May 3 (Bloomberg) -- Swine flu’s potential to reconfigure its genetic material and become more deadly is a “major concern,” said Anne Schuchat, a scientist with the U.S. Centers for Disease Control and Prevention.

The prospect of what is called reassortment is keeping Schuchat and health officials worldwide on the alert even as the newest outbreak has proved itself to be relatively mild outside Mexico, where 19 of the 20 confirmed deaths were reported. Human cases of the disease have been confirmed in 30 U.S. states and 19 countries, according to the CDC and the World Health Organization, a global agency that may declare a pandemic.

Canadian health officials yesterday reported the world’s first case of the swine flu jumping to pigs from a human, probably after a farm worker in the province of Alberta became ill during a trip to Mexico. Additional gene reassortment has the potential to change the makeup of the disease, Schuchat said today in a conference call.

“The prospect of reassortment is always there with influenza,” Schuchat said in the conference call today. The agency is especially concerned that the new flu, already a risk of causing sickness and death worldwide, may mutate in human or pig populations, she said.

‘Mixing Vessel’

Pigs are an ideal breeding ground for new forms of the flu, including the new H1N1 virus, Nancy Cox, chief of the flu division at the CDC’s Center for Immunization and Respiratory Disease, said at the briefing. The running hypothesis among scientists has been that the new flu -- a combination of four strains from swine, birds and humans -- started inside a pig, she said.

The infected pigs in Canada are recovering and there’s no need to cull herds worldwide, said Peter Ben Embarek, a food safety science with the WHO, which is based in Geneva, on a conference call today.

Pigs serve as a “wonderful mixing vessel” for bird, human and swine viruses, Cox of the CDC said.

“If pigs are infected with this new virus, and some of the swine influenza viruses that are already circulating, there could be additional reassortments of them,” she said. “Likewise if a human were co-infected with one of the seasonal influenza viruses, and this new H1N1 virus, we could have a virus reassortment which emerged that has slightly different properties than either of the two parental viruses.”

Reassortment “is of major concern” in the new virus, Schuchat said. Even if symptoms remain mild, the ease with which the illness can spread among a world population with little natural immunity still makes it a threat, she said.

Flu Toll

The U.S. yesterday reported 226 cases in 30 states, with one death, a 22-month-old child who had traveled from Mexico and died April 27 at a Houston hospital. The number of people with flu in the U.S. is increasing at a time when the typical season would be reaching its end, Schuchat said.

Data so far suggest that the virus is striking younger patients than is typical for influenza, and younger patients than usual are entering hospitals, Schuchat said. There are “very few” patients with swine flu over the age of 50, and the median age is 17. It’s possible that the elderly have greater immunity toward the disease, Schuchat said.

“I do expect more cases, more severe cases, and I do expect more deaths,” Schuchat said. “We are planning to remain aggressive with this new virus. I’m particularly concerned about what will happen in the fall,” beginning in late September, when flu season starts for the Northern Hemisphere, Schuchat said.

“We’re not out of the woods yet,” she said.

To contact the reporters on this story: Tom Randall in New York at trandall6@bloomberg.net.

Last Updated: May 3, 2009 17:03 EDT