Bats really do harbor more dangerous viruses than other species

Is there something special about bats? The question has been hotly debated among researchers studying the origins of deadly viruses. Marburg, Ebola, severe acute respiratory syndrome: They have all been linked to bats, leading some scientists to argue that something about the mysterious mammals makes them especially likely to harbor viruses dangerous to humans. “Bats are special,” is their motto. But others argue that the bat order is very well-studied and very big—one in five mammalian species is a bat—biasing results.

That debate may finally be over. A broad look at all viruses known to infect mammals suggests that bats are, indeed, more likely to carry unknown pathogens that can wreak havoc on humans. Surprisingly, the study comes from researchers who until now were bat doubters. “As a scientist, you accept the results of your own study—even if they prove you wrong!” says disease ecologist Peter Daszak of the EcoHealth Alliance in New York City, a senior author on the new study.

Daszak's group started out trying to answer a broader question: Where should scientists concentrate efforts to find as-yet-unknown viruses threatening humanity? Most emerging infectious diseases are zoonoses, diseases that originate in animals, and some may have the potential to trigger massive epidemics. But there are thousands of species of mammals, potentially carrying hundreds of thousands of viruses—so where do you start?

Daszak's colleague, ecologist Kevin Olival, collected information on all viruses known to infect mammals, 586 in total, found in 754 species. (Some viruses are known to infect only one species; others are found across a wide range of animals.) To correct for the fact that some animals have been studied more than others, the researchers counted the number of disease-related publications on each species. The researchers then calculated how many viruses they would expect to find in each species, if it were as well-studied as the animal with the most publications—the red fox. This gave them an estimate for the “viral richness” of every mammal species.

They found several patterns in the data: For instance, large animals carry more viruses than small ones. And animals ranging over wide areas carry more viruses than species with confined habitats.

In their next step, the scientists looked only at the 188 known zoonotic viruses, agents that have been found in humans and at least one other mammal. Even after accounting for factors that would increase zoonotic virus load—how closely the animal is related to humans or how much its habitat overlaps with cities, for example—bats host a significantly higher proportion of zoonoses than other mammals, the researchers report today in Nature. They estimate that there are about 17 zoonoses yet to be discovered in every bat species versus about 10 for rodent and primate species.

Fabian Leendertz, an epidemiologist at the Robert Koch Institute in Berlin, says the paper offers strong evidence that bats are unique, but cautions that it is difficult to completely correct for the bias that comes from bats having been so widely and systematically studied. "What we need now is more real-world data showing how exactly infectious diseases spill over from animals to humans.”

The study is "important for anyone interested in zoonotic spillover events,” says Linfa Wang, who heads the "bat pack,” a research team in the Emerging Infectious Diseases Programme at the Duke-NUS Medical School in Singapore. Wang has spent many years arguing whether bats are special with Daszak, and says it's exciting that the new paper comes from his group. Daszak, meanwhile, is gracious in defeat: "Linfa was right all along," he says.

But that's no reason to fear or fight them, he adds. Bats have many useful roles, from pollinating flowers to controlling insects. And virus outbreaks are not inevitable as long as humans keep their distance. "These viruses will only emerge in people if we continue encroaching into bat habitat, hunting and eating them, and otherwise making contact with them,” Daszak says.

Just what makes bats special is still unclear; there are many competing hypotheses, from a primitive immune system to echolocation creating droplet clouds that help spread viruses. The next debate is about to take off.