Spillover

by David Quammen

  Gulp, um, yeah. Date-palm sap is a seasonal delicacy in the villages of western Bangladesh. It flows in the veins of a certain palm tree, the sugar date palm (Phoenix sylvestris), and if the tree is tapped, sap will drain into a carefully placed clay pot. Like the sap of a maple tree, it’s sugary—even more sugary than maple, evidently, because it needn’t be rendered down with hours of cooking. Some people are ready to pay good takas, scarce cash, for date-palm sap offered fresh and raw. Tappers sell it door-to-door in the nearby villages, or else on the roadside, like a neighbor kid with a lemonade stand. Customers usually bring a glass or a jar of their own. They drink it down on the spot or carry it home to share with the family. The best quality sap is red, sweet, and clear. Natural fermentation sets in quickly, and the price plummets after 10 a.m., when the sap is no longer so fresh. Impurities also lower the value. Impurities, as you’ll see, have another result too.

  The investigation at Tangail found that single distinction between the sick and the well: Among those infected, most had drunk raw date-palm sap. Their healthy neighbors mostly hadn’t. It suggested a more intricate story.

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  So I went to see Steve Luby, at the ICDDR,B. He’s a tall, gaunt man with short brown hair and glasses, serious but not pompous, a former philosophy major who turned to medicine and epidemiology, and then chose to focus on infectious diseases in low-income countries. He has been in Bangladesh since 2004. He knows the place pretty well. He hears a steady tolling of preventable deaths and tries hard to prevent as many as possible. Much of his work involves familiar and mundane diseases, such as pneumonia, tuberculosis, and diarrhea, which cause far greater mortality than Nipah. Bacterial pneumonia, for instance, accounts for about ninety thousand deaths annually just among Bangladeshi children under age five. Bacterial diarrhea kills about twenty thousand newborn infants every year. Given those numbers, I asked Luby, why divert any attention at all to Nipah?

  To be prudent, he said. Classic case of the devils you know versus the devil you don’t know, none of which can you afford to ignore. Nipah is important because of what might happen and because we understand little about how it might happen. “This is a horrible pathogen,” he said, reminding me that the lethality among Nipah cases in Bangladesh is more than 70 percent. “Of those who survive, a third of them have marked neurological deficits. This is a bad disease.” And about half of all known cases in Bangladesh, he added, have acquired it by person-to-person transmission, a worrisome development that hadn’t appeared during the Malaysian outbreak of Nipah.

  Why has person-to-person spread been a major factor in some of the outbreaks but not others? How stable is the virus? What’s the chance that it might evolve into a form that’s even more readily transmissible? Bangladesh, as I’ve mentioned, is very densely populous, with about a thousand humans per square kilometer, and still increasing. That population, dispersed rather evenly across a crowded but rural landscape, with low levels of income and medical care, pressing relentlessly against the last remnants of native landscape and wildlife, puts the country at special risk of epidemics, whether from old mundane pathogens or strange new ones. So of course Nipah is an important part of our work, Luby said, even though the numbers (so far) are small.

  And there’s another reason, he added. No one in the world knows much about this virus. “If we do not study it in Bangladesh, it will not get studied.” Malaysia has seen only one outbreak. India, one in 2001, and another recently. Bangladesh, he pointed out, citing the count as of 2009, has had eight outbreaks in eight years (and more since my conversation with him). Lab work can be done anywhere, but lab work won’t solve the mysteries of how Nipah behaves in nature. “If we really want to understand how it moves from its wildlife reservoir into people, what happens in terms of human disease transmission, this is the place we’re going to do it,” he said.

  To understand how it moves from its wildlife reservoir into people requires that one basic point of reference: the identity of the reservoir. Bats were logical suspects, of course—flying foxes in particular—based on what had been learned in Malaysia, and on the parallel findings for Hendra in Australia. The only flying fox native to Bangladesh is a big thing called the Indian flying fox (Pteropus giganteus). Luby and his team knew from earlier work that members of this species too had tested positive for Nipah antibodies. But how did the virus get from bats into people, if not by way of pigs? Well, it happens that Indian flying foxes enjoy date-palm sap. Tree owners complained of hearing bats in their palms at night. As the Luby team reported, after their work in Tangail: “Owners viewed the fruit bats as a nuisance because they frequently drink the palm sap directly from the tap or the clay pot. Bat excrement is commonly found on the outside of the clay pot or floating in the sap. Occasionally dead bats are found floating in the pots.” But that’s not enough to eliminate the demand for raw sap.

