The researchers speculate that the birds contracted drug-resistant E. coli from contact with human sewage or waste in lower latitudes before migrating north.
Infected birds could spread resistant bacteria to other animals or—in rare cases—even back to humans as the birds migrate around the world, Olsen said.
(Related news: "New Bird Flu Strain Spreads Fast, Is Resistant to Vaccine" [November 1, 2006].)
Although they think it is unlikely, the researchers say it's possible that the bacteria developed immunity on their own or acquired it from their fellow microbes.
"In nature you have a lot of circulating natural resistance," Olsen said.
"This resistance has been circulating for billions of years as a means for fungi and bacteria and other microorganisms to control each other."
Michael Yeaman is a microbial pathologist at the University of California, Los Angeles, who was not involved in the study.
The idea that the birds caught the bacteria from humans is interesting, he said, but he would have been more surprised if no resistant microbes were found at all.
"There are microbes in existence today that are resistant to antibiotics that have yet to be discovered or developed," Yeaman said.
And Terry Hazen, a senior scientist at Lawrence Berkeley National Laboratory in California, notes that some bacteria are known to be resistant to substances they have never come into contact with.
"There's a lot of compounds in the environment that are actually quite similar in structure to antibiotics," Hazen said.
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