Parasites, although an old nemesis, continue to be a real concern in the raising of sheep, regardless of the production system or production goals. The parasite battle takes resources (time, pharmaceuticals) and exacts casualties (usually decreased production and thriftiness, but sometimes sheep can die from heavy infection). One can never totally win the parasite war, just control them to the extent that they do not hinder production or animal wellbeing. Sheep will have worms, its the level of infestation that becomes the key.

To control internal parasites most effectively, a system of early prevention is more labor, economical and production efficient than treating major problems as they appear. For a control and prevention program, some knowledge of type of parasites, life cycle of parasite, sheep susceptibility, anthelmintics (dewormers) available, management options available, local and regional considerations, and whole farm prevention programs are necessary to win the battle.

Who Is the Enemy?

There are a number of internal parasites that cause problems with sheep. However, the two with the most practical significance to sheep are Haemonchus Contortus (barber pole worm) and Trichostrongylus. Both of these attach to the lining of the abomasum (fourth or true stomach) and suck blood from the animal.

Symptoms are weakness, poor growth, scouring, bottle jaw, pale membranes, poor reproduction, and in severe infestations, death can occur.

The life cycle of Haemonchus is a complex one and is dependent on the animal host, proper environmental conditions, pasture consumption by the sheep and is characterized by about four stages of development. In simplified terms, the stages are: adults, eggs from adults passing with fecal material, several larval stages which occur on the pasture, ingestion of larva by sheep, development of larva into adults and the cycle starts anew. The whole process, under optimum conditions, takes from seven to 1 0 days. However, the larvae can remain in a hypobiotic (almost like hibernation) state while in the sheep and "wait out" adverse conditions, such as winter time.

A word about tapeworms. In general, tapeworms cause no major problems with sheep. They do not suck blood, and thus, they do not weaken sheep like Haemunchus and Trichostrongylus do. The white segments that are commonly seen in lamb feces are tapeworm eggs. They do seem to bother shepherds. Granted, tapeworms can cause lambs to scour some and may be of concern if flystrike is a problem, but they do not cause a production or life threatening problem like Haemonchus or Trichostrongylus do. Also, adult sheep become naturally resistant to tapeworm infection, so treating the ewe flock for tapes is probably a waste of money and effort.

How to Win the Battle!

To win the parasite battle, one needs to interrupt the cycle of events throughout parasite development, and to interrupt the parasite, animal and pasture cycle. There are several weapons one needs to be made aware of that are available to use. Actually, it is not realistic to expect a parasite free flock, a good control and prevention program is practical.

The environment ones'flock is subjected to is critical in controlling worms. A program that will work in Indiana may be useless in Alabama and not needed in west Texas. Thus, take stock of temperature conditions, humidity, rainfall, lushness of forage, animal stocking density, etc. before planning a program. Usually, regions with warm, moist weather, lush forages and mild winters have the most potential for internal parasite problems.

Lambs are at most risk of adverse affects of parasites, followed by lactating ewes, ewes in late gestation, dry ewes and rams. Internal parasites can be a serious problem in lambs and lactating ewes. Thus, these are groups that should be targeted.

To control parasite infestation, one needs to prevent infection. This is accomplished by timely anthelmintic treatment of the flock and pasture and animal management procedures.

Dewormers that are approved for use in the United States for sheep are:

Levamisole (Tramisol, Levasole), lvermectin (Ivomec), and albendazole (Valbazen). Each of these can be effective if used properly and at the recommended dosage. Levamisole is available in either drench, bolus or injectable form. lvermectin is available in either drench or injectable form, although only the drench is approved for sheep. Albendazole is a white liquid available in drench form. All of these wormers have unique characteristics in regards to dosing rate, withdrawal dates prior to slaughter, what parasites they are effective against, possible side effects, and of course, cost. One needs to visit with their veterinarian and carefully read labels before using these products. There are other anthelmintics available to use in sheep, but they are not FDA approved. However, all three of these dewormers can be used to base a treatment program on.

Parasites can develop resistance to anthelmintics. Thus, they should be used judiciously. The current recommendations are to use one type of dewormer for one year and then switch to another type the next year and still another type the third year. The process would start anew in the fourth year. The main problem with this strategy is that not all of the (albendazole kills tapes) aforementioned dewormers control tapeworms. Those producers that do wish to control tapeworms will have to give an annual dose or two of Albendazole during tapeworm season.

Fecal exams by an experienced veterinarian can be of assistance in determining when to deworm or if parasite resistance to antheimintics is a problem. Take a sample of fecal material directly from a representative number of sheep in the group (about five to 1 0 percent), store in plastic bags in a cool place and deliver it to your vet in a reasonable amount of time.

Pasture management is critical in preventing parasite problems. One should keep a short record of when and how long pastures have been grazed and by what specie or class of animal. If one can identify "safe" pastures they can graze the most parasite susceptible animals in them. Safe pastures are those last grazed when weather was hot, those that have been grazed by cattle or horses before sheep turnout, those that have had a cutting of hay made before grazing or newly established pastures.

Lets walk through what I would recommend as a parasite control program for a Midwest or temperate climate condition for a producer lambing in the spring and going to pasture with ewes and lambs.

1 )    Worm ewes at or two weeks before lambing.

2) Seven to 14 days later, treat again.

3) Turnout should be to the safest pasture available.

4) Two to three weeks after turnout, run fecal exams on ewes and deworm lambs.

5) Seven to 14 days later, deworm lambs again.

6) Run fecal exams on ewes and lambs at regular intervals throughout the summer to assess treatment needs.

This program, to be most effective, needs to be combined with pasture rotation. The key element in a program is to prevent pasture contamination by eggs or larva. Thus, the early worming in short sequences is to help prevent pasture contamination. When safe pastures are used with early deworming, the battle is half won.

I cannot stress enough that each individual farm and each region of the country have a different situation and thus may need a different program. However, use of effective dewormers at critical periods to prevent pasture contamination is common to all environments. Use of pasture rotation, safe pastures, regular fecal exams and subsequent treatment as needed will help each producer develop their own parasite control program.

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