  On a long list of possible risk factors that Luby’s team took to Tangail, sap drinking was just another hypothesis, added to the interview protocols almost as a hunch. The first investigators on the scene were social anthropologists, Luby told me; they were very simpatico with the local people, very low-key, asking open-ended questions, not so formal and quantitative as epidemiologists. “And the anthropologists said, ‘Everybody with a case drank date-palm sap.’ ” He meant everybody with a case of Nipah, not a case of bottled sap. The epidemiologists came next, confirming that hypothesis with hard data. “The Tangail outbreak was the epiphany moment for us,” he said. The epiphany seems obvious in retrospect, as epiphanies often do: Yes, drinking raw date-palm sap is an excellent way to infect yourself with Nipah.

  He explained the context. That western area of Bangladesh, in which most of the outbreaks occurred, could be considered the Nipah Belt. Possibly that’s because it’s the Date-Palm Belt. The bats range widely, but the west is where sugar date palms grow well and are much prized for their sap. The harvest begins in mid-December, with the first cold night of what passes for winter in Bangladesh. The tappers are known as gachis, tree people, from the Bangla word gach, meaning “tree.” Other people own the palms, and the owners typically get a half share of the product. The gachis are poor, independent operators, generally agricultural laborers who do this as a seasonal sideline. To harvest sap, a gachi climbs a tree, shaves away a large patch of bark near the top to create a V-shaped bare patch (from which sap oozes out), places a hollow bamboo tap at the base of the V, and hangs his small clay pot beneath the tap. The sap flows overnight; the pot fills. Just before dawn, the gachi climbs up again and brings down a pot of fresh sap. Maybe he gets two liters per tree. Bounty! Those two liters are worth about twenty takas (US $0.24) if he can sell them before 10 a.m. He empties the clay pot into a larger aluminum vessel, mixing the sap and the bat feces (if any) and the bat urine (if any) and the virus (if any) from one tree with the sap (and its impurities) from others. Then off he goes to sell his product. Some gachis are complacent about the risk of adulteration. One told a colleague of Luby’s: “I do not see any problem, if birds drink sap from my trees. Because birds drink a slight amount of sap. I would get God’s grace by giving bats and other animals a chance to drink sap.” He gets God’s grace and the customer gets Nipah. Other gachis do care, because clear reddish sap brings a better price than foamy, gunky sap full of drowned bees, bird feathers, and bat shit.

  The whole investigation, for Steve Luby, leads in two very different directions—one practical and immediate, the other farsighted and scientific. On the practical side, he and his people have been exploring low-cost methods for helping gachis keep bats away from their clay pots. A simple screen made of woven bamboo scraps, costing about ten cents, can be placed around a tapping wound and its clay pot, fencing the bats out. That’s a simple fix, and probably more humane than passing a law against harvesting date-palm sap. On the scientific side, Luby told me, there are crucial unanswered questions about Nipah virus. How does it maintain itself in the bat population? Why does it spill over when it does? Is it
readily capable of human-to-human transmission, or just under special circumstances? Has it emerged recently, a new pathogen, or is it something that’s been killing Bangadeshis, unnoticed, for millennia?

  Those questions lead to another. How have changes to Bangladesh’s landscape, and the density of people upon it, affected the fruit bats, the virus they carry, and the likelihood of spillover? In other words: What’s new in Nipah ecology? For more a more eloquent answer to that, Luby said, you could talk with Jon Epstein.

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  Eloquence is good but field time is better. I left Dhaka with Jon Epstein the next morning, headed west toward the river crossing that would take us into the southwestern Bangladesh lowlands.

  Epstein is a veterinary disease ecologist, based in New York. He was employed at the time by an organization called Wildlife Trust, under its Consortium for Conservation Medicine (the same organization as Aleksei Chmura, and more recently rebranded as EcoHealth Alliance). In addition to his DVM, Epstein has a master’s in public health and a lot of experience handling big Asian bats. He worked with Paul Chua in Malaysia, capturing flying foxes amid the coastal mangroves, sometimes while chest-deep in seawater. He led the team that found evidence of Nipah among flying foxes in India, after the first outbreak there, and was part of a multinational group that identified bats as the reservoir of the SARS virus in China. He’s a large sturdy fellow with a crew cut and lozenge glasses, looking like a former high school quarterback grown fortyish and serious. He was in Bangladesh, not for the first time, to gather data toward understanding when, where, and how the Indian flying fox carries and sheds Nipah.

