healthHealth and Medicine

The Next Pandemic? Online Tool Shows Top Viruses That Threaten To Spill Over To Humans


Tom Hale

Senior Journalist

clockApr 8 2021, 13:29 UTC

Many of the highest threats are coronaviruses found in bats. Image credit: Independent birds/

In late 2019, a vicious virus with pandemic potential somehow spilled over into humans, laying the way for one of the worst disease outbreaks in recent history. While COVID-19 has been dubbed a "once-in-a-century pandemic" by some, it's apparent 21st-century livestyles and humanity's broken relationship will make disease outbreaks increasingly common in the future. Where will the next virus spillover come from, though?

Scientists at the University of California, Davis have developed a new web application that ranks the great viral threats that currently lurk in wildlife but run the very risk of zoonotic spillover into humans and pandemic potential. 


Known as SpillOver, the app is freely available. It’s the product of a recent study appearing in the journal PNAS that ranked the threat of 887 wildlife viruses using 32 risk factors, such as the environment it can be found in, its current host, and how it might interact with human behavior. 

The top 12 of the list are known human pathogens, including SARS-CoV-2 (ranked 2nd), Ebola (ranked 3rd), and rabies (ranked 10th), while the top spot was taken by the Lassa virus, a viral hemorrhagic fever that's relatively common in certain parts of West Africa.

Beyond the top 12, we start to see viruses that have yet to be identified in humans but run the risk of spillover. Ranked at 13th is Coronavirus 229E (Bat strain), for instance, an alphacoronavirus that lives within bat populations of the Congo, Cameroon, Guinea, Rwanda, Sierra Leone, Senegal, and Uganda. The virus is ranked so highly due to a number of factors, namely the environment it inhabits. It lives in forested areas that are being increasingly encroached on by humans through deforestation and urbanization, upping the chances of spillover from a bat to a person. 


Bear in the mind, the research is only looking at virus spillover from wildlife species. The study did not intend to look at potentially disease outbreaks from domestic species used in agriculture, which is also a common source of pandemics

Viruses that dwell within bats commonly appear among the top ranks. Many of the highest threats are also coronaviruses, a wider group of viruses that cause respiratory tract infections, ranging from a common cold to lethal infections. The threat of coronaviruses is all too clear. Since 2000, three highly pathogenic and deadly human coronaviruses have emerged: SARS, MERS, and SARS-CoV-2, the virus responsible for COVID-19.

"SARS-CoV-2 is just one example of many thousands of viruses out there that have the potential to spill over from animals to humans," Zoë Grange, who led the development of SpillOver as a postdoctoral researcher with the One Health Institute at the University of California, Davis (UC Davis), said in a statement. "We need to not only identify, but also prioritize the viral threats with the greatest spillover risk before another devastating pandemic happens."


Worryingly, the ongoing COVID-19 pandemic was caused by viruses that were previously unknown to science and scientists are still on the hunt for the direct evolutionary precursor of SARS-CoV-2. Although we had the warning shots of SARS and MERS, the outbreak was not predicted. The researchers behind this new project hope their work will inform future efforts to better understand potentially zoonotic diseases and emerging pandemic threats.

"SpillOver can help advance our understanding of viral health threats and enable us to act to reduce the risk of spillover before pandemics can catch fire," said corresponding author Jonna Mazet, a professor at the UC Davis School of Veterinary Medicine and founding director of the One Health Institute and former global director of PREDICT.

healthHealth and Medicine