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Bat-Eating Linked to Neurological Illness

John Pickrell in England
for National Geographic News
June 13, 2003
 
Throughout the last century the Chamorro people of Guam suffered alarmingly high incidences of a mystery neurological illness found to be a hundred times more common than in the United States. Last year researchers suggested this sickness is linked to the tradition of feasting on the Mariana flying fox, a type of bat, which accumulates neurotoxins in its diet.

Now scientists report in the June edition of the journal Conservation Biology that the recorded increase and decrease in prevalence of the disease was closely mirrored by the rate of human consumption of these bats. The team predicts that originally dense populations of the herbivorous flying fox (Pteropus mariannus) may have once dwarfed the human population six-fold and have plummeted from more than 60,000 animals to fewer than 200 today.

"Through the consumption of cycad [seed]-fed flying foxes, the Chamorro people may have unwittingly ingested large quantities of cycad neurotoxins," says the study.


Bowl of Bat

Following World War II when Guam changed hands from Japanese to American control, researchers began to notice a higher incidence of a neurological disease similar to Lou Gehrig's, Parkinson's, and Alzheimer's. Known as amyotrophic lateral sclerosis-Parkinsonian dementia complex (ALS-PDC), or "lytico-bodig" by the Chamorro, the illness causes muscle weakness, paralysis, dementia, and death. In the village of Utamac alone, one-quarter to one-third of all deaths between 1944 and 1953 were attributed to the disorder.

Genetics, pollution, and other possible causes were all ruled out until researchers thought they'd found the key in the form of cycad seed flour, which is used traditionally to make tortillas, said Sandra Banack, ethnobotanist and study co-author at California State University in Fullerton. (Ethnobotanists study the plant-related traditions of indigenous peoples.) The disease had previously appeared to crop up more in those that lived a traditional lifestyle, she said, and it ran in families.

The seeds of cycad plants common on Guam and the surrounding Pacific islands contain high levels of chemicals toxic to the nervous system. However, that theory fell out of favor when studies showed the quantities of the chemical ingested from tortillas were suspiciously low.

That was until ethnobotanist Paul Alan Cox at the Hawaiian National Tropical Botanical Garden in Kalaheo, and other colleagues, began to wonder if cycad-seed-eating flying foxes might be involved. The bats have been especially desirable food items to the Chamorro, possibly because the tradition is one of few retained from older times before four centuries of upheaval and cultural oppression which began with Spanish colonial rule in 1565.

Served at weddings, fiestas, birthdays, and alike, the etiquette of bat-eating and preparation involves "rinsing off the outside of the animal like you would a cucumber and tossing it in boiling water," said Banack. The animals are then served whole in coconut milk and are consumed in their entirety. Meat, internal organs, fur, eyes, and wing membranes are all eaten, she said.

Last year Cox and well-known neurologist Oliver Sacks of the Albert Einstein College of Medicine in New York City reported in the journal Neurology the first evidence for a link between bat eating and the disease. Bats consume large quantities of cycad seeds, and—like some eagles, which were shown to build up levels of the pesticide DDT in fat tissue—probably accumulate the toxins to dangerous levels.

Flying Foxes Fed to Pigs

Now, Banack, Cox, and geographer Clark S. Monson of the University of Hawaii in Honolulu provide further compelling evidence for that explanation.

The team reports that in the early 20th century prior to regular firearm use, people would sneak up upon the bats and catch them with nets or thorny branches used to snag wing membranes. Due to the inefficiency of the method, this probably had little impact on population numbers. Flying foxes were so abundant a hundred years ago, that farmers fed them to pigs during hard times.

By extrapolating the density of bat populations in the forests of nearby uninhabited islands to the forested area of Guam a hundred years ago, the team have predicted that island could easily have supported more than 60,000 flying foxes.

However with the arrival of firearms the Chamorro were able to catch the animals in unprecedented numbers. "When disturbed, the bat colonies fly out of the roost [tree] and circle around," said Banack, "[and] hunters could easily shoot hundreds at a time."

The persecution of flying foxes continued throughout the last century and even after the species was listed as endangered by the IUCN and its hunting and trade became prohibited. "Flying foxes are still now frozen and shipped in from other places, and their trade can be very lucrative" despite frequent fines, said Banack.

The researchers document the close correlation between an increase in the consumption of bat from the 1920s and rapidly increasing prevalence of lytico-bodig disease. They also document the decline in the illness, which has gone hand-in-hand with plummeting bat numbers. "The disease is petering out, which is wonderful for the people, but frustrating for the scientists trying to work out exactly what causes it," said Banack.

"It's a very interesting idea" that bat-eating is linked to the illness, comments Michael Heinrich, ethnobotanist at the University of London School of Pharmacy in England. Other studies have also linked plant chemical consumption by indigenous people to diseases, he said. However there needs to be more evidence that bats accumulate toxins in high enough quantities, and that Chamorrans gorge on enough of them to get a nasty dose of neurotoxins, said Heinrich.

Time may be running out for the flying foxes of Guam, however. The last remaining colony of fewer than 200 individuals is found on the grounds of the U.S. Anderson Air Force Base, said Banack. The future doesn't look good—an introduced species of brown snake has been eating baby bats, meaning that virtually none have made it to adulthood since 1982.
 

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