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 Knowledge for the CommonWealth

Managing Internal Parasites

Livestock Update, May 2004

Scott Greiner, Extension Animal Scientist, Sheep, VA Tech

A significant health issue faced by sheep producers in the Mid-Atlantic region is internal parasites. As the level of parasite drug resistance increases, control programs based solely on anthelmentic drugs are becoming less effective. The prevalence of drug resistant worms is increasing, and new drug products to control worms are generally not available.

The most important worm parasites reside in the stomach and intestine. They are nematodes and belong to a family called trichostrongyles. 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 that causes anemia, leading to poor performance and frequently death. Bottle jaw is a result of H. contortus infection, but unlike other parasites H. contortus does not usually cause diarrhea.

In order to effectively control parasites, it is important to understand the life cycle of the parasite. Adult female worms produce eggs that are passed in manure. Larvae hatch, and go through several stages of development in the environment before they infect the next host. During the warm months of the year enormous numbers of larvae can build up on pastures. Virtually all worms need pasture for successful development; as they do not survive well on dirt lots or in the barn. The success of larvae outside the host depends on the climate. Moisture and warmth are necessary for development and survival. Barber pole worm does not survive cold winters well, but in eastern Virginia with its mild winters larvae will probably survive better over the winter. Dry weather is very hard on these larvae once they are out on the grass. Haemonchus larvae also undergo a process called arrested development where they sit quietly in the stomach following infection and don't develop into adults until several months later. This is an important adaptation for keeping the worm population viable through cold winters when eggs and larvae don't survive well on pasture. The worms that became arrested in the fall resume development in the spring and reproduce. Worm parasites are a part of the natural sheep world. Since we cannot eradicate them completely, the goal is to maintain the parasites at a level that will not produce any illness or economic loss.

Drug resistant Haemonchus are widespread throughout the world in sheep and goats and the problem is increasing in the U.S. Drug resistance in parasites is passed from one generation to the next. As the proportion of resistant worms increases generation after generation, a drug will become less and less effective. Continuous exposure of a population of worms to the same drug has also been demonstrated to hasten resistance. Drug resistance in not easily diagnosed in a flock, since drug ineffectiveness is also caused by such factors as under dosing, using expired drugs, inappropriate administration, and very high levels of parasitism (fast reinfection due to high worm loads on pastures). Often, resistance is not readily obvious until a drug is virtually ineffective and significant losses in production or even deaths occur.

The definitive method of determining resistance is to perform a Fecal Egg Count Reduction Test. This process involves collection of initial fecal samples from approximately 10 sheep. The sheep are then dewormed, and 7-10 days later a second fecal sample is collected from the same sheep. Some untreated sheep should also be sampled at the same times. The number of parasite eggs is counted in each set of fecal samples and the percentage reduction after treatment is determined. Fecal egg counts are currently not performed by the state labs. The parasitology lab at the Virginia-Maryland Regional College of Veterinary Medicine can assist producers with these tests (contact Dr. Anne Zajac, 540-231-7017,

Given the widespread development of resistance, step should be taken to minimize and delay the onset of parasite resistance. The following procedures are important in minimizing and slowing down the development of parasite resistance:

  1. Reduce the number of deworming treatments (see below).
  2. Use the correct dose according to label directions, and do not under dose. Divide sheep into weight categories, dose for the heaviest animal in each category.
  3. Use a product that is fully effective. All of the available modern dewormers fall into 3 major groups of drugs- Benzimidazoles (BZD), Macrolides (also called macrocyclic lactones) and a third group referred to as nicotinic agents. Recognize which products are in each group because once worms become resistant to one member of the group, they will be resistant to the other members of the group.
  4. Chemical NameFamily Approved for Use in Sheep?Trade Name
    Fenbendazole BZD No Safeguard
    Albendazole BZD Yes (not first 30 days pregnancy) Valbazen
    Levamisole Nicotinic Yes Tramisol
    Pyrantel Nicotinic No Strongid T
    Ivermectin Macrolide Yes (drench only) Ivomec
    Doramectin Macrolide No Dectomax
    Moxidectin Marcrolide No Cydectin
    Note: Some of the drugs listed here are not FDA approved for use in sheep and, as such, can only be used following consultation with your veterinarian with appropriate consideration of withdrawal times.
  5. When giving a product orally, make sure you administer it in the back of the mouth. If you deposit it in the front of the mouth it is more likely to stimulate the closure of the esophageal groove. This groove is important in lambs because it allows the milk to go directly from the esophagus to the stomach and bypass the rumen. With dewormers it is much better if they go into the rumen because they will be more slowly absorbed and stay in the body longer.
  6. When giving a benzimidazole or ivermectin orally it is better to hold the sheep off feed for 12 to 24 hours before treatment (do not hold off water, only feed). By doing so, the drugs will not pass so quickly through the GI tract, and active levels will be maintained in the body longer.
  7. Rotate dewormers on an annual basis. To reduce the selection for resistance it is best not to use any single drug group for too long.
  8. Avoid bringing resistance on your farm through new sheep that are purchased. Always quarantine new animals and immediately deworm them with at least 2 drug classes. Keep them separated, preferably away from any pasture, for a week until no further eggs would be passed in the manure from imported drug resistant parasites.

As suggested earlier, 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 reduce their selection for resistance. One method of reducing the number of dewormings is to monitor eye color. Infection with the blood-sucking results in anemia which can be diagnosed by pale mucous membranes around the eye. A South African researcher has produced an eye color chart, called the FAMACHA system, in which sheep are checked on a regular basis and the color of the mucous membranes is checked against a chart that then directs which sheep should be treated. This system is beginning to be used in the U.S. Secondly, reducing stocking density will likely reduce the number of dewormings since the parasites will effectively by spread over a larger area. Thirdly, research has demonstrated that animals on a high nutritional plane are more resistant to the adverse effects of parasites than those on marginal diets. Protein and minerals, as well as energy, are important in resisting the effects of barber pole worm because new red blood cells must be generated to replace those lost to the parasites. Using the sheep's natural immunity to parasites will also be beneficial. Levels of resistance vary with age and reproductive condition, with lambs the most susceptible and ewes in early lactation generally more susceptible than dry ewes. Concentrate worm control efforts on the sheep that need it the most (lambs). The pasture with the lowest number of parasite larvae should be used for ewes and lambs, not for rams or dry ewes. Similarly, pasture management will reduce parasite loads. Rested pastures and pastures that have been recently cut for hay generally have fewer parasites. The most susceptible animals should graze these pastures first. Grazing with cattle or horses also can be effective; as most parasites are species-specific will not infect other animal species. Lastly, consider genetic selection as a component of the parasite control program. In any population of sheep there will be some highly resistant sheep and some very susceptible sheep, which is likely related to the immune response of the animal. You should eliminate the highly susceptible ones from your flock (sheep that always develops bottle jaw before the others). Some sheep breeds, especially the Caribbean hair sheep breeds, appear to have a high level of resistance to gastrointestinal nematodes.

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