More than 50 percent of emerging infectious diseases originate in animals — including SARS, avian flu, and Ebola. But scientists believe that many outbreaks could be detected earlier and perhaps avoided altogether. The method, they say, is monitoring for virus outbreaks before they jump to humans — in other words, in the wild, when they spread among animal populations.

In a new study published in the journal EcoHealth, scientists analyze previous outbreaks of zoonotic illnesses — those that spread from animals to people — in order to see if they could have been detected before they inflected humans.

"The fact is that pathogens are not seen by the naked eye," Isabelle-Anne Bisson of the Smithsonian Conservation Biology Institute and lead author of the report told VICE News. "One important way to discover new viruses is to monitor for sick or dead animals."

Often the same pathogens that make humans sick can also lead to visible symptoms in animals. But these early warning signs — animals that display seizures, uncharacteristic aggression, and even death — are rarely monitored and reported.

Using historical data sets and newspaper reports on past outbreaks, the study examined 143 zoonotic viruses occurring between 1940 and 2004. While 75 of the illness had symptoms that cause visual signs in their animal hosts, only 13 of the illnesses were identified before they made the jump to humans, meaning an important warning sign that a disease might soon infect humans was overlooked. The study estimates that globally, zoonotic disease outbreaks are believed to have cost over $20 billion in direct and $200 billion in indirect costs between 2000 and 2010.

The study was funded through USAID's PREDICT program, which identifies emerging threats in global viral hotspots — the Congo Basin, the Amazon Basin, China, and South Asia — sampling live animals and creating viral databases. Bisson's study was commissioned to round out the program by exploring the effectiveness of monitoring sick and dead animals, an area that's been previously overlooked by those studying emerging pathogens.

"Live animal sampling is critical. But it's expensive," Bisson told VICE news. "You need a lot of highly trained people. By monitoring sick and dead animals, you can get your citizens involved. Farmers, hunters, rangers — people who interact with these animals every day."

'Sick and dead animals are our first clue.'

Bisson and her team tested their theory in Uganda, where rangers in Queen Elizabeth Conservation Center were given cell phones and a simple data collection program. They were asked to record information on any sick or dead animals they encountered during their daily patrols, such as where the animal was located and what kind of symptoms it was displaying. The information was then transferred directly to management, where the data was analyzed for any significant patterns.

"We were able to identify two diseases using these techniques," Bisson told VICE News. She would not reveal which ones but plans to release their findings in another study. But in both cases, she said, early identification proved critical for stopping the virus in its tracks. "In the end they were easily treatable, but it proved that monitoring sick and dead animals, where they live, can work."

Rangers are used to observing animals in the wild. But ordinary citizens can be used to help identify emerging pathogens too. In Pennsylvania, public health officials noticed that infected mosquitos weren't just passing West Nile virus on to people, but also on to birds. The disease causes swelling of the brain and can prove fatal if left untreated. Crows seemed to be particularly susceptible to the illness. The state created a website and an education campaign, encouraging people to report dead crows whenever they spotted them.

The New York State Department of Health took reporting a step further, creating a "dead bird hotline." By tracking the number of dead crows, public health officials were able to determine if West Nile Virus posed a significant health risk to the local population and take appropriate preventative measures.

But monitoring for signs and symptoms in animals isn't a perfect solution. Not all diseases that can prove fatal in humans exhibit symptoms in animals. Take H7N9, the latest avian flu outbreak. According to the World Health Organization, as of October, the virus had infected 453 people, killing 175 of them, mostly in China.

But H7N9 is virtually asymptomatic in the poultry from which the disease originates. While it's original host has yet to be identified, scientists speculate that the virus likely jumped to humans at a food market where poultry was being sold. Fortunately, the disease does not appear to transmit easily between humans and a pandemic outbreak seems unlikely.

Identifying potential pathogens before they become outbreaks is high stakes work. The World Health Organization says at least 6,000 people have died from current Ebola outbreak. Economic losses from the disease could run as high as $32.6 billion dollars by the end of 2015, according to World Bank. Preventative monitoring programs, while complex, could prove to save both money and lives.

"Sick and dead animals are our first clue," Bisson told VICE News. "And we need to pay attention to these clues and identify pathogens before they jump to humans, not after."