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The worm that wants to be eaten

By David, 25 October 2013 News

Written by Sarah Kellett

A European hare.

According to new research, European hares carry the black scour worm, which commonly attacks sheep.
Image: Thinkstock

Atop a blade of grass waits a baby worm. Sheep graze all around in the South Australian pasture, ripping up mouthfuls of juicy greenery. The worm quivers as a mouth nibbles nearby. Then finally, the moment arrives. In a flurry of teeth and gums the worm is swallowed. But not by a sheep. Instead, it’s eaten by a wild hare.

Introduced to Australia around the same time as rabbits, European hares are larger and have bigger ears to help cool them in hot weather. They are also carriers of the black scour worm commonly found in sheep, according to research by the University of Adelaide.

Parasitic worms are a major source of sheep disease, causing diarrhoea and even death. An adult worm in a sheep’s small intestine lays up to 200 eggs a day, which pass out in poo onto the field. Worms can be killed with a sheep drench – a dose of de-worming chemicals. But some worms will be more resistant to the drench than others. It only takes a few survivors to produce hundreds of eggs that will hatch into drench-resistant worms.

“It’s a major problem worldwide,” says Philip Stott, from the University of Adelaide. “Usually within five or so years of a new chemical drench being developed, somewhere in the world the worms are starting to show signs of resistance.”

Farmers can delay resistance by giving sheep the correct dose of medicine and keeping visiting rams in quarantine. But hares can move easily between paddocks, spreading worms around.

This can be good or bad depending on the circumstances, explains Philip. If the next door neighbour is saving money by only giving sheep half a dose of the drench, those worms will become resistant quickly. “Hares happily hopping from one property to another can spread resistance, grazing on the poorly drenched farm and defecating on the other.”

Hares can also be helpful. By providing a refuge for untreated worms, they can delay resistance. The young of untreated worms compete with drench-resistant worms and eventually outnumber them. When a sheep swallows the waiting baby worms, the parasites are more likely to be killed by the drench next time, making it easier for farmers to manage the problem.

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