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Managing parasites, Selective treatment recommended for sheep deworming

by Wyoming Livestock Roundup

Blanket worming a flock of sheep or a herd of goats is no longer a treatment method recommended for controlling parasites, a veterinary epidemiologist with the Great Plains Veterinary Education Center at the University of Nebraska told ranchers. 

“Timed, blanketed deworming does not work anymore. Goats and sheep cannot be completely cured of parasites,” Brian Vander Ley said. 

Sheep and goat parasites are becoming more resistant to dewormers, and with only a few deworming products remaining on the market, he recommends selectively treating only affected animals. 

“Resistance is defined as the inability of the deworming product to reduce the fecal egg population by 95 percent,” he explained. 

Effective option

Vander Ley told producers during the Midstates Hair Sheep Producers Farm Tour that mass treating an entire flock of sheep or goats is a mistake. 

“Some areas have no effective wormers left in their arsenal. With sheep and goats making up such a small share of the pharmaceutical market, the likelihood is small any new dewormers will be developed in the future,” Vander Ley explained. “Even the likelihood new cattle dewormers will be developed is small.”

“What we have left is selection criteria,” he stated. 

The bottom line is, sheep and goats will always have parasites, Vander Ley said. The key is selecting for animals that can tolerate them. 

Breeding resistance

If animals are mass-treated and turned onto a new pasture, they will shed resistant worms and re-infect themselves with the resistant bugs. 

“If we mass treat all of our animals, we are reducing the number of dewormers that will work,” Vander Ley said. 

After a deworming product is used, Vander Ley added, 100 percent of the animals with resistant parasites will be left on the pasture to shed eggs from resistant worms. 

“While we still have a few products left that work, we need to use that time to develop sheep and goats that are tolerant to parasites,” he said. 


Vander Ley recommends two management strategies to prevent resistant parasites from developing.

  “Don’t deworm animals at the same time and don’t deworm animals and move them into a new pasture,” he stated. “What we want to do is breed parasites that maintain some level of susceptibility. We want them to produce eggs with susceptible genetics.”

He continued, “Try to dilute down the genetics to replace resistant parasites with susceptible parasites. We want as many animals in our flock that haven’t had dewormers as possible.”

In most cases, 20 percent of the animals in the group cause 80 percent of the problems. 

“That is why we need to select animals with some parasite tolerance,” Vander Ley mentioned.

“Animals will never totally be void of parasites. They will always have some,” he said. “Management-wise, I would keep a list and gradually take animals I treat the most off that list by culling them.”

Infected animals

“Only 20 percent of the animals in a given population are infected with parasites,” Vander Ley continued. “Producers should only deworm the goats or sheep that are infected.” 

Isolating infected animals is not something he recommends, because it creates another location on the farm where animals can shed eggs and create more problems. 

“We should leave those animals in the herd so they can shed eggs that will mate with resistant worms and hopefully help develop animals that can tolerate parasites,” he explained. 

“Don’t blanket deworm all of the animals. It only creates resistant worms, especially in a new area,” he continued. “It can even kill the kid goats, if they are in that area, because they are exposed to nothing but resistant worms, and they don’t have the immunity to fight them.”

Invest in a scale

Animals that need deworming should be dosed accurately. 

“It is really important to use the right dosage when treating these animals. That’s why I would recommend investing in a good scale,” Vander Ley explained. “If we under-dose animals, they will become re-infected with resistant parasites.” 

“I don’t recommend half-dosing them either. It is not a good way to save money,” he stated.

Multi-drug therapies

If producers decide to mass treat their animals, Vander Ley said not to use multi-drug therapy. 

Multi-drug therapy is mixing different classes of dewormers. 

“Multi-drug therapy will have a higher kill rate, but it can also make the animals produce worms that are resistant to the different classes of dewormers,” he said. “I recommend strategic worming, which is defined criteria I would use to deworm any animal.”

Vander Ley recommends deworming products be given to goats or sheep orally by mouth. 

“Never use a pour-on or an injectable with goats or sheep,” he explained. Using pour-ons or injectables spread out the duration of the dewormer, killing all of the worms except the resistant ones.”

“The duration is so long, it actually selects for resistant worms,” he said. 

The veterinarian recommends administering the dewormers in combination with one another and alternating classes for the most impact. He also recommends putting as much selection pressure on parasite tolerance as possible. 

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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