With millions of birds descending on Delaware Bay during migration, the propensity for bird flu (H5N1) to spread among flocks—and potentially among humans—has been a pressing concern. And as animals, from gray whales to monarch butterflies make epic treks of thousands of kilometers each year, the role of these travelers in spreading highly pathogenic diseases along the way has been a key question for ecologists and epidemiologists alike.
At first brush, a long-distance voyage might sound like an excellent opportunity for substantial disease spread. But perhaps unexpectedly animal migration might actually help keep super-pathogenic diseases in check, argued a team of researchers behind a new review paper.
"By placing disease in an ecological context, you not only see counterintuitive patterns but also understand the advantage of disease transmission," John Gittleman, dean of University of Georgia's Odum School of Ecology and who was not involved in the paper, said in a prepared statement.
Birds that migrate across continents tend to have a higher viral load than those that stay at home year round. But for some of those species, migrating might help to avoid the more virulent pathogens that can accumulate in one environment. Other animals might use this migratory escape method as well. For example, previous research has suggested that reindeer (Rangifer tarandus) shrug off pesky warble flies (Hypoderma tarandi) by finding new summer pastures away from where fly larvae have been shed in the spring.
Research has also shown that some sick animals tend not to travel as far, which can help prevent more extensive spread of infections. A 2007 study showed that Bewick's swans (Cygnus columbianus bewickii) infected with strains of avian flu viruses migrated later in the season and traveled shorter distances than healthier birds. And simply preparing for migration can compromise animals' immune systems. For example, research in redwings (Turdus iliacus) showed that captive birds anticipating migration were more likely to succumb to latent Lyme disease (Borrelia) infections. These sick birds, in turn, might not voyage quite as far a field as healthier fowl.
Better understanding these disease dynamics will be important in stemming potential pandemics. But migrations, like the animals themselves, are moving targets, influenced by human impacts to the physical and climatic landscape. With less available land and more impediments to movement—such as fragmented habitat—more animals might end up in a few close-quarters stopovers like Delaware Bay or curtail their migration, leading to fewer opportunities to escape local infection or leave the sick behind. Climate change is already shifting migration routes into new areas and seasons, likely changing the pathogens to which animals are exposed or carry.
As populations of flying foxes (Pteropus fruit bats) have had their 50- to 1,000-kilometer trips shortened by deforestation in Australia and Southeast Asia—and as increased animal agriculture has brought them in closer contact with pig farms—the bats have brought more zoonotic diseases to humans. And cultivating salmon near natural salmon migration routes have spread sea lice (Lepeophtheirus salmonis) to passing wild juvenile fish that otherwise would not otherwise be exposed to this parasite.
"A lot of migratory species are unfairly blamed for spreading infections to humans," Barbara Han, also of the University of Georgia and a co-author of the new paper, said in a prepared statement. "But there are just as many examples suggesting the opposite—that humans are responsible for creating conditions that increase disease in migratory species."
The paper was published in the January 21 issue of Science.