Bats have an image problem even when they aren’t linked to a pandemic. Some people resent them for occasionally inhabiting attics and chimneys, for example, while others just typecast them as spooky or dangerous.
As the novel coronavirus rampages around the world, however, its suspected link to bats has presented these misunderstood mammals with yet another public relations fiasco. Although the origins of SARS-CoV-2 remain shrouded in mystery, many experts believe it likely began in bats, then passed through another animal host — pangolins, maybe — before making the fateful leap to us.
It’s critical to trace the source of the virus, both to help us manage this pandemic and to minimize the risk of other zoonotic outbreaks in the future. Of more than 1,400 viruses and other pathogens known to infect humans, more than 60% are zoonotic, meaning they’ve jumped to our species from some nonhuman animal.
We don’t know for sure if this virus came from bats, but even if it did, it’s still worth resisting the urge to further demonize these wild animals, who benefit us far more than many people realize. While bats are a source of some dangerous viruses — including SARS-like coronaviruses and Ebola — there is nothing special about bats that makes their viruses more likely to sicken humans, according to a recent study in the Proceedings of the National Academy of Sciences (PNAS).
That doesn’t mean we should pay less attention to bat viruses, of course, but it does suggest we should focus on a wider range of animals, not just the ones where we expect to find zoonotic viruses. And even when a bat virus does jump to humans, whether directly or through another species, it’s worth noting how little influence bats have on that relationship, especially compared with us.
Hosts with the most?
Bats and rats are both small, furry mammals whose English names differ by one letter, but they aren’t closely related. In fact, bats are closer relatives of primates than of rodents, despite some superficial similarities with the latter.
The two mammal groups do have some things in common, though, including reputations for spreading diseases. Rodents are infamous for carrying fleas that spread plague, for instance, and for other zoonotic diseases like rat bite fever, tularemia and hanta virus. Bats are commonly associated with rabies, as well as zoonotic viruses like Nipah, Ebola and SARS-like coronaviruses.
It’s true that bats and rodents are reservoirs for an array of zoonotic pathogens (also called zoonoses), but according to the PNAS study, both groups are “unexceptional” among warm-blooded animals in that regard. The authors found no significant differences in the proportion of viruses that are zoonotic across 11 major taxonomic orders of birds and mammals, including bats and rodents.
For each of those 11 orders, the number of viruses correlates with “species richness,” the researchers found. In other words, if certain animal groups seem unusually rife with zoonotic viruses, it might just be because they contain a wide range of species. More species can mean more niches for viruses to exploit, so diverse animal groups often host a higher number of viruses overall. Many of those viruses may pose no risk to humans, but since the proportion of zoonotic viruses is roughly the same for all 11 orders, an order with more total viruses should also have more zoonotic viruses.
Bat viruses have understandably drawn lots of attention in recent decades, and many experts have wondered if bats are disproportionately likely to harbor zoonotic viruses, maybe due to some quirk of their biology or ecosystems. Bats have powerful immune systems that help them tolerate many viruses, for instance, and that may push the viruses to evolve faster than in other hosts, thus becoming more dangerous if they later leap to other species. But that doesn’t mean bat viruses are any more likely to leap to humans. According to the PNAS study, there may be so many zoonotic bat viruses simply because there are so many kinds of bats.
“The recognition that several high-profile viruses originated from bats triggered tremendous interest in whether there was something special about their ecology or immune systems which makes their viruses disproportionately likely to infect humans,” said co-author Daniel Streicker, senior research fellow at the University of Glasgow’s Institute of Biodiversity, in a statement. “Our finding that the number of zoonoses that have emerged from bats is about what would be expected for any mammalian group of their size casts doubt on the idea that traits of bats produce viruses with a heightened propensity to infect humans.”
The Zika virus, which occurs naturally in monkeys and apes, was first discovered in central Uganda’s Zika Forest in 1947. (Photo: Rod Waddington [CC BY-SA 2.0]/Flickr)
It’s clearly important to study bat viruses given their track record, since their secrets could be key to fighting this pandemic as well as preparing for the next one. In general, though, the study supports “a host-neutral explanation for observed variation in the number of zoonoses among animal groups,” the researchers write. Rather than focusing on a few animal groups with bad reputations, they argue, we should be monitoring viruses in all kinds of wildlife.
“Although bats will and should remain a focus for viral reservoir research … our work shows that the proportion and number of zoonotic viruses in bats is not unusual compared to other mammalian groups,” says co-author Nardus Mollentze, research assistant at the University of Glasgow’s Center for Virus Research. “This means that ongoing efforts to identify potential future threats to human health by screening animals for undiscovered viruses will need to focus on a much wider range of species than is currently the case.”
When researchers are trying to identify which animal viruses might become zoonotic, they should focus more on the biology of the viruses themselves than which animals serve as their natural reservoir, Mollentze adds. “Our study also highlights the need to find new traits of viruses that can help us anticipate their zoonotic potential, since knowledge of the current reservoir was not helpful to predict whether a virus might infect humans — even when the reservoir is closely related to humans.”
