Sheep Worms

Sheep worms live in the stomach and small intestine of sheep. They feed on the contents of the stomach and small intestine, but they are primarily bloodsuckers. The barber pole worm (Haemonchus contortus) and the hairworm (Trichostrongylus) are two of the most common types of sheep worms. Both the barber pole worm and the hairworm are fairly common in the state of Indiana, but they can be found in other areas as well. These types of sheep worms can cause anemia if they are allowed to multiply. Sheep worms can also decrease growth rate, increase susceptibility to other diseases, and lower the value of sheep wool due to fiber breaks and scouring.

Sheep worms have a complex life cycle. Mature worms living within sheep shed eggs within the sheep’s feces and onto the pasture. The eggs usually hatch within two to three weeks into a larval stage called “L3.” The L3 larval stage is then consumed by the sheep. The larvae can either develop into the mature larvae (L4) or go into a dormant stage called hypobiosis, within the sheep. The L3 larvae go into hypobiosis to survive adverse climactic conditions, such as winter. This means there are two sources that infect sheep – those that overwinter on pasture and those from animals.


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To successfully control sheep worms, proper timing of treatment is important. The key to controlling parasites is to prevent pasture infestation so strategic worming of the flock with the appropriate dewormers is critical to winning the war on sheep worms. The four critical treatment times are

  • Two to four weeks before lambing
  • Before turning ewes onto spring grass
  • Two to four weeks before breeding
  • When coming off pasture in the fall

Dewormers are quite effective for worming sheep. There are a number of dewormers that are approved for sheep. These include: Levamisole, Thiabendazole, Phenothiazine and Ivermectin. Prescription drugs used for sheep include Albendazole and Fenbendazole. Levamisole, Ivermectin, Albendazole, and Fenbendazole are effective against larval, adult, and larval stages in hypobiosis. This makes them the most effective agents to use. Albendazole should not be used immediately prior to or after breeding, as it can prevent uterine implantation of embryos.

There are many different management strategies to help control worms in sheep. Strategic deworming, using “safe” pastures for younger animals, pasture rotation, and utilizing more than one species of livestock to assist in control of parasites are some management strategies that can aid in controlling sheep worm and other parasites that threaten sheep.

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Author: The Top Worm

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