Sweet and Deadly: Bat-Borne Virus Brews in Bangladesh’s Date Palm Pots

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Indian flying foxes are widespread in Bangladesh. When their forest habitat becomes too fragmented, these giant fruit bats can spread an unusually deadly virus to humans.  (Photo by Fritz Geller-Grimm)

New research from the University of Wisconsin suggests that deforestation is promoting the spread of a disease called Nipah virus in Bangladesh. The virus has no cure, no vaccine -- and a mortality rate of more than 70 percent.

After Nipah virus first appeared in Malaysia in 1998, epidemiologists traced it back to Indian flying foxes -- giant fruit bats that are widespread in South Asia. In Malaysia the bats infected domesticated pigs, which in turn infected their farmers. But the latest outbreaks have been in Bangladesh, where pigs are rare. A 2006 study by the CDC and other groups concluded that the Bangladesh outbreaks were caused by drinking contaminated date palm sap, a sugary syrup humans and bats both love.

Sap collectors slice into date palm trees with machetes, then hang clay pots to catch the sweet syrup that drips out. Bats drinking from the pot can contaminate the sap with Nipah virus. Photo courtesy of Micah Hahn.
Sap collectors slice into date palm trees with machetes, then hang clay pots to catch the sweet syrup that drips out. Bats drinking from the pots can contaminate the sap with Nipah virus. Photo courtesy of Micah Hahn.

Date palm sap is collected from tree trunks, like maple syrup: collectors tap the trees with machetes and let the syrup run into clay pots overnight. During the night, when the bats are out foraging, they find these pots, drink from them, and sometimes leave behind the Nipah virus in their saliva, urine, or feces. Cooking or fermenting the sap could destroy the virus, but in Bangladesh the sap is commonly sold raw at street markets, a practice the government banned after a 2011 outbreak that killed 21 children.

That law isn't enforced, though, especially in rural areas, and the sale of raw sap continues. The Institute of Epidemiology, Disease Control, and Research in Dhaka, the capital of Bangladesh, reported 18 more cases before February 11 of this year. Patients usually show up with a fever, headache, and neurological symptoms like confusion and seizures. There isn’t much doctors can do beyond keeping them comfortable and helping them breathe once the disease spreads to their lungs.

Understanding which villages are most vulnerable to Nipah would facilitate more targeted prevention efforts, but the virus' geographic distribution was puzzling. Outbreaks seemed to be clustered around a strip of territory in central and northwestern Bangladesh that’s come to be called the “Nipah belt.” Population density is higher in the Nipah belt than outside it, and forest density is lower, but the bat population -- presumably the source of the outbreaks -- is the same. And even within the Nipah belt, some villages escaped the virus entirely when similar ones, with the same number of bats, had outbreaks.


That was the mystery that intrigued Micah Hahn, then a graduate student at the Nelson Institute for Environmental Studies at UW-Madison. “Why here, but not there? How does the environment help determine who gets Nipah virus and who does not?”

To answer that question, Dr. Hahn and her colleagues combined high-tech remote-sensing techniques with low-tech door-to-door surveys. They also hung infrared cameras to monitor bats feeding at night. All this data enabled them to create a high-resolution map that compared the geographic distribution and density of people, bats, and trees.

Bat roosts. Photo courtesy of Micah Hahn.
During the day, fruit bats roost in colonies of hundreds. But surprisingly, just having a bat colony nearby doesn't necessarily increase the chance that Nipah virus will spill over to humans. Photo courtesy of Micah Hahn.

That led to the discovery of a surprising culprit: deforestation. The United Nations’ Food and Agriculture Organization estimates that Bangladesh has lost nearly three-quarters of its forest in the last 30 years as its population has expanded. In the Nipah belt, as Hahn explains in this video from the Nelson Institute, what remains are small, uneven patches of forest instead of large swaths of jungle. The places where the forest is most fragmented are the places most vulnerable to Nipah virus.

For every 10 percent reduction in tree cover at the sites in the Nipah belt where the bats were roosting, Hahn found that a nearby village was twice as likely to have an outbreak. Why? Even though there were the same number of bats in these fragmented forests as in thicker ones, Hahn observed that they “tended to settle in several small roosts scattered throughout the villages, rather than in one large roosting colony.” In areas where the human population density is high, like in the Nipah belt, Hahn speculates that this dispersion increases the likelihood that the bats will find human food sources. When bats and humans start sharing food, disease transmission becomes much more likely.

Date palm sap is collected in clay pots like these. Covering the pots with fabric to keep the bats out can prevent contaminationDate palm sap pots. Photo courtesy of Micah Hahn.
Date palm sap is collected in clay pots like these. Covering the pots with fabric to keep bats out can prevent contamination. Photo courtesy of Micah Hahn.

But Hahn is quick to point out that the bats themselves aren’t the problem. In fact, since bats help regenerate forest by strewing seeds all over their territory, they could be part of the solution. “This is not just about having bats,” she said. It’s a combination of bat behavior and human behavior that sparked the emergence of Nipah virus in Bangladesh. “The disease risk is a result of humans changing the landscape in ways that create opportunities for human/wildlife interactions,” Hahn said

Nipah virus isn’t the only disease whose spread is influenced by the way humans manage the landscape. In Uganda, replacing natural swamps with cropland increased the risk of malaria. Yellow fever, leishmaniasis, and Hantavirus have also been shown to behave differently when the landscape changes. Hahn’s research will help target Nipah prevention and surveillance efforts to the most vulnerable villages, but it’s also evidence that the consequences of reshaping our environment could be more complicated than we expected.

When the forest is broken up into small patches with lots of holes in the canopy, bat and humans start to find food in the same areas. Photo courtesy of Micah Hahn.
When the forest is broken up into small patches and interspersed with human settlements, like it is in the Nipah belt, humans and bats can start sharing food sources. Bat-borne diseases are the result. Photo courtesy of Micah Hahn.