Missing the forest
Deforestation robs wildlife of ancient habitat, often forcing animals to adapt by living in closer proximity with people — along with any unknown zoonotic viruses they might be carrying. (Photo: Aizuddin Saad/Shutterstock)
The traits of viruses may be a key factor in whether they become zoonotic, but that’s still just part of the equation. For a virus to jump into humans from bats or other wildlife, the two hosts usually need to come into fairly close contact, something most wild animals will take great pains to avoid. Humans, on the other hand, force an array of wildlife to share space with us, both inadvertently and on purpose.
We still have a lot to learn about the new coronavirus, including whether it really came from a Wuhan wet market. The market is a compelling candidate for the source of the virus, though, especially compared with conspiracy theories suggesting the virus was engineered by people. The evidence so far points to a previously unknown wildlife virus that found a new niche to exploit.
“This is a product of nature,” Robert Garry, a professor of microbiology and immunology at Tulane University School of Medicine, told NPR’s Emily Kwong last month. “It’s not a virus that has arisen in a laboratory by any scientist, purposely manipulating something that has then been released to the public.”
That said, while the virus itself might not have been made by humans, it did need our help to start a pandemic. That includes our failures to limit its spread, but also the conditions under which it jumped to us in the first place.
“Mixing large numbers of species under poor hygienic and welfare conditions, and species that wouldn’t normally come close together gives opportunities for pathogens to jump species to species,” Andrew Cunningham of the Zoological Society of London recently told the BBC.
Nipah virus occurs naturally in flying foxes, but when deforestation in Malaysia forced these large fruit bats to resettle in barns, their guano spread the virus to pigs, who then passed it on to people. (Photo: Panu Ruangjan/Shutterstock)
Wet markets may generate more of those opportunities for viruses, but they aren’t the only human tradition that raises our risk of zoonotic disease. Domesticated animals are a major source, as another recent study found, accounting for eight of the 10 mammal species with the most zoonotic viruses. That includes pigs, cattle, horses, sheep, dogs, goats, cats and camels, which in some cases may only be intermediaries passing a virus from wildlife to us. As with wet markets, keeping these animals in cramped, dirty conditions is likely to worsen the threat of zoonoses.
On top of bringing other animals into our midst, we also have a long history of destroying, degrading or invading wild habitats, which often erodes the natural buffers between wildlife pathogens and us. The Nipah virus, for one, reportedly spread to people after deforestation in Malaysia forced bats to inhabit mango trees on pig farms, letting the virus leap from bat guano to pigs to people. And according to the study above, published in the Proceedings of the Royal Society B, wild mammal species who suffer from poaching or habitat loss at human hands are the most likely to repay us with viruses.
“Among threatened wildlife species, those with population reductions owing to exploitation and loss of habitat shared more viruses with humans,” the authors wrote. “Exploitation of wildlife through hunting and trade facilitates close contact between wildlife and humans, and our findings provide further evidence that exploitation, as well as anthropogenic activities that have caused losses in wildlife habitat quality, have increased opportunities for animal-human interactions and facilitated zoonotic disease transmission.”
Buffers and biodiversity
If bats and rodents have more viruses due to their “species richness,” it might stand to reason that biodiversity is bad. That would require ignoring many important benefits of biodiversity, though. Not only do zoonotic viruses usually need human help to reach our species, but the biodiversity that enabled them offers far more pros than cons for humanity.
Preserving wild habitats and biodiversity can actually reduce the risk of zoonoses such as Lyme disease, malaria and acute respiratory infections, research has shown, due to the “dilution effect.” This phenomenon is still not well-understood, and may vary by disease and context, but it at least seems to warrant more research, especially given widespread declines of biodiversity. And regardless of whether it can protect us from zoonoses, we know biodiversity provides many irreplaceable health and economic benefits, from natural resources and ecosystem services to the inspiration for biomimicry and life-saving medications.
That includes bats, who support us with valuable ecosystem services we often take for granted. Small insect-eating bats provide free pest control, for example, devouring huge quantities of insects that can annoy us, spread disease and damage crops. Bats eat a lot of mosquitoes, research has found, potentially reducing our risk of mosquito-borne diseases. They help gardeners and farmers, too, as they gobble up troublesome pests: By eating corn earworm moths alone, bats save U.S. corn farmers an estimated $1 billion per year.
Many fruit-eating bats also play vital roles for the plants they visit, namely pollination and seed dispersal. More than 300 species of plants reportedly rely on bats for pollination, especially in tropical and desert climates.
We still don’t know for sure whether the new coronavirus came from bats, but even if it did, that has far more to do with us than it does with them. Bats don’t want to hurt us, and they tend to help us when we leave them alone. This coronavirus is rightfully frightening, but it’s worth noting that bats are not only blameless, but also unworthy of our fear in general. The world is scary enough already.
Don’t blame bats for their zoonotic viruses
Bats don’t pose an unusual risk among mammals, research suggests, and bat viruses we do get often rely far more on us than bats.