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