WVU experts chase sheep-killer worm

CHARLESTON (AP) — A silent killer is attacking sheep all over the country, but sleuths at West Virginia University are hot on its trail.

This deadly assailant is known to researchers as “Haemonchus contortus,” although laymen call the parasite a “red stomach worm” or “barber pole worm.”

The nasty little critter earned that last name because its intestines are wrapped around its ovaries and the worm resembles a barber’s pole when it is full of blood.

The worms feed on blood, eventually killing most host sheep. Even worse, the parasites are becoming resistant to drugs once used to keep them at bay.

The parasites do not affect all sheep, however. St. Croix sheep, a breed native to the U.S. Virgin Islands, has a strange resistance to the barber pole worm.

Researchers at West Virginia University are now studying St. Croix sheep for clues about how to beat the parasite.

“The parasites have never developed a resistance to the St. Croix like they have to the drug,” said Scott Bowdridge, a WVU assistant professor of food animal production.

Bowdridge began studying the parasites while working on his doctorate at Virginia Tech.

“The St. Croix can clear it themselves in five weeks after the infection. The second time we give it to them, it never shows up,” he said.

Bowdridge and his team are studying the St. Croix’s immune system response to the worms.

“We know that they produce a ‘kitchen sink’ approach to immunity. They throw everything they have at it,” he said.

Regular domestic sheep, meanwhile, don’t put up a fight. And no matter how many times the sheep get the parasite, their immune system response never improves.

“They can recognize a lot of other pathogens, but when that parasite gets in there, they just shut down,” Bowdridge said.

Researchers are now looking for a way to “wake up” domestic sheep’s immunity to barber pole worms. Bowdridge said the St. Croix sheep would offer some clues about which chemicals and protein signals trigger a response to the invaders.

“They have all of the same physiology. They have the same organs and the same lymph nodes,” Bowdridge said.

WVU recently received a $150,000 grant from the U.S. Department of Agriculture to fund research on the parasites and the sheeps’ response to them.

Researchers get samples of the parasite from feces produced by chronically infected sheep.

In the pasture, the parasite is spread from sheep to sheep through feces. The worms develop in the feces and crawl up blades of grass where they are eaten by other sheep, setting off other infections.

WVU researchers prevent new infections by keeping sheep on raised metal floors.

“All their feces fall through the floor,” Bowdridge said.

Instead, they intentionally spread the virus by extracting parasites from sheep excrement and administering them orally to otherwise healthy sheep.

WVU’s farm has around 50 sheep dedicated to Haemonchus contortus research, including 25 St. Croix and 25 commercial crossbred ewes.

Bowdridge said the Haemonchus contortus has lived in sheep bellies for as long as researchers have studied that kind of thing.

“It’s always been killing sheep,” he said.

Farmers used to consider a sheep’s hardiness against disease when deciding which animals to breed.

That focus shifted when safe, effective drugs for barber pole worms hit the market in the 1960s. Farmers began using these medications on their flocks and paying less attention to breeding as a prevention method.

In the early 1980s, a drug called Ivermectin became available. Bowdridge said this drug became very popular because it’s safe to give animals — even up to 10 times the recommended dose — and the drug can be used on pregnant animals.

“Giving producers that much power was probably a mistake in hindsight,” he said.

Farmers started overusing Ivermectin. And since the drug never killed 100 percent of parasites in a sheep’s stomach, the remaining bugs eventually built up a resistance to the drug.

Ivermectin is still widely used, Bowdridge said, and many farms still have wireworms that are susceptible to the medication.

Other farms have had to return to older medications, ones used in the ‘60s and ‘70s, because the parasites haven’t built up a resistance to them.

Pharmaceutical companies no longer make many of those older drugs so there is not enough supply to meet the new demand, Bowdridge said.

Drug-resistant worms are mostly an East Coast problem, though Bowdridge said researchers believe the parasites are moving farther west.

Until researchers find a solution to the parasite problem, Bowdridge said farmers should keep using medications, but only in a very selective manner.

He said only sheep with a very bad infection should receive drugs. The animals become anemic during an infection, so farmers are instructed to wait until their lower eyelid goes from blood-red to very pink or pale.

“Those are the ones we want to treat very quickly. You’re only treating the animals that need it.”

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