The researchers also studied the virulence of two strains of the pandemic H1N1 virus in a nonhuman primate model as a way to predict how the strains would affect humans. Comparing an isolate from California with one from Mexico, Richt and his collaborators found that the California isolate was more virulent than the Mexico isolate. Both pandemic H1N1 viruses are more virulent than seasonal H1N1 flu viruses.

"With different isolates, there are different clinical outcomes," he said.

Establishing animal models for pandemic H1N1 is important, Richt said, because physicians have two types of antiviral medications to treat influenza. One type, called adamantine-like drugs, targets the M2 protein; the other type includes drugs like Tamiflu that target the neuraminidase protein. He said that this pandemic H1N1 is already resistant to the M2 inhibitors but still is sensitive to Tamiflu.

"Some pandemic flu isolates from humans have now shown resistance to the Tamiflu," Richt said. "So the big issue now is if these Tamiflu-resistant strains take over, we have no drug to treat infected patients. And because we don't have a vaccine yet in the United States, this might be a problem.

"Pandemic H1N1 is another example of how important it is to work on the nexus of human and animal health," he said.

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