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Control of Internal Parasites in Sheep

Livestock Update, May 1998

Scott P. Greiner, Extension Animal Scientist, Virginia Tech

Internal parasites may cause serious economic losses for sheep producers if an effective parasite control program is not implemented. 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. 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.

  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.

  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. 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.

  5. 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.

  6. 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. Any sheep left untreated and then mixed with sheep on a rigid deworming schedule may result in reinfestation.

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