[This was written by Fran Siebrits and published online by Wild Magazine http://www.wildcard.co.za/, 2011]

 It
has previously been assumed that migratory animals spread diseases from
one area to another. But a recent study has suggested the opposite. 

Every year animals all over the world migrate – some of them over
thousands of kilometres and over many months. In Tanzania the wildebeest
migration is a well-known wildlife spectacle and along South African
shores the annual sardine run attracts attention.

It’s been thought that migration increases disease. Surely when animals
migrate their diseases travel along with them? Research done by the
University of Georgia Odum School of Ecology points to a different
pattern.

In the case of some parasites, migration enables animals to escape an
environment that has become infested. While the hosts are away the
parasite load decreases and the animals can return to a habitat that is
relatively free of disease.

Migration also exacts a physical toll and disease-ridden animals can’t
survive long journeys. The result is that infected individuals are
removed from the group and the most virulent strains are eliminated.

These new insights come from a study on monarch butterflies. Butterflies
in cool climes like Canada migrate south to spend the winter in central
Mexico. In more temperate places like Florida the monarchs don’t
migrate. Researchers discovered that the parasite load is highest in the
butterflies that don’t migrate and lowest in the ones that journey the
furthest.

This research emphasises the importance of migration to the health of a
species. Unfortunately, nowadays successful migration is hampered by
deforestation and urbanisation. Many migratory routes are blocked by
dams, fences and agricultural land, where livestock and migratory
species are exposed to each other’s pathogens.

“Migration is a strategy that has evolved over millions of years in
response to selection pressures driven by resources, predators and
lethal parasitic infections,” says researcher Barbara Han from the
University of Georgia Odum School of Ecology. “Any changes to this
strategy could translate to changes in disease dynamics.”

Source:

S. Altizer, R. Bartel, B. A. Han. Animal Migration and Infectious Disease Risk. Science, 2011; 331 (6015): 296 DOI: 10.1126/science.1194694
Viewed online [http://www.sciencedaily.com/releases/2011/01/110120142323.htm]