  He brought along Jim Desmond, another American veterinarian, newly recruited to the organization, whom Epstein would train in the particular delicacies of searching for Nipah virus in bats as big as crows. The fourth member of our party was Arif Islam, also a veterinarian, one of very few in Bangladesh who works with wildlife and zoonotic diseases, and the only member of our group who spoke fluent Bangla. Arif was crucial because he could draw blood from a bat’s brachial artery, negotiate with local officials, and order curried fish for us in a local restaurant.

  It was almost 9 a.m. by the time we cleared the traffic of Dhaka, where the busses grind against one another like chummy elephants and the green motorbike taxis dodge through the gaps, seeming ever at peril of getting squashed. Finally the road opened. We rolled westward toward the river, relieved to be away. Behind us, the low sun shone feebly through the smog of the city, orange as a bloodied yolk.

  We made the ferry crossing into Faridpur District—dry season, the Padma River was low—and proceeded on a two-lane between the rice paddies. We stopped in Faridpur city to pick up more personnel, a pair of field assistants named Pitu and Gofur, with special skills. Both were small men, as compact and agile as jockeys, expert climbers and bat catchers who had worked intermittently with Epstein for several years. Their bat-catching expertise came from an earlier career in poaching, but now they were on the side of the angels. With them aboard, we turned south, snacking on oranges and spicy cracker mix along the way. We eased through small towns clogged with rickshaws and busses and motorbikes; down here in the southwest, I noticed few private cars. One community seemed to specialize in the quarrying, bagging, and shipping of sand, a resource available in abundance. It was transplanting time for the new rice crop, and we could see men and women bent double, digging the dark green shoots from their thick nursery patches along the river bottoms, bundling them, moving them, replanting them carefully in flooded paddies. On drier ground grew small patches of other crops—corn, beans, grain—and the occasional cluster of banana trees or coconut palms. Drier ground, though, was becoming more scarce as we moved farther south. Straight ahead was the Sundarbans swamp, where the Ganges delta dissolves into mangrove islands and braiding channels and crocodiles and wet-footed tigers, but we weren’t going that far. Already the land was so flat and low, the water table so high, that sumps of stagnant water surrounded every village and town we passed.

  Along here we started to see more date palms, their smooth trunks scarred with barber-pole striations showing where gachis had tapped them in years past. It was mid-January now and the sap harvest was on, perfect timing in case we wanted to sample a glassful. We didn’t. Bangladeshis call the stuff kajul, I learned from Arif. They believe that it’s a salubrious beverage, killing parasites in the gut. But you’ve got to drink it fresh, Arif said. Boiling the sap ruins not just its taste but also the medicinal effect. He drank it himself as a boy, yeah, sure—but not anymore, no way, not since he’s been working on Nipah.

  In midevening we reached a city called Khulna, found rooms in a decent hotel, and the next day went out looking for bat roosts, of which Arif had prescouted several during an earlier trip. West of the city, the land seemed lower still, and water was plentiful—water in paddies, in sumps, in lagoons, in shrimp-raising ponds. Village people and their livestock lived on patches of dirt reached by footpath causeways, and the road itself ran along an embankment, material for which had presumably come from borrow pits that were now the funky greenish and brownish pools alongside. If you wanted high ground here, you had to build it. There were plenty of trees but nothing to call forest, just a scattering of coconut palms, bananas, papayas, tamarinds, a few hardwoods, and many more date palms, into one of which I saw a gachi climbing. He was barefoot, using his hands and feet plus a belt rope to ascend, like a Wichita lineman. He wore a lungi (a sarong, knotted at his waist), a turban, and over his shoulder a woven quiver, which held two long, curved knives. Nearby, a small boy on the roadside carried four red clay pots, empty and ready for placement to catch tonight’s drippings.

