Tips for Successful Internal Parasite Control

Livestock Update, June 2003

Scott Greiner, Extension Animal Scientist, Beef, VA Tech

Internal parasites are a potential 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 mortality in extreme cases. The two most significant parasite impacting sheep in Virginia is Haemonchus contortus. 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. Approved dewormers for use in sheep include Levamisole (Levasole and Tramisol), Ivermectin (Ivomec Sheep Drench), and Albenazole (Valbazen). Keep in mind that all other products are currently not labeled for sheep, and must be prescribed and administered under veterinary direction. Following are a few tips for a successful sheep deworming strategy:

1. 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. Frequency of deworming will be related to stocking rate, age of sheep, breed of sheep, weather, and pasture worm load. In many cases, deworming only a few days late can result in anemia and reduced performance, particularly in lambs.
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. Lower stocking rates will reduce the intensity of the deworming program. Fewer sheep result in fewer shed worm eggs within a given area, thereby reducing parasite loads. This in turn may reduce the frequency of deworming, and help minimize developed resistance.
4. 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.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. Deworm the entire flock (or group of sheep grazing the same pasture). 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.
6. When introducing new sheep to the flock, deworm with the most effective product available. New sheep should be isolated a minimum of 30 days prior to introduction. Mixing untreated sheep with sheep on the deworming program may destroy earlier efforts to minimize worm loads in the flock.

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