A famous unsolved medical puzzle of last century–why a neurological disease spiked on Guam–may hinge on the local tradition of serving boiled bat.
After World War II, doctors noticed that the Chamorro people of Guam experienced 100 times as high an incidence of diseases resembling amyotrophic lateral sclerosis than people in the continental United States do. Last year, scientists proposed that when large local bats called flying foxes (Pteropus mariannus) dine on seeds of the cycad plant, they accumulate high concentrations of neurotoxins, which transfer to people who eat the bats. Now, ethnobotanist Paul Alan Cox of the National Tropical Botanical Garden in Kalaheo, Hawaii, and his colleagues report that the rise and fall of the disease tracks an increase and then decrease in human consumption of local bats.
In the Guam disease, people typically lose muscle strength and control and then waste away. Studies of these cases might reveal a chemical trigger for certain neurological conditions, comments Peter Spencer at Oregon Health Sciences University in Portland.
Cox adds a new reason to study the Guam cases: Animals that haven’t traditionally played a large role in human diets now show up in markets around the world in great abundance. “We’re worried,” says Cox.
Early investigators of the Guam disease ruled out genetic explanations and found links to traditional lifestyles. The Chamorro people make tortillas of flour from seeds of cycads, which carry potent chemicals such as the neurotoxin BMAA. However, researchers found that cooks soak the seeds in a way that leaches out much of the neurotoxins.
In a fresh approach, Cox and New York neurologist Oliver Sacks sketched out the basics of the bat hypothesis in the March 26, 2002 Neurology. Cox and other colleagues more recently correlated hunting and trade of bats with the disease’s incidence, they report in the June Conservation Biology.
The Chamorro people have a long tradition of serving flying foxes as delicacies, says coauthor Sandra Banack of California State University at Fullerton. The bats, complete with fur, eyeballs, and viscera, are usually eaten in coconut milk.
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Traditionally, hunters snag the bats with thorny vines, not an easy feat. During the past century, though, increased military presence on the island made guns more widely available and bat hunting more efficient. As a cash economy emerged, the bat market boomed. Hunters sold the flying foxes for up to $30 apiece.
Eventually, the relentless hunting and the ravages of the introduced brown tree snake drove the bats on Guam near extinction. People then relied on imported bats. Banack reports that bats on other islands don’t eat cycads as much, and thus wouldn’t accumulate as much of the neurotoxins as the Guam bats do. Cox says that he and his colleagues are now measuring high concentrations of cycad compounds in Guam bat tissue.
Whether the cycad’s active component comes from bats or tortillas, “it’s the structure of the chemical that’s important,” says Spencer. Because he’s failed to produce the full, progressive disease in monkeys by dosing them with BMAA, he’s now pursuing cycasin, another cycad compound.
The flying fox is “a potentially interesting additional source of cycad exposure,” says Spencer.
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