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Parasites should be controlled to avoid resistance, manage sheep health

by Wyoming Livestock Roundup

Internal parasite control for sheep is rapidly evolving, says Bill McBeth, a veterinarian with Zoetis Animal Health, but it is a topic that many sheep producers don’t have on the forefront of their mind.

“Parasites rapidly evolve, and they have high reproductive rates,” he continues. “They genetically want to preserve themselves.”

Resistance to anthelmintics

Routine anthelmintic use kills only those parasites that are susceptible, but the small number of resistant parasites survive, meaning that resistance can be rapidly built. 

“The real message here is that parasites are developing resistance faster than we are developing compounds to combat that resistance,” McBeth says.

Drug resistance was first seen in 1964, and parasite resistance to Ivermectin was detected in the sheep and goat industry in 1981.

“Ivermectin was used by the sheep and goat industry off-label, but by the time it was approved, we already had resistance from parasites,” he adds. “No chemical class has been introduced for use in the U.S. since.”

Anthelmintic resistance is an issue worldwide. A new compound introduced in New Zealand in 2009 saw parasites that were resistant to it within four years.

“This just illustrates how rapidly these bugs move,” McBeth says.

Parasite populations

Parasites are passed on in the sheep herd as sheep graze.

McBeth explains that infected sheep pass the eggs of parasites in their feces when they defecate.

“The eggs turn into larvae on pastures, and they are consumed by the animal,” he says. “There are two populations of worms we have to think about managing – the larval load on the pasture and the adult load in sheep.”

While important to control populations, McBeth also notes that maintaining refugia is also important.

“We have to provide a refugia for the worms and larvae that are susceptible to our anthelmintics, so we can keep their gene pool alive to overcome or dilute the effect of resistant parasites,” he says.

Reducing parasites

There are several important considerations for avoiding the risk of anthelmintic resistant parasites in sheep flocks.

“First, don’t buy resistant parasites,” McBeth says. “Quarantine purchased or returning animals and don’t commingle sheep and goats. That is a recipe for resistance.”

For producers who are able, McBeth also suggests doing egg count reduction tests as a part of the strategy to monitor resistance.

“Worm new animals and new bucks with a couple of classes of antibiotics and do fecal egg count tests,” he adds. “Bascially, we want to clean them out. Then, we should put them on our pastures contaminated with the worms we have for a minimum of 48 hours.”

McBeth continues, “The idea is that we want the worms in our sheep and the larvae on our pastures to have a high percent of worms susceptible to our anthelmintic.”


When treating for parasite populations, McBeth also suggests using targeted and selective treatments.

“About 20 percent of the sheep in any flock generate 80 percent of the eggs,” he says. “Certain individuals are genetically more susceptible to parasites, so targeted, selective treatment is important. Treat only the affected animals.”

Affected animals can be determined using a resource called FAMANCHA. The card provides a color-coded comparison chart to allow producers to determine if sheep are affected by Haemonchus, a common blood-sucking parasite.

“Anemia is one of the big symptoms of parasite infections,” McBeth explains. “There are also several other clinical scoring systems, but FAMANCHA is used quite often and is less labor intensive.”


In conjunction with treatment, McBeth also notes that pasture management and management of the larval load is important for producers.

“We have to look at stocking density, managing hot spots around water, the concept of clean pastures and using different species – like horses or cattle – on our pastures,” he says, emphasizing that goats should not be run on the same pastures as sheep.

He also explains that sheep that perform poorly may provide a reservoir to parasites in a flock, and those animals should be culled.

“Susceptibility to parasites is partially genetic,” he says. “Eliminate the bottom 20 percent of the flock.”

Combining parasite therapies with management can reduce parasite populations and reduce the occurrence of anthelmintic resistance in sheep.

McBeth spoke during the 2015 West Central States Wool Growers Convention, held in Park City, Utah in mid-November.

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

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