The deadly strain of bird flu currently in southern Asia could spread to the United States or be introduced into poultry flocks here intentionally by terrorists, said Dr. Greg Poland, an infectious diseases expert at Mayo Clinic in Rochester, Minn.

"There's no reason to think that terrorists couldn't use what's happening with the bird flu to their advantage," Poland told United Press International. "Imagine, infecting flocks deliberately."

He said all U.S. infected flocks would have to be destroyed -- more than 100 million birds in Asia have been destroyed or killed by the flu virus.

"Think of the chaos that would create" and the economic implications, he said.

Poland, who is a member of the Infectious Diseases Society of America's pandemic influenza task force, said this situation could interfere with the supply of chicken eggs needed to make several vaccines. Tens of millions of chicken eggs are required to produce flu vaccine, so there could be no or only limited supplies of either a special vaccine being developed to prevent humans from contracting bird flu or the regular annual flu vaccine.

In a typical year, flu kills 36,000 people and hospitalizes about 200,000.

Health experts fear the strain of bird flu in Asia could adapt to humans and spread around the globe, causing a worldwide pandemic. The virus has killed at least 50 people in several Asian nations, including Vietnam and Thailand, and it may be spreading. Russian officials reported this week that flocks in the Siberian region have contracted a strain of avian flu, but officials have not been able to determine if it is the same strain that is involved in the outbreak in southern Asia.

Bruce Gellin, director of the National Vaccine Program Office in the Department of Health and Human Services, said health officials have taken precautions to protect chicken flocks that produce eggs for manufacturing vaccines.

"These flocks are well-protected," Gellin told UPI. "There are other diseases of birds, not just avian flu, and this is designed to prevent infections of flocks that provide eggs regardless of what that infection is."

In addition to keeping the chickens isolated from other flocks and wild birds, there is limited access to the farms where they reside, Gellin said, adding that the farms are subject to regular inspections to ensure all of the required biosecurity provisions are in place.

He noted that redundancy is built into the system. There are more chickens than need, so in the event a flock does become infected, there would be surplus fowl available to meet egg-production needs.

The vaccine manufacturers also are focused on preventing infections of the flocks they use to produce eggs for vaccines.

"It's a risk that we are very aware of and it's a risk that we take great precautions to protect against," said Lenn Lavenda, spokesman for Sanofi Pasteur, the only currently licensed supplier of flu vaccine in the United States.

"To date, we have never had infection of one of the flocks that produce eggs for flu vaccine production," Lavenda told UPI.

Sanofi Pasteur plans to produce 60 million flu shots for the upcoming flu season and this will require "hundreds of thousands of eggs per day," Lavenda said. Ultimately, the company will go through millions of eggs to produce the vaccine supply, he said.

In addition to the precautions cited by Gellin, manufacturers also keep the egg-producing flocks in multiple locations, Lavenda said. This helps reduce the risk of a widespread infection, "so in the event one flock did get infected, it wouldn't spread and jeopardize the entire supply," he said.

Lavenda said the spread of bird flu by agroterrorism is less of a concern than an accidental infection.

"The main risk is that it would happen unintentionally by somebody accidentally bringing a virus in or by some other means," he said.

New chickens are periodically brought in to the flocks to replace birds that are no longer producing eggs, so that could be one way the bird flu or another infection could accidentally be introduced into the flocks.

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