Migration may reawaken Lyme disease.

Lyme disease may hide in healthy-looking birds until the stress of migration drives it into a potentially infectious state, warn Swedish researchers.

According to this new scenario, migratory birds could be powering more of the spread of Lyme disease than expected, suggests Bjorn Olsen of Umea University in Sweden. He and his colleagues describe the migratory flare-up of Lyme disease in the Feb. 17 NATURE.

Since 1975, the disease has grown from a local medical puzzler in Lyme, Conn., to what the Centers for Disease Control and Prevention calls "an important public health problem." The more than 10,000 U.S. cases reported annually cluster in the Northeast, in north-central states, and on the West Coast. Now that doctors know what to look for, they recognize the disease in temperate regions of Asia and Europe.

Lyme disease often begins with a circular skin rash, followed by a nasty mix of flu and arthritis symptoms.

Researchers eventually fingered corkscrew-shaped bacteria: a spirochete called Borrelia burgdorferi and closely related species. After sucking infected blood, Ixodes ticks spread these bacteria.

Olsen and other researchers have shown that some birds can pass bacteria to ticks. Still, even in regions known to be squiggling with spirochetes, "100 out of 100 birds can test negative," Olsen says. "That's very strange."

To see if the infection hides in birds, he and his colleagues collected a common European species, the redwing thrush (Turdus iliacus), in northern Sweden. Researchers kept nine thrushes on an unchanging schedule of 12 hours of darkness, then 12 hours of light. Another 10 birds got conditions mimicking the shrinking daylight of autumn. About day 50, the faux fall triggered migratory restlessness.

At the start of the project, researchers injected a European Lyme strain called Borrelia garinii into six birds under constant conditions and eight under changing day lengths. No sign of infectious bacteria showed up in cultures of skin and blood samples from the birds on a constant schedule. Yet after the fidgeting set in, five out of eight infected birds in the migration group yielded bacteria.

Andrew Spielman of Harvard School of Public Health cautions that "they haven't put any ticks on the birds." He says that to be convinced of infectiousness, he needs to see ticks pick up bacteria from the birds.

Despite the absence of ticks, Thomas N. Mather of the University of Rhode Island in Kingston welcomes the Swedish research as "bringing up a point that needs study." The idea that hormones surging for migration activate Lyme infections raises questions about a host of other work, he says. For example, he found that gray catbirds exposed to the disease did not infect ticks. Now, he wonders if he'd get different results in stressed birds.

The bacterial strain the Swedes used seems more adapted to birds than does the Borrelia in the United States, Mather notes. Migratory flare-ups might be peculiar to European Lyme disease, he remarks, but researchers need to check.

Migratory reactivation of Lyme disease "could be important," comments tick-disease specialist Durland Fish of Yale University School of Medicine. "I think birds are probably underestimated."