CRACKING THE CODE
Bats’ Link to Ebola Finally Solved
A new paper outlines five steps required for a virus to ‘spill over’ from bats to humans. But don’t just blame the bats—deforestation and hunting are to blame, too.
Every night, they emerge from their roosts, taking to the skies on silent wings. Their nightly flights bring with them the powers to pollinate plants and control insect populations. From this perspective, bats seem like a universal good.
Many cultures, however, have an uneasy relationship with bats. These nocturnal fliers might do some good, but their association with night, rabies, and All Things Creepy means that, at best, we tolerate them. Adding to their negative aura is recent research showing that bats can be the source of infectious diseases like SARS and Ebola, as well as lesser-known pathogens like Hendra and Nipah virus.
It’s all too easy to blame bats for causing these human pandemics, including the most recent (and deadliest) Ebola outbreak. After all, these viruses hang out in bats in between outbreaks—trace any outbreak of these viruses back far enough and you will find a bat.
But a new study reveals that the picture is much more complicated. Far from being hapless victims in outbreaks of Ebola and other infectious diseases, researchers are showing that human actions are at least as much to blame as bats.
“We really know very little about what precipitates the spillover of pathogens,” said the study’s first author, Raina Plowright, a researcher at the Center for Infectious Disease Dynamics at Penn State University. “What we do know, however, is that there are clear links to environmental change.”
Take the ongoing Ebola outbreak. The leading hypothesis for the ultimate source of the Ebola virus, and where it retreats in between outbreaks, lies in bats. The source of the current Ebola outbreak is unknown, but some have speculated that the virus may have jumped to humans from the hunting or consumption of a bat or other wild animal infected with Ebola.
Although bats may have creeped us out for centuries, their links to emerging infectious diseases are much more recent. Aside from rabies, scientists didn’t think that bats were a major source of zoonoses—the infections that jump from animals to humans. That opinion slowly began to shift, starting in the mid-’90s, thanks to a disease outbreak in Hendra, a suburb of Brisbane, Australia.
Several horses in Hendra had fallen ill and died, followed by one of their trainers. Scientists figured out that a virus had caused these deaths, although they had never seen this particular virus before. The virus had to come from somewhere, but no one could figure out its origin. Then, scientists tested a group of bats called flying foxes and found antibodies against the (aptly named) Hendra virus. This indicated that the bat had been infected with Hendra at some point in its life, but it didn’t provide the conclusive proof that bats were the disease reservoir. It wasn’t until 2000, when scientists isolated pieces of genetic material from the Hendra virus in flying foxes that they could confirm that the virus originated in bats.
In Malaysia and Bangladesh, a devastating infectious neurological disease emerged just a few years after Hendra. Population booms in those countries caused the clear-cutting of forest to make room for people and their farms. Bats that had once lived deep in the forest were now eking out a living on mango trees and near pig farms. In 1999, pigs began falling ill near the town of Nipah in Malaysia, and humans shortly followed. The Nipah virus, scientists quickly realized was similar to Hendra, which quickly led researchers to bats.
However, it wasn’t until SARS was linked to bats in 2005 that research—and its accompanying worries—began to skyrocket. Scientists increasingly began hunting for viruses in bats—and finding them. “If you look for new viruses, you will find them,” said Bucknell University disease ecologist DeeAnn Reeder.
Studies have shown that the vast majority of viruses remain undiscovered, so perhaps it shouldn’t be surprising that researchers have found an array of new viruses in bats. But other studies have found that not only do bats harbor a disproportionate number of new viruses, their immune system may make them especially good at dispersing the viruses far and wide.
Plowright knew, however, that the viruses still had to travel from bats to humans, and her work indicated that the jumps weren’t straightforward. In a paper published Tuesday in Proceedings of the Royal Society B, she and her colleagues set out to identify these pathways and the different layers of transmission. They identified the five different steps required for viral spillover from bats to humans.
“There’s a pattern of how a pathogen flows through the system,” Plowright said. For starters, both the bats and the virus have to be present. It sounds painfully obvious, but it’s actually a complicated question. Because of the bat’s unique immune system, viruses tend to spread through bats in large pulses, where nearly all bats become infected before the virus naturally subsides. Also due to their unusual immune system, bats can remain healthy and able to travel even while infected. This allows the bats to spread the viruses to other bat populations in distant areas. “The more mixing there is between bat populations, the more potential there is for viral transmission,” said Olivier Restif, an infectious-disease biologist at the University of Cambridge.
While the bats are infected, they shed large quantities of virus that can infect other animals. Humans are exposed to viruses from other species all the time, and we almost never get sick. On rare occasions, however, viruses can manage the jump. Statistically speaking, the more humans and other animals are exposed to these viral pulses, the more likely it is that viruses will spillover from bats to humans.
“Spillover isn’t easy. We have a biased picture because we only know of the success stories,” said Uriel Kitron, an infectious-disease specialist from Emory University. But bats have co-existed with these viruses for thousands or even millions of years, which led scientists to ask why they have only been spilling over to humans on a significant basis since the mid-1990s. The answer, it seems, is us. Human actions increasing our exposure to wild bats are playing an important, but underemphasized, role in the emergence of new infectious diseases.
“Everything we’re doing right now seems to be stirring the pot,” said Reeder.
The growth in human population in recent years has led to large-scale deforestation, especially in Southeast Asia, to make room for human settlements as well as crops and livestock. As a result, many bat species are finding their homes and food sources gone. Some species may simply move out of the area or go locally extinct, but other species can adapt to humans by roosting in our buildings and eating the fruit on our trees. They leave their urine, feces, and saliva behind, all of which may contain viruses. If livestock become infected, they are often kept in close quarters with other animals and near humans, all of which gives a bat virus ample opportunity to spread.
“As we have more intensive agricultural practices, which place lots of different animals together, we will have more possibility for spillover,” says Kitron.
Other human practices, like the hunting and consumption of wild animals for food, provide other opportunities for spillover. Plowright pointed out that bats have been a popular food source for people in Ghana for a long time, and the practice has a long cultural history and significance in many parts of the world. However, this practice does potentially expose hunters to the pathogens carried on these animals.
Despite the growing importance of bat-related disease, scientists know very little about how bats live their lives and how they have managed to co-exist with these viruses and not show signs of illness. Knowledge about bats and Ebola is even more limited. “Basically everything we know about Ebola in bats comes from one scientific study. That’s it,” Plowright said.
Instead of blaming bats for causing disease, Plowright and colleagues say that we need to look at the chain of transmission events for each virus and figure out the best place to break that chain. For Hendra virus, scientists have developed a horse vaccine that both prevents horses from becoming ill and keeps the horses from infecting humans.
Bats are crucial to the ecosystem, performing extremely valuable jobs like pollination and insect control. Living with bats is far preferable to living without them, Plowright says. “If we do stupid things like get rid of bats, we’re going to be in big trouble,” she said.
It’s now primarily up to us humans to identify and break the chain of events that lead to outbreaks of diseases like Ebola.