Dealing with Sheep Parasites

Livestock Update, May 2009

Dr. Scott P. Greiner, Extension Animal Scientist, VA Tech

As sheep producers we welcome the onset of spring with return of the flock to pasture and access to lush forages. Along with this, however, we recognize that parasite season is also upon us. The most significant health issue faced by sheep producers is internal parasites. Throughout the U.S., and especially in the mid Atlantic and southern states, the most important member of this family is the barber pole worm (Haemonchus contortus). The barber pole worm is a bloodsucking parasite found in the stomach. Infected sheep become anemic, leading to poor performance and frequently death. Bottle jaw is a classic symptom of H. contortus infection; along with loss of condition, weakness, and rough appearance (parasitic infection may or may not be accompanied by diarrhea.

Control of internal parasites has been complicated by the development of resistance to many of the dewormer drugs available. This resistance has been brought about by several factors, including improper use of dewormers. Our dependence on dewormers as the primary mechanism to control parasites has resulted in prolonged exposure of the parasites to the drugs, and over time the parasites have developed resistance. Overuse and improper use of dewormers also contributes to development of resistance, and over time parasites which are susceptible to the dewormers have been killed off leaving a population of parasites which are highly resistant. This extent of resistance will vary greatly from farm to farm. Given the prevalence of drug resistant worms, and lack of new products entering the market in the near future, parasite control programs must utilize strategies in combination with dewormers to control parasites is necessary. These strategies are important even for flocks which do not have a resistance problem, as they will slow the development of resistance and prolong effective use of dewormers. The following outlines several factors regarding an integrated approach to parasite control:

Pasture Management and Grazing Strategies
The life cycle of the worm involves the shedding of eggs in the feces of the sheep. Given the right environmental conditions (warm and humid), these eggs hatch and the larvae migrate up the blades of forage and are then ingested by grazing sheep. The majority of the larvae are found in the first 2 inches of forage. Consequently, grazing management strategies which minimize overgrazing and leaves a residual amount of forage (>2 in.) are conducive to reducing parasitism. Stocking rates are closely related to these factors, and flocks which have lower stocking densities tend to have lower parasite loads. This is a result of less grazing pressure, and the dilution effect of having fecal eggs and resulting larvae spread out over a larger land area.

The use of clean pastures has long been a strategy to control parasites. A clean pasture is one that is not contaminated with parasite larvae. This may be a pasture that has been cut for hay, grazed by another species (cattle or horses), or rested. Recent research indicates that the rest period needs to be at least 3 months, and 6-12 months in some cases. Most farms lack the acreage to rest pastures this amount of time.

Multi-species grazing of sheep with cattle or horses is an additional strategy that can be implemented. Since the parasites that affect these species are different, co-grazing helps to reduce the population of infective larvae available to the sheep since some of the population is consumed by cattle (and therefore do not propagate).

Proper Use of Dewormers
Dewormer products available for sheep fall into three drug classes:
Benzimidazoles- includes albendazole (Valbazen) and fenbendazole (Safeguard)
Macrolides- includes ivermectin (Ivomec) and moxydectin (Cydectin)
Nicotinics- includes levamisole (Tramisol, Prohibit)

Resistance has been documented in all of the above drug classes, and commonly reported for the white dewormers (albendazole, fenbendazole) and ivomectin. The only definitive way to determine if a flock has resistance is to conduct a fecal egg count reduction test, which will objectively determine the effectiveness of a particular dewormer in the flock. This test can be performed with the assistance of a veterinarian or extension agent.

Ideally, dewormers should be rotated on a yearly basis, using a product from different drug class each grazing season. This is becoming more challenging with the development of resistance, and reduced market availability of levamisole.

When administering dewormers, be sure to accurately dose the animal, which requires accurate assessment of weight. Always dose for the heaviest animal in the group. Deworming sheep on an empty stomach may assist in the effectiveness of the deworming. Hold sheep off feed or pasture 24 hours prior to treatment (do not withhold water).

Strategic and Selective Deworming
To effectively control parasites we have evolved our approach from deworming all animals at regular intervals, to a strategic approach in which we deworm less frequently and concentrate on high-risk animals. The most recent approach includes strategic deworming, which involves evaluating and treating individual animals based on their parasite load.

As compared to ewes, lambs are at much higher risk of parasitism as a result of less immunity. The previously mentioned grazing strategies are important particularly with grazing ewes nursing lambs. Stocking rate and forage management in conjunction with well-timed dewormings should be utilized for this production group. Lambs will exhibit the effects of parasitism well before ewes, so monitoring of grazing lambs should guide treatment protocols. Weaning and grazing lambs separate from mature sheep assists in parasite management for both groups of sheep. When separated, lambs should graze “cleaner” pastures with more forage availability.

It has been demonstrated that the majority of the parasite problems in flocks are the result of a small proportion of the sheep. Methods which identify these problem animals and eliminate them from the flock assist in controlling parasites and reducing resistance. Animals which are chronically wormy are good candidates to cull.

An important step in controlling the development of resistance is to reduce the number of deworming treatments. By reducing the number of treatments, the goal is to reduce the number of worms that are exposed to the drug and thereby become resistant. The FAMACHA system has been developed for this purpose, and utilizes color of the eye membranes to assess anemia (related to parasite load), and allows for treatment decisions to be made on an individual animal basis. To implement FAMACHA, producers need to attend an educational session to obtain training and the eye color chart used as the decision-making tool. Contact your local extension agent for details regarding training.
FAMACHA also provides a mechanism for identifying and selecting both parasite resistant and highly susceptible sheep. Since each animal is scored individually, keeping records over time will assist producers in identifying the genetics in their flock which are problematic and/or most adaptable to their parasite management program.

Summary
Managing parasites is essential to sheep enterprise profitability. A number of strategies are available which reduce the dependence on dewormers, and implementation of these strategies is necessary to address drug resistance which has become widespread in recent years. Each flock will be unique in the techniques which equate to a successful parasite management program. Forage and grazing management and prudent use of dewormers need to be matched to the production system and resources of an individual farm. Stocking rate, forage quantity and quality, grazing practices, and flock genetics are all contributing factors which will impact a planned parasite control management program.