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Parasite resistance in sheep continues to grow, proves problematic

by Wyoming Livestock Roundup

“The big problem, and one we want to focus on, is the issue of anthelmintic resistance, which is a worldwide problem,” said Will Getz of Fort Valley State University. “It is not just in the southern U.S., either. Incidents have come up as far north as Canada.”

Parasite resistance to anthelmintic drugs provides a number of challenges to producers, who have noticed that resistance to multiple classes of drugs is occurring.

“Our options are rapidly decreasing as we move through this matter,” he continued. “We need to define resistance and what we are looking at.”

Getz and his colleague Tom Terrill discussed parasite resistance in sheep during a recent webinar sponsored by the American Sheep Industry Association.


Parasitic worms have been around since Americans started raising sheep, said Getz.

When dewormers are utilized, they target the different genetic components of the parasites, effectively killing them. 

However, Getz noted that not all worms were killed with each treatment. Those worms that remained alive were resistant to the drug and continued to propagate that resistance in their progeny. 

“Every time we deworm, we are deselecting the worms that have a high frequency of resistance genes,” he explained. “Every time we deworm, we kill those worms that are susceptible to our drugs, and the ones that remain that are responsible for the next generation are resistant.”

Before resistance can be detected, several generations pass until critical levels are reached.

“At the point that we reach a critical level, it is too late to reverse the situation,” Getz noted. “We have developed a resistant population, and resistance is forever.”

Research done by Ray Kaplan in 2005 found that 17 percent of farms in the southeast U.S. were experiencing total anthelmintic failure – a figure that is troubling for the industry. 

Changing perspectives

The sheep industry, said Terrill, needs to change its perspective on dewormers.

“We can no longer think of dewormers as being a cheap, simple input we can use to maximize productivity,” Terrill commented. “We need to realize that they are extremely valuable and should be treated as a limited resource.”

By taking a more medical approach to using anthelmintic drugs, using the treatments only when they are necessary, Terrill said producers should be able to slow resistance. 

“We need to take a long view and look at the reality of the situation to reduce our dependence on drugs that are routinely used,” he added.

Smart use

While the situation with parasite resistance is serious, Terrill noted that a smart approach to utilizing drugs can still enable them to be effective while minimizing creation of resistant parasites. 

“When we talk about smart drenching, we are talking about trying to come up with way to intelligently use the drugs we have available,” he continued.

In a smart drenching program, Terrill said producers must know what the resistance status of their flock is, which drugs the parasites are resistant to and which parasites the sheep have.

Resistance status

To determine what a flock is resistant to, Terrill noted that there are two methods ranchers can utilize.

“A fecal egg count reduction test can be done by any rancher or farmers with access to a microscope,” he explained. “A Drench-Rite analysis has to be done in a laboratory.”

Farmers should repeat this process every two years, said Terrill, who added that resistance can happen very quickly. 

“When a rancher knows what resistance he or she has in their flock, it has to be managed appropriately,” Terrill said.

Other tips

There are a number of other considerations that sheep producers should take into consideration to improve management of flocks.

“Generally, ranchers should avoid overstocking,” noted Terrill. “As a general rule, five to eight sheep is one animal unit.”

He added that fewer animals are better because it reduces exposure to parasites that may be present.

“The more animals in a field, the more feces they drop and the more larvae are present,” Terrill said. “It also means more animals will be exposed.”

By rotating pastures, Terrill also noted that exposure can be decreased.

“There is a whole lot of evidence that says rotating pastures helps from a parasite standpoint,” he explained. “It is important to keep animals away from risky areas where parasite populations are high.”

He also emphasized that producers should be careful in purchasing new sheep.

“Be careful not to buy resistant worms,” said Terrill. “All new sheep should be quarantined and aggressively dewormed.

When producers carefully manage their flocks and use dewormers appropriately, Terrill and Getz noted that anthelmintic resistance can be slowed.

“We have to be diligent with parasite management,” Getz commented. “We have the potential to be more effective and sustainable when we better understand the parasite better than we have in the past.”

Saige Albert is managing editor of the Wyoming Livestock Roundup and can be reached at

History of resistance

“When modern chemical dewormers became available, phenothiazine became the drug of choice,” said Will Getz of Fort Valley State University during a recent webinar. “We didn’t have sophisticated dewormers, but that was the first effective, cheap, safe dewormer.”

Phenothiazine was prominent for nearly 20 years.

“We became over-reliant on dewormers and forgot about management,” Getz commented. 

Because it was easier to deworm a whole herd, rather than focus on infected animals, Getz noted that drugs became ineffective over time.

“Whenever the old drug became less effective, a new one was on the market,” he said. “We don’t have any new drugs available now, so we have to make some wise choices and deal with resistance.”


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