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The risk of animals passing diseases to humans could increase in some cases, but decrease in others, as people encroach on and disrupt wildlife migration paths, according to a review in Science (21 January).
Climate change is also affecting migration patterns, and the review says there is an urgent need for research on how changes in habitat and climate will affect disease in migratory animals, to predict risks for both people and wildlife.
Although there is a general assumption that long-distance movements of migrating animals can increase the spread of pathogens, including zoonotic pathogens that jump from animals to humans, such as Ebola virus in bats and avian flu viruses in birds, the evidence for this is scarce, the review says.
"There are examples that suggest that most wild birds aren’t likely to spread the most pathogenic strains of avian flu over long distances, as was previously suspected," Sonia Altizer, at the Odum School of Ecology, University of Georgia, United States, told SciDev.Net.
There might even be a decrease in transmission risk for some diseases, according to the review, but more research is needed to make accurate predictions.
"One of the biggest surprises is that there aren’t a lot of clear, published cases of migratory species carrying infectious diseases. This could be partly because of the challenges of studying species across international borders," Altizer said.
Some long-distance migrations are a known disease threat to humans and livestock. For example, the deadly Ebola outbreaks in the Democratic Republic of the Congo have been linked to an influx of migratory fruit bats.
But now shifts in natural migration patterns may change where and how such disease outbreaks occur.
Destruction of habitats by urbanisation or agriculture can eliminate stopover areas and herd more animals to the few remaining sites. This, in turn, could create hotspots for disease transmission, the review says. Human encroachment on these natural sites could also increase the risk of contact with diseased animals.
Outbreaks of the Nipah and Hendra viruses in Malaysia and Australia, affecting pigs and horses respectively, have been traced to changing habits of previously migratory fruit bats — instead of migrating in search of ephemeral food sources they now settle around fruit orchards that provide year-round fruit. The location of the orchards brought the fruit bats close to pigs and horses.
Migration could reduce the amount of disease as infected animals are left behind to die, but changes in migration might prevent such natural purging of infections, the review says.
Fences and dams might also force some animals to stop migrating, which could increase the prevalence of pathogens in the population.
Jeff Waage, a biosafety expert and director of the UK’s London International Development Centre, told SciDev.Net: "This is the first time that people have looked at migratory species from a disease point of view, including how migration may affect humans and livestock. It is interesting because only ten years ago we had no idea that bats were an alternative host of some of the most serious human diseases, such as Ebola."
Kate Jones, a wildlife epidemiologist at the UK’s Institute of Zoology in London, told SciDev.Net this review "has pulled together all of the complex issues regarding migration and infectious diseases, and has laid the foundation and a direction for future research."
Science 331, 6015 (2011)