Parasitism is one of the most damaging diseases in sheep.

Round worms and coccidia are two of the worst parasites, according to Reid Redden, North Dakota State University Extension Service sheep specialist.

Almost all sheep have some level of parasitic infection. Various factors, including animal health, diet and level of infection, affect animals’ ability to resist the detrimental effects of parasites.

A good control program starts with understanding the parasites’ lifecycle, Redden says.

The round worm is the most damaging. The adult round worm attaches itself to the lining of the abomasum, or fourth and final stomach compartment in ruminants such as sheep. The worm feeds on the sheep’s blood and lays thousands of eggs. The eggs develop into larvae in the sheep’s feces, and rain transfers the larvae from the feces to plants such as grass. Sheep eat the larvae, which turn into adults in the animals.

Anemia (low red blood cells) is the most damaging result of round worm parasitism. It can cause paleness of the gums and lining of the eye, bottle jaw, weight loss, weak wool and even death. Round worms also can cause diarrhea in sheep, but sheep can be severely anemic without having diarrhea.

Larvae density on plants is greatest near the ground; consequently, overgrazing pastures increases infection rates. Larvae development can vary from five days to many months, depending on the weather. For example, infection rates spike in sheep during the spring and summer rainy season because the larvae thrive in warm, wet conditions.

Round worms have three mechanisms to survive in a sheep flock:

• Larva can arrest development (hibernate) in the sheep until the animal is more susceptible to infection.
• Larvae can survive on pasture for many months during very adverse weather conditions.
• Parasites develop drug resistance to wormers very quickly.

Examining feces for eggs is the best way to determine the types of parasites infecting the sheep and the severity of the infestation, Redden says. The FAMACHA anemia test also will help producers assess parasite problems. Postmortem exams will show whether parasites are in the digestive tract.

“Sheep have the ability to prevent larvae development, expel adult worms and inhibit egg production,” Redden says. “However, the ability to resist parasitic infection can vary greatly among sheep. Resistance depends on prior exposure to parasites and genetic background. Genetic selection for parasite resistance can be very effective to reduce parasitic problems.”

He recommends each sheep operation develop its own parasite control program because no single program works for all farms. He says most management plans should include an application of a dewormer two weeks prior to or just after lambing.

Redden also suggests using dry lots to hold sheep 24 to 48 hours after deworming so the animals can drop all the parasite eggs they may be carrying before being moved to new pasture. Returning sheep to the same pasture immediately after drenching them only provides two weeks of relief from parasitic infection. Also, moving sheep to new pastures immediately after drenching them provides some extended relief from parasites, but it eventually infects the new pasture with parasitic eggs.

Reducing parasite infections on pastures is critical to a successful management program. Here are some other ways to accomplish that:

• Maintain low stocking rates to prevent sheep from having to graze in areas with high fecal defecation and keep the animals from grazing grass down close to the ground.
• Divide pastures into smaller segments and rotate pastures.
• Hay pastures before or after grazing with sheep.
• Graze pastures with horses or cattle before or after sheep.

Anthelmintics (wormers) can be a very powerful tool to help fight parasitism, but they must not be the only prevention or treatment method because parasites will develop resistance to them, Redden warns. Producers should follow the directions on the product label and work with their veterinarian for any off-label use.

Coccidiosis is the second most damaging parasite to sheep. Coccidia are very common in sheep’s intestinal tract. The single-cell protozoa damages the lining of the small intestine and causes severe diarrhea that may contain blood and/or mucous. Infections typically affect young sheep that were stressed by shipping, handling and adverse weather conditions. Feed and water fecal contamination from sheep and most avian species increases the infection rate.

Several products (Decoquinate, Lasalocid, Rumensin and Sulfaquinoxaline) can be added to the animals’ feed or water during times of stress. These products are very effective in preventing coccidiosis, Redden says.

“Any treatment should be based on environmental conditions that promote parasitism,” he adds. “So work with your local veterinarian and Extension agent to develop the best parasite management protocol for your farm.”

NDSU Agriculture Communication - May 10, 2012