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WASHINGTON (Reuters) - Patients who became severely ill with H1N1 swine flu last year often developed kidney failure, which worsened their illness and raised costs, Canadian researchers reported on Wednesday.Doctors should be on the lookout for kidney damage in patients who are hospitalized with the virus, they told a meeting of the National Kidney Foundation."It's concerning that so many people got some form of kidney injury, although it was reversible in the majority of them," Dr. Manish Sood of the University of Manitoba in Winnipeg said in a statement.Sood's team looked at the cases of 47 critically ill patients with confirmed H1N1 infections who were admitted to one of seven intensive care units in Manitoba, a Western Canadian province.Two-thirds had kidney injury or kidney failure. Eleven percent of them needed dialysis and 16 percent died."Patients who come to the ICU (intensive care unit) with critical illness who also have kidney injury stay longer, take up more resources, and have a much higher chance of dying."Swine flu emerged a year ago in the United States and Mexico and spread around the world in six weeks, killing thousands of people. It has hit children and young adults especially hard.The World Health Organization's Emergency Committee says it is as severe as influenza pandemics in 1957 and 1968 and remains a threat, especially to healthy young adults. The U.S. Centers for Disease Control and Prevention estimates H1N1 has killed 12,000 Americans and put more than 265,000 in the hospital.Any type of serious infection or critical illness can cause kidney injury, Sood said, who added that viral infections like influenza also caused muscle breakdown. Kidneys are damaged as they filter out the broken-down muscle cells.