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Managing parasites, Approach can decrease resistance

by Wyoming Livestock Roundup

“Total anthelminthic failure was reported in 2005 by Dr. Ray Kaplan,” said Will Getz of Georgia’s Fort Valley State University in a recent webinar. “Some survey work indicated that 17 percent of farmers in the southeast U.S. in particular were experiencing total anthelmintic failure.”

Failure of drugs to address parasite populations in U.S. sheep flocks means that changes in management must occur, he added.

“We can no longer think of dewormers as a cheap and simple input we can use to maximize production,” Getz added. “We need to realize that these dewormers are extremely valuable and treat them as limited resources.”

Resistance problems

To address resistance problems he noted that dewormers must be used and treated as a medical solution for individually infected animals rather than as a whole-flock, broad spectrum solution to parasite problems.

“If resistance is inevitable, we need to consider the ways we can slow it down,” he continued, “and we need to consider the ways in which we can slow parasite resistance.”

Getz noted that slowing resistance may include deworming less frequently to maintain sensitive populations and developing integrated pest management strategies.

Smart drenching

Getz’s colleague Tom Terrill noted that smart drenching programs, or programs to intelligently utilize the handful of anthelminthics that remain effective, are available and important to use.

“We are not saying, ‘Don’t use drugs,’” Terrill said. “What we are saying is use them properly, use them wisely and treat them as a valuable resource.”

Smart drenching involves knowing what the resistance status of the flock is, identifying the drugs that parasite populations are resistant to and utilizing appropriate management strategies.

Resistance status

Determining the resistance status of the flock is very important, and there are two methods that can help a producer determine their flock’s status.

“One method is the fecal egg count (FEC) reduction test,” Terrill explained. “Each farm or ranch can do the FEC with access to a microscope.”

To perform this test, producers should take a fecal sample from at least 10 animals and determine the numbers of parasite eggs per gram of feces.

After 10 days samples should be taken from the same set of animals and reanalyzed.

“A 90 to 95 percent reduction should be seen,” Terrill noted. “If that number drops below 85 percent, that indicates resistance.”


There are a number of options to manage sheep to reduce the possibility of parasite resistance.

“Be careful when buying animals or in bringing new sheep into your operation. Try not to buy resistant worms,” Terrill cautioned. “Keep new animals isolated before allowing them onto the farm or ranch.”

Additionally, Terrill noted that appropriate stocking rates can help to alleviate parasite concerns. 

“Don’t overgraze,” he said, “and temporary or permanent fences can be built to provide safe pastures where livestock can be moved away from a high concentration of larvae.”

Grazing multiple species in the same pastures can also alleviate some parasite stress.

“Cattle can pick up sheep parasites, and they don’t hurt the cattle,” Terrill explained. “At the same time, sheep and goats don’t share the same parasites as cattle and horses.”


Utilizing pasture rotations can also help alleviate parasite concerns. 

“There is a whole lot of evidence that rotating pastures helps from a parasite standpoint, and it also helps with the animal’s nutrition, which helps them deal with the parasite,” Terrill said.

By avoiding contaminated pastures, he added that infection can be avoided. 

Dewormer strategies

Before using dewormers, Getz noted that some measures can be taken to increase vaccine efficacy. 

“One of the things producers can do is to restrict feed 24 hours prior to treatment because it decreases the amount of matter in the digestive tract, allowing the dewormer to have more contact with the digestive tract and increasing efficacy,” Getz explained. 

Ewes in late stages of pregnancy should not be feed restricted for 24 hours. Rather he encourages providing a second dose 12 hours later. 


Along with appropriately managing pastures, Getz said that good nutrition is a key factor in protecting against parasites. 

“Animals that are well fed and have a nutrient balance will be able to withstand the challenges of a parasite infection much better,” Getz said. “This is true regardless of the products being used.”

Animals with poor nutrition, low energy and low protein will have increased difficulty dealing with parasites. 


Getz further noted that rotating dewormers can also cause more problems than it solves.

“This idea was promoted for many years, but it is not a replacement for proper resistance prevention measures,” he said. “It will not make the problem of resistance go away.”

At the same time, rotating dewormers may hide a resistance problem that could be present.

“Rotation will tend to mask the development of resistance until it reaches a point where the producer has to use multiple drugs,” Getz added. 

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

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