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Parasite control provides challenge

Sheep producers are encouraged to stick with a class of dewormers until they see resistance in their sheep, before switching to another class of dewormers, a sheep specialist with West Virginia University explained. Scott Bowdridge presented a webinar on looking towards the future of parasite management through “host-colored” glasses. 

During the webinar, Bowdridge said it is becoming more difficult for sheep producers to find an effective dewormer because the parasites build up immunity against it until it is no longer effective. 

Bowdridge recommended that producers purchase an effective dewormer, follow the label directions and continue to use it until it is no longer effective, before switching to another class of dewormers. 

“Overusing dewormers got us into this situation,” he said. “It is due in large part to using them for prophylactic treatment.” 

Bowdridge is currently conducting research in attempt to help sheep producers better control parasites in their flocks. While some research involves developing more effective methods of parasite control, Bowdridge is also looking at breeds of sheep that have more natural resistance to parasites.

Understanding parasites

During his presentation, the sheep specialist discussed three classes of parasites sheep producers need to worry about. 

The Trichostrongylidae family are parasites that infect the small intestine causing symptoms like diarrhea in infected animals. The Teldorsagia circumcinta parasites infect the abomasum, and Haemon-

chus contortus is a blood-sucking parasite that also lives in the abomasum. 

This third category of parasites is the one producers should most be concerned about, he said.

Discussing the life cycle of sheep worms, Bowdridge said the adult worms lay eggs, which pass onto pasture in the dung. The larvae need warm, moist weather to hatch, which is typically in the spring in this area. 

Once hatched, the larvae migrate in films of moisture from dung pellets onto pasture grass. Then, the infective larvae are eaten by the sheep, and the larvae develop into adults in the gut in about three weeks. 

Finding parasites

Bowdridge said these parasites can cause anemia in sheep and can eventually kill them if an effective dewormer isn’t given. 

“The sheep will appear thin, due to lack of blood. The body’s response is to fill these areas with any kind of fluid it can,” he explained. 

Producers can monitor their flocks for parasites through management. 

“Worms are very seasonal,” he explained. “They may be inside the sheep, but they know what’s going on in the outside environment the animal is in. They are a summer parasite in areas where it gets cold during the winter.”

Bowdridge said producers should consider using the Five Star Worm Plan, which includes:

Manage the level of pasture contamination,

Use anthelmintics (dewormers) appropriately,

Monitor and treat animals selectively,

Quarantine and treat new introductions,

Investigate treatment failure.

Bowdridge also recommends that sheep producers utilize FAMACHA, which is a diagnostic tool producers can use to help identify parasitic infections in sheep and goats. 

“The tool is a chart that matches eyelid color to anemia levels, an indicator of parasite infection. This type of diagnosis allows farmers to target treatment only to infected animals, which in some systems has reduced use of deworming agents by 90 percent. Not only do farmers save money, they significantly reduce the likelihood of causing parasites to become resistant to dewormers. FAMACHA was developed in South Africa and is distributed in the United States through the Southern Consortium for Small Ruminant Pest Control,” according to its website. 

Other control strategies involve rotational grazing, use of copper oxide wire particles (COWP), high-tannin cultivars and the use of parasite-resistant sheep. It can also help to keep flocks off pastures when they are shedding parasitic eggs.

Finding solutions

Bowdridge is currently doing research looking at parasite-resistant sheep. These sheep are ones with lower fecal egg counts, numerically lower FAMACHA scores and perform better through the grazing season, he said. 

“Lower is relative to the contemporary group average, not your neighbor,” he explained. “Parasite-resistant sheep are not devoid of parasites, they are just more resistant to the effects of parasitism,” he said. “In the Katahdin breed, heritability has been reported to be near 0.5, which indicates that parasite-resistance has a large genetic component that should respond well to selection. If used appropriately, crossbreeding with a parasite-resistant breed should improve parasite resistance.”

The problem lies in the breeds that show some resistance to parasites. Many producers don’t want to crossbreed to breeds like St. Croix, Barbados Blackbelly or Katahdins. 

“We need to develop the ability to identify resistant individuals without requiring that they become infected,” Bowdridge explained. “We need to develop a test that can be done anytime regardless of age, production system, parasite exposure, or breed.” 

“It is not that susceptible sheep are incapable of responding to parasite infection, rather that their response is blocked by either a parasite-derived mechanism or by lack of immunological awareness,” Bowdridge mentioned.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..