Steps help rid sheep of roundworms

Roundworm is the most damaging parasite that affects sheep, says Reid Redden, North Dakota State University Extension sheep specialist.

The adult roundworm attaches itself to the lining of a sheep’s stomach. 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.

Key Points

The adult roundworm feeds on the sheep’s blood and lays thousands of eggs.

Deworm sheep and hold them in a drylot for 24 to 48 hours.

Moving sheep to new pastures after drenching them provides some relief.


Anemia is the most damaging result of roundworm parasitism. It can cause paleness of the gums and lining of the eye, bottle jaw, weight loss, weak wool and even death. Roundworms 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.

Roundworm survival

Roundworms 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 type and severity of parasites infecting sheep, 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 before or after lambing.

Redden also suggests using drylots 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 grazing areas with high fecal defecation and keep 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, or 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.

Source: NDSU Extension Communications

This article published in the June, 2012 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.