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Sheep Update: Control of Internal Parasites

Livestock Update, May 2000

Scott Greiner, Extension Animal Scientist, Sheep, Virginia Tech

Internal parasites are perhaps the most significant source of economic losses for sheep producers during the spring and summer months. Parasitic infestation can result in decreased production of ewes and lambs on pasture through reduced milk production and poor weight gains, and even death may occur in extreme cases. The two most significant parasites impacting sheep in Virginia are the barber pole worm and the brown stomach worm. These worms thrive under warm and moist conditions of late spring and summer, which emphasizes the importance of an effective parasite control program as sheep go to pasture. Currently there are two approved products for use in sheep- Levamisole (Levasole and Tramisol) and Ivermectin (Ivomec Sheep Drench). Keep in mind that all other products are currently not labeled for sheep. Following are a few things to keep in mind for this year's deworming program:

  1. Pasture-lambing ewes should be dewormed two weeks prior to lambing. Egg numbers increase significantly in ewes just before and after lambing. Due to the life cycle of the worms, this means that larvae increases around the same time lambs start grazing. If deworming is not done prior to lambing, treatment should be done at lambing and the ewes and lambs moved to clean pasture if available and a stringent deworming program implemented thereafter.

  2. Use pasture management to enhance the effectiveness of a deworming program. The practice of "dose and move" can reduce the dependence on anthelmentic drugs to prevent and treat parasites by reducing the number of parasites sheep are exposed to. Using the dose and move technique, sheep are moved to a clean pasture after treatment. A clean pasture may be one that has been harvested for hay, previously grazed by cattle, or been without sheep for a year. A clean pasture does not ensure that infective larvae are not present, but has infectivity low enough that susceptible sheep do not become infected rapidly. A strategic deworming protocol must still be followed after moving the sheep.

  3. Deworm the flock on a regular basis. Be sure to record the date of treatment so a schedule can be followed. This is especially important when the dose and move system is not applicable due to limited pasture availability. Normally sheep should be treated every three to four weeks. Keep in mind that worms may develop resistance to a drug if exposed frequently.

  4. Lower stocking rates will reduce the intensity of the deworming program. Fewer sheep result in fewer shed worm eggs within a given area, and thereby reducing parasite loads. This in turn may reduce the frequency of deworming, and help minimize developed resistance.

  5. Rotate dewormers annually. This means that if you used Ivermectin last year, switch to Levamisole this year. Rotating anthelmentics on an every other year basis will help prevent parasites from developing resistance to the product.

  6. Administer the proper dose. Be sure to estimate the weight of the sheep accurately. Dose the sheep for the heaviest in the group, not the average. Dosages given that are inadequate for the body weight of the sheep are not only less effective on decreasing worm loads, but may also enhance parasite resistance to the drugs.

  7. Deworm the entire flock. For a parasite control program to be effective, it is important to include all of the sheep. Lambs should be treated beginning at around six weeks of age. Mature ewes are more tolerant to high worm loads than are lambs. Not grazing lambs will significantly reduce the intensity of the deworming program for the ewe flock.

  8. When introducing new sheep to the flock, deworm with the most effective product available. Mixing untreated sheep with sheep on the deworming program may destroy earlier efforts to minimize worm levels in the flock.

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