A guest post by Jen Guyton
The last fragile wing finally came free from the threads of my mist net. I sank into the sand on the riverbank, took a deep breath, and tugged off my yellow deerskin gloves. Eight cotton bags wiggled as they hung from the line that tethered my mist net to a tree. We’d gotten a swell of banana bats (Neoromicia nana), tiny creatures, no heavier than a large grasshopper, that are thought to roost in the furled leaves of banana plants. They had come in low over the river like a pack of tiny, flittering wolves, hunting the gnats and mosquitoes that hovered in a veil over the water. I’d had to work fast, because the longer the bats stayed in the mist net, the more tangled they became. Some even chewed their way through the nylon thread, escaping in a flurry of teeth and leaving behind a yawning hole for me to mend, its edges fringed with fragrant urine. Now, I just had to wait for my subjects to leave me a few fecal pellets in the cotton bags so that I could analyze their diet.
I looked with relief at Kaitlyn, my assistant for the day, as we took swigs of our beers. I watched the net billow and catch the moonlight, shining silvery-black like a benighted spider web, and listened to the sound of elephants crashing their way through the dense riverine vegetation in the distance.
Out of the corner of my eye, I saw Kaitlyn startle and look down. “I think something just peed on me,” she said, sounding perplexed. I shone my headlamp above her and was greeted by the glittering eyes and bulging cheeks of a large bat, hanging from the branch of the tree above us and happily chomping away on a piece of fruit. From the white patches below its ears and its fawn-colored fur I recognized it as an Epauletted fruit bat, a member of the genus Epomophorus.
I ran to grab my hand net, a long mesh bag on a circular frame with a handle. The bat was a dozen feet above us, and I didn’t have the handle extension sections, so I quickly duct taped the net to a mist net pole. I raised the net slowly, very slowly, sure that the bat would see me coming and take flight. But it continued to munch merrily, and it disappeared into the net with little more than a metallic peep! of protest. As I collected a fecal sample from it, Kaitlyn cleaned the urine from her clothes.
Stories like these have gotten me into trouble lately. “I study bat communities in Africa,” I’ll say, only to be greeted by wide eyes and mouth poised to speak the word that’s on everyone’s mind: Ebola.
Luckily, I work on the other side of the continent, thousands of miles from where Ebola has now taken almost 5,000 lives. My field site in Mozambique, on the southeastern coast of Africa, is safe from bat-borne diseases, as far as we know. But it’s no secret that bats have been implicated frequently in emerging zoonotic diseases – diseases of animal origin – that are now cropping up among humans: rabies in the Americas, Marburg virus in Africa, Hendra virus in Australia, and Nipah and SARS viruses in Southeast Asia are all harbored by bats.
The recent Ebola outbreak, too, has tenuous ties to our fluttering friends: scientists have found its antibodies in several species of West and Central African fruit bats. We can’t be sure, though, that they are “reservoir” species – organisms that consistently maintain a virus in their bodies without showing signs of illness. This would allow the bats to harbor Ebola, giving it the opportunity to spill over into humans. But, since we haven’t isolated live virus particles from the bats, all we know is that at some point in their lives,they were infected with or exposed to the virus that left its signature on their immune system.
So far, there’s no record of a bat transmitting Ebola to humans. Humans can get it from other humans, and we have solid evidence that people have become infected through ape carcasses, scavenged and eaten. People in parts of Africa eat bats too, but whether humans can catch the bug directly from bats is still a mystery. Some bat-borne diseases need to pass through what’s called an “intermediate” host – another species that amplifies the virus, allowing it to multiply and become more virulent – before humans can catch it. That is true of Hendra virus, which is found in Australian flying foxes. Contact with the bats poses little known threat to humans, but four people have died after interacting with sick horses. The horses, it seems, fed on fruits from trees where bats roosted.
All of this adds to bats’ undeservedly bad reputation. Their mystical association with vampires, nocturnal habits, their seemingly erratic flight pattern, a slew of spooky superstitions, and now a misperception that bats are unusually disease-ridden have earned them a less-than-exalted place in the human consciousness. In some cases, this negative image arouses persecution. In 2007, hysteria stemming from a Marburg virus outbreak in Uganda led to mass extermination of Egyptian fruit bats, leaving heaps of them piled on the floor of the forest. This wasn’t an unprecedented reaction – people have been slaughtering vampire bats in Peru since the 1960s in an effort to control rabies, and a few years ago, the four human deaths from Hendra virus in Australia led to widespread culling of flying foxes.
But does reducing bat populations actually help reduce the risk of bat-borne diseases jumping to humans? Surprisingly, the answer is: probably not. In fact, there’s evidence that it could make things worse. In Uganda, the fruit bat extermination led to a much larger outbreak of Marburg, which is closely related to Ebola, among humans. As it turned out, fruit bats recolonized the caves from which they’d been exterminated, and the new population had a much higher prevalence of Marburg infection than the exterminated one. We’re seeing a similar effect in the Peruvian vampire bats – rabies prevalence is higher in populations that are subjected to culling by a poison called “vampiricide”, which preferentially kills adult bats. That’s probably because killing adults removes individuals that have already been exposed to the disease, making them immune. That allows “susceptible” juveniles, with no immunity, to proliferate, and the infection spreads like wildfire.
It’s not clear whether bats really are different from other animals that could potentially carry diseases, or whether we’re just paying more attention to them now; there’s currently a debate raging among scientists about whether bats are special as disease reservoirs. Some say yes. This may be because many bat species are very social, which would allow pathogens to spread easily. Or, it could be that bats have a long evolutionary relationship with some virus families. Some scientists hypothesize that it’s linked to bat physiology: an unusual immune system, or the remarkably high body temperatures that bats experience during flight, could play a role in their ability to survive infections and, in the end, become reservoirs of pathogens.
Others argue that the numbers just don’t add up and that bats aren’t any more disease-ridden than other mammal groups. Given that bat research is on the increase it could be the simple result of a twisted treasure hunt: the harder we look, the more we find.
What we do know is that bats are special in a lot of other ways, and they deserve a boost in popular image. They’re the only mammals that have evolved true flight. They’re also one of the few groups, along with some whales, shrews, and birds that use echolocation – the ability to “see” a landscape using reflected sound waves. The combination of flight and echolocation allows them to fill a special role as nocturnal predators of aerial insects, with the potential to suppress insects like mosquitoes or some agricultural pests that aren’t active during the day. That does us humans an important service, and scientists have estimated that bats save U.S. agriculture $53 billion dollars in pest control every year. The high diversity of bats – they’re the second most diverse mammal group after rodents – allows them to fill a number of other important roles in ecosystems, such as dispersing the seeds of rainforest trees or pollinating flowers, including the agave used to make tequila.
We don’t yet know as much about bats and their diseases as we should, but the little evidence we do have suggests that killing bats will actually worsen the problem. It also suggests that the same things that are driving some bats toward extinction are also driving spillover events. Deforestation, for example, forces bats to find new homes in cities and increases the probability of their contact with humans. And eating bats gives their pathogens even easier access to people. We can reduce those risks if we protect bat habitats, halt culling efforts, and convince people to stop hunting and eating bats. None of these are trivial endeavors, but we need to try. In the time of Ebola, bat conservation is more important than ever.