BEIJING // Scientists believe they have solved the mystery of why hundreds of villagers in a poor part of rural China died of heart attacks over the past 30 years. Tiny white mushrooms are thought to be the cause of most of the 400-plus sudden deaths in an area of Yunnan province in the country's south-west. But the death rate has dropped off dramatically, as villagers are now being warned not to eat the apparently deadly fungi. The solution to the mystery of Yunnan Sudden Death Syndrome, announced by scientists from the China Field Epidemiology Training Programme, part of China's Centre for Disease Control and Prevention, was described in the journal Science. Sudden deaths had been occurring since the 1970s, principally during the rainy season, but efforts by Yunnan-based scientists to find the cause of the deaths had failed. In late 2004, researchers from the Centers for Disease Control and Prevention (CDC) in the United States were called in to investigate. Robert Fontaine, an epidemiologist with the CDC who has been assisting the Chinese scientists on the project, described the phenomenon as "quite unique". The deaths kept occurring, he said, "year after year" in the same villages in the same months. After visiting the remote highland areas and interviewing villagers at length, overcoming the fact many locals spoke their own language or a different dialect, the scientists identified factors linked to the deaths. "Everybody had their own pet hypothesis," Dr Fontaine said. Genetic factors were ruled out because no clear patterns of deaths in certain families were found, while another possibility was Keshan disease, linked to a lack of the trace element selenium, deficient in the area's soil. However, postmortem examinations did not show heart lesions expected from Keshan disease, and the virus that causes the disease was not prevalent in many of the affected villages. "There were many potential underlying factors. In every country there are people who die suddenly of heart failure. That background is also present in Yunnan," Dr Fontaine said. "What characterised this was that these deaths would occur in July and August and were usually associated with high rains from the rainy season. When a village was affected, it would be affected over one week. Within 24 hours you would have several deaths." The picking and eating of mushrooms was "far more frequent" among those who had died, as was the drinking of surface run-off water. By 2005, warnings were being given to villagers to be careful about eating mushrooms, and the number of deaths dramatically dropped. When deaths did occur, the researchers often found victims had been eating the tiny mushrooms. Sometimes mushrooms were left over in their homes. Yunnan province is known for supplying mushrooms - they are sent as far as Japan and Europe - but the tiny white mushrooms scientists believe cause the sudden deaths are eaten locally, rather than exported, because they are not valuable commercially. They are too small and they turn brown soon after being picked. The mushrooms were eaten as part of some poor villagers' normal diet, as unpalatable as others might find the fungi. In early tests, when the mushrooms, from the genus Trogia, were fed to mice in the laboratory, the creatures did not die. "We thought the mushroom might contain a low-level poison," Shi Wu-Xiang, an epidemiologist involved in the investigation, told Science. "Some people may eat this, no problem. Other people who eat too much, or who have underlying heart disease - they may have trouble." Chemicals extracted from the mushrooms were fed to mice, and this time the animals died. The researchers believe rare types of amino acids, which are the building blocks of proteins, are toxic. As well as the amino acids, the trace element barium, known to disrupt heart rhythms, is thought to be a factor. "We suspected barium for a couple of reasons," Dr Fontaine said. "Barium is known to be a heart toxin and mushrooms concentrate barium." Barium levels in the blood and urine of family members of those who had died were up to five times higher than average, he said. The mushrooms themselves had levels of barium that, while on their own would not be expected to kill a healthy person, could set off a fatal alteration in the heart rhythm in someone with an underlying cardiac problem. The suggestion that mushrooms are to blame is not accepted by everyone. Some local doctors believe that the local water supplies could be responsible. Dr Fontaine himself said "a lot more work needs to be done", in particular to characterise the toxin in the mushrooms. He described the whole investigation as "a great learning experience for me". He added that the process also appears to have yielded benefits for the villagers: with ongoing warnings not to eat the mushrooms being made, so far this year there have been no deaths linked to the syndrome.