This fact sheet was developed by students enrolled in Purdue's ANSC 442 Sheep Management course in Spring 1998, as a semester project. These fact sheets provide useful information on various topics related to sheep. View the list of fact sheets.


Internal Parasites of Sheep


Internal parasites damage the health of sheep, causing significant production losses. In some areas of the United States, internal parasites are a major limiting factor to sheep production on pasture. These parasites can lead to death, reduced lamb production, and reduced wool yield.

The 3 most prevalent forms of internal parasites are the worms: Haemonchus contortus, Ostertagia circumcinta, and Trichostrongylus colubriformis. H. contortus is the parasite that most affects sheep. It is easily identified by its barber pole appearance.

bottlejaw.gif (62448 bytes)Some of the signs that indicate a parasite problem include sudden death, scours, weight loss, and weakness. Anemia and "bottle jaw", the accumulation of fluid under the lower jaw occur primarily with an infestation of H. contortus.

The infection of sheep usually begins with the ingestion of a 3rd stage larva (L3) in most species and normal development from an L3 to a mature stage (L4) usually takes 2 to 4 weeks. The L4 begins its reproductive cycle by laying eggs that are passed through the feces. This is where pasture contamination occurs and ensures the life cycle will continue. Once in a free environment, it may take 2 to 3 weeks for the worms to develop into their infective stage. The infected larvae are picked up off of grass and the cycle continues. Once inside the digestive tract, they can continue to produce eggs or they can enter an "arrested" stage known as hypobiosis. This guarantees their survival during harsh winter conditions. They can survive in hypobiosis for 6 months or longer.

Controlling Internal Parasites

The key to parasite control is to break the life cycle of the parasite. This is accomplished through good management practices and deworming at strategic intervals. Some of the more common deworming strategies are: Pre-lambing treatment, prophylactic treatment in the spring, and the treat-and -move strategy. Each of these strategies has different guidelines and goals.

Pre-lambing treatment calls for treating bred ewes 2 to 4 weeks before lambing. They should be wormed on a drylot so they do not infest their pastures. A wormer that is commonly used for this is levamisole, because it is effective against worms in hypobiosis and it will prevent their further maturity.

The prophylactic treatment in the spring refers to treating sheep every 2 to 3 weeks in early spring to reduce the summer buildup of pasture contamination. This treatment should be done 4 times on a dry lot or winter pasture to prevent contamination of spring and summer pastures.

The treat and move strategy is designed to extend the effectiveness of a single treatment, by treating the sheep in one area, letting them stay there over night to expel the worms and then moving them to a clean pasture. This will help limit reinfection and cut cost by using a single treatment.

There are several different types of wormers on the market for internal parasites in sheep. Some of the more commonly used wormers include Levamisole, Thiabendazole, Phenothiazine, Ivermectin, and Fenbendazole. The one or the combinations that you use will depend on what type of parasites you have and which will be most cost effective to use.

One problem that many producers' experience is that their sheep build a tolerance to the dewormer that they use. They will build a resistance if a dewormer is used repeatedly or if they do not receive the full-recommended dose. To help avoid this problem you should rotate you wormer on an annual basis and you should group your sheep according to size and give each sheep in the group the dose recommended for the largest sheep in the group. This will ensure that they get a full dose and it is better to overdose them than to underdose.

Currently there is research being done to develop a vaccine that will reduce the parasite load in sheep. However, it could be several years before this vaccine is put on the market. Until then it will be in the best interest of the producer to continue with good management practices and use strategic deworming methods.