In 1819, the French corvette Uranie called in at Guam, the largest and southern most of the Mariana Islands in the western Pacific. It was one of many landfalls during the Uranie’s four-year voyage around the world, but Jacques Arago, the expedition’s artist, had lost none of his enthusiasm for observing native islanders. The Chamorros of Guam were tall, robust people whose ancestors had sailed from south-east Asia more than 3000 years earlier. Arago made plenty of drawings and, as this was a scientific voyage, he also kept detailed notes of the islanders’ lives – right down to the diseases they suffered. He reported that they had cholera, leprosy and a clutch of other illnesses. But he made no mention of a terrible sort of paralysis. A century later, Guam’s graveyards were filling up with victims of a catastrophic disease that attacked the nervous system. How could Arago have overlooked such a common and appalling sickness? He hadn’t. When the Uranie called, there was no such disease on Guam.
HARRY ZIMMERMAN had been on the island of Guam only a few months when he realised there was something strange about the place. Zimmerman was a doctor with the US Navy, and when the US recaptured Guam from the Japanese in 1944, he had been assigned to the civilian hospital in the capital, Agana. The war had taken a terrible toll on Guam and its people, but Zimmerman was shocked by something else. In just a few months, he had seen eight patients with advanced amyotrophic lateral sclerosis – a devastating disease that attacks the brain and leads to paralysis, dementia and death.
Zimmerman was no stranger to the disease: before the war he had specialised in neurodegenerative disorders. What shocked him was how common it was among the native Chamorro people. On Guam, the disease was complicated: in some patients it was like ALS, but others had symptoms of Parkinson’s or Alzheimer’s or both. It became known as ALS and Parkinson-dementia complex (ALS/PDC) and it was a hundred times as common among Chamorros as ALS was elsewhere. Why? Was it to do with their genes? Or was it something to do with their way of life?
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The disease was certainly more common in parts of the island where people still led a traditional lifestyle. It ran in families. Outsiders didn’t get it – but Chamorros who left the islands sometimes did. The disease was a strangely recent phenomenon. Spain claimed the islands in 1565 and occupied them until 1898, when Guam was handed over to the US at the end of the Spanish-American war. In all that time, not one explorer, missionary or colonist ever mentioned a fatal paralysis among the Chamorros.
In 1902, the American vice-governor, William Safford reported that the Chamorros were “remarkably free from disease and physical defects, and lived to a great age”. The first record of the disease appears two years later, when three death certificates mention some form of paralysis. The numbers grew until, by the 1940s, ALS/PDC was the leading cause of death among adult Chamorros.
The disease was appallingly obvious, but the cause was not. There were plenty of suggestions: a dodgy gene, a virus, aluminium in the water, heavy metals in the soil, a mineral deficiency. All were ruled out. Perhaps it was something in the diet? An early suspect was the flour used to make tortillas. In Chamorro villages, people made flour from cycad nuts, which are packed with toxins. The case against the cycad grew stronger when chemists isolated a potent nerve poison, an amino acid called BMAA (
-methylamino-L-alanine). The case collapsed again when researchers found that the process of preparing flour removes almost all the BMAA. It would be impossible to eat enough cycad flour to damage the brain.
In 2002, New York neurologist Oliver Sacks and ethnobotanist Paul Cox, director of the National Tropical Botanical Garden in Hawaii, suggested that BMAA could yet be the culprit. People couldn’t eat enough cycad flour to damage their brains, but they might acquire bigger doses from flying foxes. These large fruit bats are a traditional part of Chamorro diet and cycads are one of the bats’ favourite foods. If BMAA from cycads accumulated in the bats’ tissues, people could be exposed to damaging doses.
There was some circumstantial evidence to implicate flying foxes. The disease appeared early in the 20th century, reached a peak in the 1940s and then began to decline. No one born since 1961 has developed the disease. This rise and fall mirrors the consumption of flying foxes.
Eating flying foxes is central to Chamorro life, a tradition that has survived through centuries of occupation and regular attempts to stamp out the indigenous culture. “Flying fox is the most prized food item in the Chamorro diet,” says Cox. “They boil them whole in coconut milk and eat the entire animal, hide and everything.” Men eat more bats than women and the disease is more common among men.
The traditional way to catch flying foxes was to climb the breadfruit trees in which they roost and net them. But flying foxes have keen sight and are hard to sneak up on. “Hunting bats was a very uncertain enterprise,” says Cox. So bats were reserved for feasts, weddings and religious events. When the Americans arrived, the Chamorro began to acquire guns, making it easier to kill flying foxes. At the same time, the island began to adopt a cash economy, allowing hunters to sell their bats. Consumption of flying foxes soared and so did the number of cases of ALS/PDC.
Demand eventually outstripped supply. One of the two native species of flying fox became extinct. The second, Pteropus mariannus, almost followed it. Today a tiny colony of about 50 bats survives on the US’s strategic air command base. The Chamorro now import fruit bats from islands where there are no cycads. Even so, the urge to eat authentic Guam fruit bat remains strong. “There have been two incidents where men have gone into the base with guns to shoot the bats. And this is a strategic air command base with B-1 bombers,” says Cox. “It shows how far people will go to get flying foxes.”
Cox needed to find out if flying foxes built up dangerous concentrations of BMAA in their tissues. In cycads, the toxin is concentrated in the seed and its protective coat, which on average contains 9 micrograms per gram of tissue. Unable to test live bats, Cox and his colleague Sandra Banack turned to 50-year-old museum specimens and found that bat tissue contained almost 400 times as much as the cycad’s seed coat. One bat contained as much BMAA as a tonne of processed cycad flour. Analysis of the brains of people who have died from ALS/PDC shows that some BMAA reaches the brain, where there are concentrations of around 7 micrograms per gram. “That’s not enough to cause acute poisoning but we think that this constant low level in the brain is enough to start killing the motor neurons,” says Cox.
In November, Cox, Banack and Canadian biochemist Susan Murch revealed a new twist to the story. Closer scrutiny of the cycad showed that BMAA is produced by a cyanobacterium that lives inside the cycad’s “coralloid” roots, which grow up through the soil and form clumps at the surface. The microorganisms fix nitrogen, providing the cycad with a key nutrient. They also manufacture BMAA, which the cycad diverts to its developing reproductive organs where it may protect them from herbivores.
The cyanobacterial connection might eventually explain why two other remote places are hot spots for similar diseases. The people of the Kii peninsula in Japan and the Auyu and Jakai people of Irian Jaya are stricken in much the same way as the Chamorros. There are no fruit bats in Japan and the Kii people don’t eat cycads, although both they and the people of Irian Jaya occasionally use them in medicines. “But now that we’ve traced the origins of BMAA to cyanobacteria,” says Cox, “that opens up the possibility that it could find its way into the diet by different routes.”