Outbreaks Show Bird Flu Virus Is Changing

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Underreporting of prior outbreaks means it also impossible to say that the avian flu is mutating in any one direction, Siegel said. "There's just no way of telling from these clusters that this virus is evolving in the direction of easier transmission -- we can't tell if these clusters are anything new, or if there was a precedent for them," he said.

Finally, he said, H5N1's genetic "leap" to human-to-human transmission -- if it ever happens -- will be much tougher than media reports have let on.

"I've talked to a top expert at the U.S. National Institutes of Health," Siegel said. "He has tried [in the lab] to manipulate H5N1 to make it transmit more easily human-to-human, and he hasn't been able to do it. He's tried different mutations, including using proteins from the 1918 Spanish flu."

While that doesn't mean the right combination of random mutations won't happen in the natural world, it suggests that a bird flu pandemic is a possibility -- but not a certainty. "There's no sense of 'imminence' here," Siegel said.

All of the experts agreed that more needs to be done to curb the spread of the virus among birds, however.

"H5N1 viruses are a 'moving target' and are evolving globally," Uyeki said. "Therefore, what is needed is ongoing, expanded surveillance of highly pathogenic avian influenza A (H5N1) viruses in animals (including poultry and wild birds) and humans in many countries."

Webster and Govorkova noted that countries that have implemented tough, bird-focused interventions did reduce the threat. But with winter approaching, they worry that H5N1 will finally make its way from Eurasia to the Americas via migrating flocks.

However, Siegel said, vaccinating every bird in the United States does not make sense right now. That's because the virus would simply go "underground," infecting fowl but not producing outward symptoms.

"You want to vaccinate susceptible populations, and then control outbreaks by killing affected birds," Siegel said.

But he also stressed that, "here, in the U.S., we as yet have no birds that have this virus. We don't even have a problem yet, except for fear."

More information

There's more on bird flu at the U.S. Centers for Disease Control and Prevention (www.cdc.gov ).