Migrations can resemble an extreme endurance sport, in some cases taking animals thousands of miles across continents or oceans to feeding, breeding and overwintering grounds. New research shows these feats may offer a surprising benefit: protection from disease.
Ecologist Sonia Altizer of the University of Georgia and her colleagues first saw evidence of this among monarch butterflies. A single-celled parasite that plagues monarchs does not affect those that migrate to the same degree it infests year-round residents. By reviewing other studies, the scientists found evidence of similar phenomena among other animals.
Disease could potentially influence how far animals travel and how long they spend in a particular place, giving it a more prominent role in shaping migrations than scientists have recognized, Altizer says.
"Animals have to move to where the food is, and they have to move to where they can tolerate the climate. But escaping from a parasite could be just as important as escaping from predators," Altizer told LiveScience. "Maybe it doesn't determine whether or not animals migrate, but it certainly can determine how far they migrate and where they migrate to and from. ... It is really a big open question right now."
The researchers’ review also suggests cosmopolitan animals transmit diseases to other species. Research has shown that at sites where birds stop to refuel on long flights, they crowd together, creating plenty of opportunity for disease, such as bird flu, to spread among individuals and species. Yet other research indicates that shorebirds and waterfowl are unlikely to spread bird flu along their routes, and there is a lack of proof that migrating animals enhance the spread of infectious diseases, particularly those that infect humans, such as bird flu and the West Nile virus, by carrying them on their long-distance routes.
Monarchs in warm habitats such as Florida may stay put all year round, while others make spectacular trips that can involve succeeding generations. Among resident butterflies, the researchers found that as many as 95 percent were heavily infected by the parasite, which can cause deformities, interfere with the butterflies' flight and shorten their lives. Migrating populations fared better, and the long-distance champs were the healthiest. The generation that travels from southern Canada to central Mexico in fall and then begins the return trip in the spring, before passing the baton, had annual infection rates varying from 2 percent to 15 percent, Altizer said.
The scientists found other clues that the regular journey might strengthen the migrants: Infections among the Mexico-bound butterflies actually decreased as they traveled south, possibly because highly infected individuals or those carrying the more serious strains of the parasite couldn't complete the journey.
There was also evidence that the migration helped the migrants escape disease. In areas of resident populations, the monarch’s preferred food, milkweed, collected high densities of parasite spores over time. Migrating monarchs would be able to feed off clean plants each spring.
Looking into other studies, the team found some parallels in other animals. The summer migration that reindeer undertake is believed to reduce transmission of parasitic warble flies, by taking the reindeer away from their spring calving grounds, where the parasites' larvae are shed and develop into adults.
Another study found that milder strains of avian influenza can affect swan migration, delaying it and reducing its distance, ultimately affecting the birds' ability to reach their destination.
Of course, there is a flip side: Animals may be exposed to disease along the migratory route. Shorebirds that stop in Delaware Bay provide an example. The birds mingle as they refuel on the way between their Arctic breeding grounds and their South American overwintering sites, reaching a density of more than 200 birds per 10 square feet. The avian influenza virus is 17 times more prevalent there than at any other surveillance site worldwide, according to a study published in 2010.
There is also evidence that migrations have a negative effect on immunity, because migrating animals may have less energy to dedicate to their immune systems, the researchers write in a review article published in Thursday's issue of the journal Science.
The human factor
Bats, birds and other migrating animals have been blamed for spreading infections that are potentially deadly for humans or their livestock, including Ebola, avian influenza, West Nile virus and Lyme disease. But there may be a poor foundation for placing the blame, Altizer said.
While West Nile virus is believed to have spread along a corridor traveled by migratory songbirds, for example, there is no direct evidence that infected birds carried the virus from point A to point B along that route, she said.
Similar allegations have been made about shorebirds and waterfowl regarding avian influenza. Although the birds can carry strains of avian influenza, including a type that can be lethal to humans, recent research has shown they are unlikely to spread it long distances, the authors write.
Humans are likely to play an increasing role in how diseases are spread through migrating populations or species. Dams, deforestation and development put up barriers to migration, potentially crowding more animals into fewer stops and also bringing them into closer contact with humans and livestock – potentially leading to outbreaks of disease in humans. Furthermore, climate change may encourage some species to stop migrating or to shift their routes, the authors write.
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