Estimating the total number of mammal viruses

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Close to 70 percent of emerging viral diseases such as HIV/AIDS, West Nile, Ebola, SARS, and influenza, are zoonoses. But until now, there has been no good estimate of the actual number of viruses that exist in any wildlife species, according to a release from Columbia University’s Mailman School of Public Health. 

Scientists there have reported on a novel new study that has estimated a minimum of 320,000 viruses in mammals awaiting discovery. That is a manageable number, they believe, and identifying and collecting information on those viruses could be a cost-effective strategy for early detection and mitigation of disease outbreaks in humans.

“Historically, our whole approach to discovery has been altogether too random,” says lead author Simon Anthony, D.Phil, a scientist at the Center for Infection and Immunity (CII) 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.”

To develop an estimate of total mammal viruses, the team studied flying foxes, the world’s largest bat species, in Bangladesh. The bats are known as a source of several outbreaks of Nipah virus in humans.

The team collected 1,900 biological samples from the bats and used polymerase chain reaction to identify 55 viruses in nine viral families. Of these, only five were previously known, including two human bocaviruses, an avian adenovirus, a human/bovine betacoronavirus, and an avian gammacoronavirus. The other 50 viruses were previously unknown, including 10 in the same family as Nipah. The researchers also used a statistical technique to estimate there were another three rare viruses unaccounted for in the samples, upping the estimate of viruses in the flying fox to 58. They used that figure to extrapolate to all 5,486 known mammals, yielding a total of at least 320,000 viruses.

Based on the flying fox study, the researchers were able to develop cost estimates for collecting virus information on a broader scale. They estimated their cost for surveillance, sampling, and discovery of all 58 flying fox viruses at $1.2 million, and used that figure to extrapolate a total cost of $6.3 billion for all mammals. Given the disproportionate cost of discovering rare viruses, they estimate that limiting discovery to 85 percent of estimated viral diversity would bring the cost down to $1.4 billion.

“By contrast, the economic impact of the SARS pandemic is calculated to be $16 billion,” says Anthony. “We’re not saying that this undertaking would prevent another outbreak like SARS. Nonetheless, what we learn from exploring global viral diversity could mitigate outbreaks by facilitating better surveillance and rapid diagnostic testing.”

The team plans to repeat the process in two follow-up studies—one in a species of primates in Bangladesh in order to see if their viral diversity is comparable to the flying fox’s, and another in Mexico, where analysis of samples from six species of bats that share the same habitat, to determine the extent to which they share viruses. With additional resources, they hope to expand the investigation to other species and viral families.

The paper is published in the journal mBio.

Read more from Columbia University’s Mailman School of Public Health.


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