Some of the biggest and most alarming public health outbreaks in recent decades, such as HIV/AIDS, West Nile and Ebola, originated in animals, leading doctors and researchers to wonder how many other disease threats might exist among non-human mammals. A new study has, for the first time, put a number to this speculation, and it is quite large: more than 300,000.

Scientists believe there is a minimum of 320,000 viruses in mammals awaiting discovery. About 70% of these—called zoonoses—have the capability to cross over into humans. Given the sheer size of this number, the researchers say it is more vital than ever to devote resources to identify possible threats to human health and find ways to mitigate them before large numbers of people become sick.

The cost of this work won’t be cheap. It is estimated it could require $6.3 billion.

But the experts say this expense is relative small compared to what one major pandemic can cost. For instance, the 2003 SARS (Severe acute respiratory syndrome) outbreak consumed $16 billion.

“Historically, our whole approach to discovery has been altogether too random,”said  lead author Simon Anthony, a scientist at the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health. “What we currently know about viruses is very much biased towards those that have already spilled over into humans or animals and emerged as diseases. But the pool of all viruses in wildlife, including many potential threats to humans, is actually much deeper. A more systematic, multidisciplinary, and One Health framework is needed if we are to understand what drives and controls viral diversity and following that, what causes viruses to emerge as disease-causing pathogens.”

The study utilized an examination of the flying fox, a species of bat that was tracked down in Bangladesh. Scientists collected 1,897 virus samples from the bat, including Nipah, which is lethal to humans. From those samples, they discovered about 60 never-before-seen viruses, from which they extrapolated the minimum total number of viruses that exists in all mammals, most of which have not yet been detected.

“For decades, we’ve faced the threat of future pandemics without knowing how many viruses are lurking in the environment, in wildlife, waiting to emerge,” said Peter Daszak, corresponding author and president of EcoHealth Alliance. “Finally we have a breakthrough -- there aren’t millions of unknown virus, just a few hundred thousand, and given the technology we have it's possible that in my lifetime, we'll know the identity of every unknown virus on the planet.”

-Noel Brinkerhoff

To Learn More:

First Estimate of Total Viruses in Mammals (Columbia University's Mailman School of Public Health)

Mammals Harbour 'at Least 320,000 New Viruses' (by Rebecca Morelle, BBC)

A Strategy To Estimate Unknown Viral Diversity in Mammals (by Simon J. Anthony, Jonathan H. Epstein, Kris A. Murray, Isamara Navarrete-Macias, Carlos M. Zambrana-Torrelio, Alexander Solovyov, Rafael Ojeda-Flores, Nicole C. Arrigo, Ariful Islam, Shahneaz Ali Khan, Parviez Hosseini, Tiffany L. Bogich, Kevin J. Olival, Maria D. Sanchez-Leon, William B. Karesh, Tracey Goldstein, Stephen P. Luby, Stephen S. Morse, Jonna A. K. Mazet, Peter Daszak, and W. Ian Lipkin; Center for Infection and Immunity, Mailman School of Public Health, Columbia University) (pdf)

Swine Flu: Is a U.S. Company Responsible? (by Noel Brinkerhoff, AllGov)