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Sheep vs. Parasite: How immune responses in sheep breeds can improve the species

Scott Bowdridge stands among sheep at the WVU Animal Science Farm

As dewormer medication use in sheep becomes less and less effective, new research at West Virginia University reveals livestock’s inherent ways of fighting off parasites.

Scott Bowdridge, associate professor of food animal production in the Davis College of Agriculture, Natural Resources and Design, has researched parasite susceptibility in multiple breeds of sheep for a decade.

“Why is that important?” he asked. “Because none of the drugs work anymore.”

To protect sheep from parasites, drugs were introduced to the sheep industry in the 1950s and were initially effective. Through treatment failures and subsequent dosage increases, the drugs inadvertently selected for drug-resistant parasites, killing the weak and leaving only the strongest parasites to reproduce.

A prolific cause of death in America’s sheep is Haemonchus contortus, or barber pole worm. The parasite pierces the sheep’s stomach to feed on its blood. Within days, anemia sets in and in severe cases, death. Bowdridge’s recent research with Texel, Suffolk and St. Croix sheep shows parasite egg release tends to be affected by breed.

In Texel sheep, he found parasites would invade but the sheep’s cells determine the size and number of eggs laid. The breed even prevented worms from releasing eggs entirely by binding around the head and reproductive structures. In contrast, Suffolk sheep, a highly used breed in the U.S. for its fast growth and large muscle mass, had a higher egg release, showing their increased parasite susceptibility.

He also studied the St. Croix breed, which is known in the livestock community to be resistant to parasite larvae specifically. Unfortunately, the breed is not commercially valuable in the United States.

Their immune response to parasites, however, might be.

Their successful mechanism of defense is to allow adult parasites to reproduce for a longer period and then eliminate the larvae.

"The climate definitely contributed to each breed’s type of resistance,” Bowdridge explained. “Where St. Croix sheep developed, they would have been exposed to larvae year-round. Whereas Texel sheep had limited exposure to larval stages, the adult stage would have been presented to their immune system for a greater period.” 


From a producers’ standpoint, the research is invaluable. To a producer, Texel sheep’s ability to lower egg count is the preferable resistance. However, from an academic standpoint, the mechanism of the St. Croix is more desirable because it prevents the accumulation of adult parasites.

“Sheep have a low will-to-live and will succumb to death if stressors become too great,” he said. “Because we know sheep are so prone to death, reducing another cause (like parasitism) through the mechanism the St. Croix demonstrates would be great.”

Although Bowdridge’s research has focused on sheep’s immune response to parasites, a second documented discovery might expand it beyond parasites. A supplementary video taken during the project shows matricidal hatching in vitro occurring in Texel and St. Croix sheep, a previously unrecorded phenomenon.

“We discovered a sheep’s cells can actually block and prevent parasite eggs from being laid. Those eggs develop into larvae inside the female parasite, causing her death,” he said. “We were never able to prove that it occurs in vivo until now. No one has ever seen that with this species of parasite.”

Bowdridge’s assumption, one that he’ll test with further research, is the immune system link between parasite resistance and resistance to a host of illnesses.

“I want to see how and why lambs bred for low parasite susceptibility are also less prone to things like pneumonia and foot rot,” he said. “If you think about the big push nationally to reduce antibiotics use in livestock production, everyone is looking for what they can select to reduce sickness. That's what we’re working on now.”

Like previous published papers, his work was again featured on the cover of Parasite Immunology.

Citation: Immune mechanisms of Texel sheep to adult and egg stages of Haemonchus contortus





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Davis College of Agriculture, Natural Resources and Design