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September 13, 2013 | By:  Julia Paoli
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Mammals Harbor At Least 320,000 Undiscovered Viruses

Humans have been playing defense against viruses for much of history. Think about it, -people mainly take action against a virus once it has already become a threat. Just recently, researchers have switched tactics and taken the offensive. A team of scientists lead by Simon Anthony of Columbia University released a study this month in the journal mBio estimating the number of novel viruses in all mammalian species to be 320,000. Anthony and his colleagues' estimate of mammalian viruses is the first ever to be statistically supported. With this information scientists may discover potentially dangerous viruses before they transition from wildlife to humans.

Roughly 70% of all new viruses infecting humans originated in other animals. Viruses that originate in mammals are particularly hazardous because they are easily transferable to people; their exposure to other mammalian species allows them to skillfully "[navigate] our own warm-blooded bodies." The knowledge of just how many viruses may be lurking in mammals helps scientists assess the threat viruses pose to society. In order to calculate the number of viruses, scientists studied one species of flying fox (a type of bat) known as Pteropus giganteus. Found in Bangladesh, the flying fox is a known carrier of multiple viruses, such as Nipah, and was therefore well-suited for the study. Scientists repeatedly took biological samples - 1,900 in all - from bat populations over a five year span. From those samples fifty-five different viruses from nine viral families were identified. Only five of the viruses found were previously known to scientists! Calculating that another three unknown viruses were not accounted for in the study, the researchers estimated that flying foxes alone harbor 58 viruses. If all 5,486 known species of mammals carried 58 different viruses, then the total number of undiscovered mammalian viruses is at least 320,000.

To clarify, 320,000 viruses is a very rough estimate. The scientists assumed that every mammal carries 58 viruses based on their findings with flying foxes. The problem with this figure is that flying foxes, and bats in general, are virus friendly animals. The lifestyle factors that predispose bats to be good viral carriers are living in large communities, long distance travel and dispersal throughout the world. It is unlikely that the remaining 5,485 other mammals also carry exactly 58 viruses. Scientists are not sure by what factor the estimate could be off. Dr. Anthony explains "it is very likely that 320,000 viruses under-estimates the actual number of viruses, but we have no way of knowing by how much. It is for this very reason that we need to expand and repeat these systematic studies, and only then will we be able to refine our estimations with greater confidence."

This study is significant because it presents the first statistically supported estimate of mammalian viruses.

Before now, some scientists speculated there to be millions of undiscovered mammalian viruses. Only 320,000 mammalian viruses is a much more reassuring and manageable number than ten million. Now that the number of estimated viruses has dropped from the millions to hundred thousands, identifying each one is feasible. The researchers' goal is to track down every single mammalian virus and catalogue it. Dr. Anthony predicts that all mammalian viruses can be identified over a 10 year period for a mere $6.3 billion. You may not think of $6.3 billion as a small amount, but in comparison to the cost of pandemics it's not too outlandish. The SARS outbreak alone cost $16 billion dollars. For just $1.2 billion the scientists estimate that 85% of mammalian viruses can be identified. What's more, a species richness estimator known as Chao 2 predicts that 85% of the viruses can be identified from studying only 500 species of mammals. Due to their rarity, 5,000 species would need to be studied to identify the remaining 15% of viruses. Putting in the time and money to find each virus before they reach epidemic proportions could pay off tremendously in the long run. As Dr. Anthony said "If we know what's out there, we'll be a lot better prepared when a virus jumps over into a human population."

One purpose of cataloguing all mammalian viruses is to establish an "early warning system." Scientists could be on the lookout for viruses with the potential to bind to human cells. If one is identified, then it can be closely monitored in the hopes of nipping an outbreak in the bud. Just imagine what the world might be like now if HIV/AIDS was found before it morphed into a full-fledged epidemic. Some scientists have even begun to implement this new preventative method of tracking down the viruses. While studying the flying foxes, Dr. Anthony and his colleagues discovered a new virus known as GBV-D. It turns out that GBV-D is related to the virus causing hepatitis in humans. The scientists are now looking for signs of GBV-D in human populations near where the bats live. Evidence of GBV-D in people would suggest that the virus has jumped into the human population. Plans are also in the works to repeat the study performed on flying foxes using primates from Bangladesh and bats from Mexico.

Although the estimate is subject to change, it is an important step in preventative measures. Not only has it defined the number of viruses that exist, but it has also brought about a new method for early prevention. Really, the only major roadblock that stands in the way of cataloguing all mammalian viruses is funding. I personally have no doubt that funding will be granted eventually, especially after people realize the potential to save a lot of money and prevent a lot of human misery. As the old saying goes, in order to defeat your enemies one has to know them, and the first step to knowing a virus is finding it.


Anthony, S. J. et al. A strategy to estimate unknown viral diversity in mammals. mBio, 5 (2013). Scientists Tally Viruses Living in Mammals. (2013).

Morelle, R. "Mammals harbour 'at least 320,000 new viruses'." BBC News. September 2, 2013.

Simon Anthony. Personal communication. "A minimum of 320,000 viruses awaits discovery." (September 3, 2013).

Yong, E. "How Many Mammalian Viruses?" The Scientist. September 3, 2013.


Ryan Poplin (via Wikimedia Commons).

Microbe World (via Flickr)

September 17, 2013 | 12:38 AM
Posted By:  Julia Paoli
Thanks for your comment Sedeer! The study by Dr. Anthony was only a rough estimate and more research is needed to refine the exact amount of unknown viruses. The study was not concerned with the influx of viruses just merely the amount currently expected in the mammalian population. But you are right to place importance on the influx of new viruses since they do frequently jump from species to species, especially from birds. It would be interesting to see how many new mammalian viruses appear each year that jumped from birds.
September 16, 2013 | 07:56 PM
Posted By:  Sedeer el-Showk
I find the idea of generalizing from one bat species to all mammals a bit too hand-wavy, but I guess you have to start somewhere. I also wonder how they handle the influx of new viruses, particularly since many of them seem to come from birds (eg, chickens and ducks). Did they discuss that at all?
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