  The bats would be ready too. Meanwhile they were sleeping. Flying foxes, unlike insectivorous bats and some fruit bats, do not roost in caves, mines, or old buildings. They prefer trees, from the branches of which they dangle upside down, wrapped in their wings, like the weirdest of tropical fruit. We visited four or five sites. We gazed up into treetops at aggregations of sleeping bats, talked with locals, and inspected the lay of the land beneath each roost, none of which met Epstein’s exacting standards. Either the bats were too few (a hundred here, a hundred there), the nearby trees or lack of them allowed no way to erect a net, or circumstances were wrong on the ground below. In one village, several hundred bats had established their roost in some legume trees, a tempting cluster, except that they dangled just above a big green puddle that seemed to serve as drain tank and garbage dump for the village. Lowering the net after captures would drop tangled bats into that water, Epstein foresaw, and oblige him to plunge in and untangle them before they drowned. Nope, he muttered. I’d rather take my chances with Nipah than with whatever’s in that bilge.

  So we returned to a site we had spotted along the road into Khulna: a derelict storage depot within a walled compound of several acres, government-owned and once used as a repository for road-building materials. From a grassy courtyard there, among the sheds and warehouses, towered a handful of great karoi trees in which dangled four or five thousand bats. It was an especially favored roost site, evidently, because the trees were so large, the walled compound protected them from village hubbub and boys with slingshots, and each evening around dusk they could drop from their branches, take flight, circle majestically out over the Rupsha River (another branch of the deltaic Ganges), and flap away for a night’s feeding amid the villages around Khulna. Okay, Epstein decided, this is it.

  Within a day, after meetings with local officials, he and Arif had obtained permission for us to go spooking around this old depot in the middle of the night. That’s why I like working in Bangladesh, Epstein said. Simple request, reasonable people, prompt action. Go into certain other Asian countries with similar expectations and you’ll see the difference.

  Before the bat catching could begin, though, we had to do some daytime groundwork. We climbed a long rickety bamboo ladder to the flat roof of a disused warehouse, just beside the karoi trees, and from
that rooftop Gofur and Pitu kept climbing. They went high into one of the trees, nimble as sailors going to the crow’s nest, and lashed a bamboo mast into place so that it towered out vertically above an uppermost limb. Atop that mast was a simple homemade pulley. They did the same in another tree, near the far side of the warehouse, and when their clambering and their rigging were done, they could raise and lower a huge mist net between the two trees.

  Their intrusion into a roost tree, of course, disrupted the bats. Hundreds of animals stirred, woke, took flight, and circled out over the river, then back around, and then out again, like flotsam adrift on a great eddy of air. They looked big as geese against the daylight sky, soaring easily on thermals or flapping in slow rhythm. When they came over us, passing low, their features were visible—the auburn fur of their bodies, the big umber wings almost translucent, the pointy snouts. Although they didn’t like being waked, there was no sign of panic. They were magnificent. I had seen fruit bats in Asia before, but never so many in motion so close. I must have been gawking like a fool because Epstein gently advised, “Keep your mouth closed when you look up.” They shed Nipah virus in their urine, he reminded me.

  At the hotel, we set our alarms for half past midnight and then roused for the real work. As we rode to the storage depot through slumbering Khulna, Epstein gave us what he called The Safety Briefing. Goggles and leather welder’s gloves for the bat handlers, he said. Medical gloves underneath. Keep your hat on, keep your long sleeves down. When you take hold of such a large bat, you want to grasp it firmly around the back of its head, your fingers and thumb beneath its mandible so it can’t bite you. Avoid being bitten. Avoid being scratched. If a bat hooks a claw into your arm, raise that hand high, over your head; the animal’s instinct is to climb upward, and you don’t want it to climb across your face. Pitu and Gofur will untangle captured bats from the net and then place them into your grasp. Take the head with one hand, get its limbs with the other, clamping each of its strong little ankles and wrists in the gaps between your fingers—one, two, three, four—and your thumb. Four pinch slots, just enough. Trust Pitu and Gofur, they’ll help. That’s how you control a flying fox so that nobody gets hurt. Drop each bat into its pillowcase—which Arif will be holding open—then knot the pillowcase, hang it from a limb, and come back for another bat. If you get scratched or bitten, we treat that as an exposure—possibly to Nipah, possibly also to rabies. We wash the wound for five minutes with soap and then douse it with benzalkonium chloride, a potent antiviral. Immediately after that, jab, you get a rabies booster. Are you vaccinated for rabies, David? (Yes.) When was your last booster, how are your titers? (Um, don’t know.) As for Nipah exposure, never mind, because there’s no vaccine, no treatment, no cure. (What a relief.) Have I said, Don’t get bit? Our first principles are, one, safety for us; two, safety for the bats. Let’s do take good care of the bats, Epstein said. (He’s a veterinarian and a conservationist, before all.) Any questions?