one was interested in talking with her because she was a veterinarian," says Laura H. Kahn, a physician and biodefense researcher at Princeton. They should have listened: McNamara identified the first outbreak of West Nile virus in North America.

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The West Nile story is important. It is also the story of avian flu, rabies, MERS, HIV, SARS, anthrax, and Ebola. The common thread: These are all illnesses that can be passed from animals to humans. "Often, infectious diseases circulate in animals for a long time before they cause outbreaks in humans," says Wondwossen Gebreyes, the director of Global-Health Programs and a professor of molecular epidemiology at Ohio State University. "To prevent disease in humans, we should be able to address what's happening in the animal world and what is happening in the environment," Gebreyes says. Human and animal health are irrevocably linked. As a veterinarian, he says, "I've always been interested in saving human lives."

Seventy-five percent of newly emerging diseases are zoonotic, meaning they can be spread between animals and humans. And they wreak havoc: People fall ill having no natural defenses, and there is often no medicine to fill the gap. It's estimated that between 1997 and 2009, the cost of these diseases amounted to $80 billion worldwide. Every year, there are 2.5 billion cases of zoonotic illnesses in humans, resulting in 2.7 million deaths.

This concept—connecting human medical and veterinary science—is called One Health. And in this framework veterinarians are the sentinels, monitoring the animal kingdom for potential threats to humans. "Once outbreaks originate, like we are seeing with Ebola, often it is too late," Gebreyes says. Prevention of the spread from animals to humans in the first place is key.

Africa has the most to gain from a united medical front. Much of the continent is a hot spot for zoonotic disease—urbanization into biodiverse areas increases the chances for viral spillover. Combine that with poor health systems, unregulated bushmeat economies, and poor veterinary systems, and the emergence of new diseases on the continent is not that surprising.

In the case of rabies, a zoonotic disease with an almost 100 percent fatality rate, a One Health approach means working with the local government to mass-vaccinate dogs. If you vaccinate the dogs, you save the people. It means educating the population about what a rabid dog looks like. It means identifying and tracking down the bats that give the rabies to the dogs. It means educating government officials. It also means setting up a health care system to treat humans with the disease. But the overall goal is to establish a system that is able to stop disease before it touches its first human.

If that framework is in place, medical disasters can be averted. "In the future, if they face—God forbid, Ebola or MERS or another major disease—they have a working system to be able to control it," Gebreyes says.

So how do you find the next Ebola, the next rabies, the next West Nile, before it comes to infect humans? You actively look for it in the wild.

That's what the University of California, Davis, is doing by deploying teams of veterinarians into zoonotic hot spots around the world—in Africa, Asia, Central America, and South America—to detect outbreaks in animal populations before they get out of control. Through their Predict initiative, funded through USAID, they also empower local governments by giving them the tools to detect and diagnose strains without having to ship samples abroad.

"We're looking for viruses in viral families that have had a lot of zoonotic diseases, especially ones that have high pandemic potential—viruses like influenza, viruses like MERS, flaviviruses [e.g., encephalitis]," Christine Kreuder Johnson, a UC-Davis veterinarian and epidemiologist, says.

In 2012, the UC-Davis group encountered five dead howler monkeys in Boliva. The team immediately collected samples, ran diagnostics, and discovered a deadly strain of zoonotic yellow fever in the necropsies. That triggered a comprehensive response from the Bolivian government. "Before any human cases could develop, [the Bolivian government] implemented a vaccination campaign, public outreach to talk about the situation so that people knew to avoid mosquitos, and a mosquito-control effort," Johnson says. "There were zero human cases."

With greater surveillance of wildlife diseases, Johnson says, it's possible we find out that the spillover of animal viruses to humans is far more common than we currently realize. Viruses have been evolving without detection in animals for thousands of years. And humans are pushing farther and farther into natural habitats. "It's just a matter of chance that some of those viruses will be able to find the right pathway to emerge in people," she says.

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In 2000, what was then the U.S. General Accounting Office released a critical report on the West Nile outbreak response. "A consensus that the bird and human outbreaks were linked, which was a key to identifying the correct source, took time to develop and was initially dismissed by many involved in the investigation," it read. "Better communication is needed among public-health agencies." Regardless, when New York officials initially misdiagnosed the human outbreak as St. Louis encephalitis, they activated mosquito control and probably saved lives. But what if it wasn't a mosquito disease, and for three weeks CDC was fighting the wrong virus without any success?

Now, CDC has an office for One Health under the umbrella of its National Center for Emerging and Zoonotic Infectious Diseases. Casey Barton Behravesh, a veterinarian working on zoonosis at CDC, says that the agency maintains much greater collaboration and communication with the USDA, universities, and local wildlife agencies to better implement a One Health approach in the United States.

"One Health is the future," Behravesh says. "If I'm working on a salmonella outbreak, I might talk to the USDA's national veterinary-services laboratory to see what they are seeing. Those sorts of things happen every day."

While CDC is the primary agency on these issues, the surveillance of animal disease is still scattered across the bureaucracy. The USDA monitors livestock but not wild animals. The Interior Department has a National Wildlife-Health Center in Madison, Wisconsin, but that lab has a staff of 27, a number McNamara says isn't large enough for truly national disease surveillance. "I don't think you'd have a repeat of what happened 15 years ago," McNamara says. But she's still worried there isn't enough coordination among the agencies.

She isn't alone. In 2013, the National Preparedness and Response Science Board (housed under the Health and Human Services Department) found "inadequate or lack of efficient and relevant information-sharing at and across all levels and areas" on biosurveillance issues, and "strongly emphasized the need to designate an oversight authority to assure compatibility, consistency, continuity, coordination, and integration of all the disparate systems and data."

Funding discrepancies abound. Comparatively, "there is very little research going on in animal diseases," Princeton's Kahn, who cofounded the One Health Initiative, an informational resource, says. The national research budget for human diseases is $29 billion. The USDA's National Institute of Food and Agriculture has a total budget of $783 million, none of which is specifically budgeted for animal-health research.

If detecting disease threats from wildlife is a difficult prospect for the United States, what can we expect from the developing world? The current Ebola outbreak will subside, but the question remains: Will the public-health authorities in West Africa be stronger or weaker in its wake? In a globalized world, health needs to be a globalized effort.

"The key message is, the world needs to wake up and learn from this lesson and not wait for another major outbreak to happen," Gebreyes says. "But rather work together to develop a system—and strong working relationship—between the developed and developing regions in the One Health system."