Obesity emerges as new risk factor for severe flu

* Nine of 10 serious cases at Michigan hospital were obese

* Obesity not a known risk factor for influenza death

* Doctors can double Tamiflu dose in severely ill patients

By Maggie Fox, Health and Science Editor

WASHINGTON, July 10 (Reuters) - People who are obese but otherwise healthy may be at special risk of severe complications and death from the new H1N1 swine flu virus, U.S. researchers reported on Friday.

They described the cases of 10 patients at a Michigan hospital who were so ill they had to be put on ventilators. Three died. Nine of the 10 were obese, seven were severely obese, including two of the three who died.

The study, published in advance in the Centers for Disease Control and Prevention's weekly report on death and disease, also suggests doctors can safely double the usual dose of oseltamivir, Roche AG's (ROG.VX) antiviral drug sold under the Tamiflu brand name.

"What this suggests is that there can be severe complications associated with this virus infection, especially in severely obese patients," said CDC virus expert Dr. Tim Uyeki.

"And five of these patients had ... evidence of blood clots in the lungs. This has not been previously known to occur in patients with severe influenza virus infections," Uyeki said in a telephone interview.

Dr. Lena Napolitano of the University of Michigan Medical Center and colleagues studied the cases of 10 patients admitted to the university's intensive care unit with severe acute respiratory distress syndrome caused by infection with H1N1.

"Of the 10 patients, nine were obese (body mass index more than 30), including seven who were extremely obese (BMI more than 40)," they wrote in their report.

Their study was not designed to see if obesity or anything else poses a special risk factor for flu. But the researchers were surprised to see that seven of the 10 patients were extremely obese.

MULTIPLE ORGAN FAILURE

Nine had multiple organ failure, which can be seen in influenza, but five had blood clots in the lungs, and six had kidney failure.

None has fully recovered, the researchers said.

The H1N1 swine flu virus first emerged in Mexico in March and was spreading out of control in the United States by the time it was identified at the end of April. The World Health Organization declared a pandemic in June.