Spillover: Animal Infections and the Next Human Pandemic (40 page)

Three months afterward, in April 2004, health officials in Bangladesh learned of still another outbreak. Faridpur District, just adjacent to Rajbari along the Padma River’s right bank, was the latest site. Faridpur and Rajbari, reachable only by slow ferry, are where the urban clamor of greater Dhaka, groping upward in concrete and rebar, gives way to the silty, deltaic lowlands of southern Bangladesh. Rice paddies line the road. Palms and banana trees grow like weeds in a vacant lot. Among thirty-six patients in Faridpur, twenty-seven died. And the pattern of social connectedness among cases suggested another concern, which had also arisen regarding the Chandpur outbreak: that some people had caught the infection from other people. A team of investigators noted that such person-to-person transmission “
increases the risk for wider spread
of this highly lethal pathogen. In an impoverished, densely populated country such as Bangladesh, a lethal virus could rapidly spread before effective interventions are implemented.” Judicious language, by which they meant: It could go like wildfire in dry grass.

Then came still another Bangladesh outbreak, the fifth within four years, this time in Tangail District, about sixty miles northwest of Dhaka. Twelve cases, eleven deaths, all during January 2005. Now it began to seem that Bangladesh was uniquely, persistently tormented by this killer disease, recurring in the early months of each year. Malaysia had had no further outbreaks. India, just north of the northwestern Bangladesh border, had had one. Elsewhere in the world, Nipah was unknown. Again a team went out from Dhaka and did a case-control study, looking for the cause of the spillover. The team leader was Stephen Luby, an American physician and epidemiologist from the CDC, seconded to Dhaka as a program director within the International Center for Diarrheal Disease Research, Bangladesh (fastidiously initialed as the ICDDR,B but commonly known as the Cholera Hospital), where he worked closely with his Bangladeshi counterpart from the Ministry of Health, Mahmudur Rahman.

Luby’s group, like Montgomery’s earlier, questioned people about potentially risky activities—things done by patients who sickened and died, or sickened and recovered, that might not have been done by neighbors who remained healthy. For the fatalities, they got their answers from surviving relatives or friends. Had the person climbed a tree? Some had, most hadn’t, both among the patients and the healthy controls. Had the person touched a pig? No, nobody in Tangail was in the habit of touching pigs. Touched a fruit bat? No, nobody. Touched a duck? Yes, but so what, lots of people did that. Touched a sick chicken? Eaten a guava? Eaten a banana? Eaten an animal that was ill at the time of slaughter? Eaten a star fruit? Touched someone who was feverish, confused, and who later died?

The questions themselves are like pen strokes on a sketch of Bangladeshi village life. But none of those questions—not even the one about tree climbing, this time—yielded any statistically significant distinction between those who had gotten sick and those who hadn’t. Only one question asked by the Luby team did: Have you drunk any raw date-palm sap recently?

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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S
o 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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loquence 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.