Controlling Internal Parasites in Sheep
This article first appeared in Purdue Sheep Day Proc. in 1992.

by Mike Neary
Extension Sheep Specialist
Purdue University

Controlling internal parasites in sheep can be a costly and significant management practice for sheep producers. Heavy parasite infestation can decrease growth rate, increase susceptibility to other diseases, lower the value of wool due to scouring and fiber breaks, cause anemia and lower overall productivity. To effectively control internal parasites at a reasonable cost, one needs to understand the interaction between the worms and sheep, have knowledge of the parasite life cycle and use dewormers and other management practices wisely.

Parasitic Life Cycle

The two groups of worms of most importance in Indiana are Haemonchus contortus (Barber pole worm) and Trichostrongylus (Hairworm) family of worms. Both types reside in the stomach and small intestine and are blood suckers.

The life cycle of parasites are complex and of an adaptive nature. Mature worms that reside within sheep shed eggs within the sheep's feces onto the pasture. With favorable environmental conditions, the eggs hatch within two to three weeks into a larval stage (L3 larvae). The L3 larval stage is then consumed by the sheep and can either develop into the mature larvae (L4 larvae) or go into a dormant stage (hypobiosis) within the animal. The L3 larvae exhibit hypobiosis to survive adverse environmental conditions, such as winter.

Thus, there are two sources to infect sheep with parasites; those that overwinter on pasture and those from animals. Estimations of infected larvae overwintering are in the 5% range of total pasture larvae. Therefore, most pasture contamination of parasites originate from the ewe flock.

Prevention Program

Proper timing of treatment is of utmost importance in successfully controlling internal parasites. Based on the previous discussion of parasitic life cycles and routes of infection, the key to controlling parasites is to prevent pasture infestation. Thus, strategic worming of the flock with the appropriate dewormers is crucial to winning the worm war.

What are the critical treatment times? The four key times are: 1) two to four weeks before lambing, 2) before turning ewes onto spring grass, 3) two to four weeks before breeding and 4) when coming off pasture in the fall. Depending on the type of production system, some of these times may overlap or there may be other modifications necessary for an effective program.

Certainly, treating ewes two to four weeks before lambing is one of the absolute necessary treatment times. This is because of a periparturient rise in fecal egg counts. The increase in fecal egg counts indicates that the larvae that overwintered in the ewe in the dormant stage are developing into mature larvae and shedding eggs. Hypobiotic larvae within the ewe are stimulated by her hormonal changes prior to lambing. Ewes can be easily wormed at the same time they are given enterotoxemia vaccinations. Depending on when ewes are pulled from fall pasture, the worming before lambing may closely coincide with this time period.

Treatment before spring grazing is crucial to prevention of pasture contamination. If parasites are historically a problem on certain farms, it may be effective to re-treat the flock 10 to 20 days after the initial pre-grazing treatment.

Treating ewes and rams prior to the breeding season allows the flushing diet to have maximum effect.

Types of Dewormers

Dewormers approved for use in sheep include Levamisole, Thiabendazole, Phenothiazine and Ivermectin. Drugs used for sheep by veterinary prescription include Albendazole and Fenbendazole. Levamisole, Ivermectin, Albendazole and Fenbendazole are effective against larval, adult and larval stages in hypobiosis. Thus, they are generally the most effective agents to use. Albendazole should not be used immediately prior or after breeding, as it can prevent uterine implantation of embryos.

Resistance to deworming agents by the worms can certainly be a problem. If resistance is a problem, a veterinarian can be of valuable assistance in identifying the problem and recommending a solution. Monitoring fecal egg counts after treatment is often required when drug resistance is suspected.

Management Strategies

Strategic deworming, pasture rotation, using "safe" pastures for younger animals, utilizing more than one specie of livestock to assist in control of parasites are some management strategies that can aid in controlling parasites.

When initiating spring grazing, try not to go back to the same pasture that was last utilized in the fall. Ideally, utilize the pasture that was grazed during the hottest, driest part of the summer. Larvae do not withstand hot, dry conditions well, thus, lower numbers of larvae would be expected to have survived on these pastures. Granted this is not always possible due to forage type available and number of pastures.

After treating sheep for parasites, rotate to pastures that have been rested and should have lower numbers of infective larvae. Pastures that have been harvested for hay would be less likely to contain high numbers of larvae.

Pastures that have had cattle or horses grazing them prior to grazing sheep, would be considered safe. There is minimal animal cross-specie infection by parasites, thus, one animal specie serves as a dead-end host for the parasites of a second type of livestock.

Lambs are the most susceptible class of sheep to internal parasite infections. Because of their age, stage of growth and lack of a developed resistance, they can develop high levels of infection. Therefore, it is imperative they have access to the cleanest pasture possible and are monitored closely for infection.


Prevention of a problem with internal parasites in sheep through strategic treatments, proper drug use, working with a knowledgeable veterinarian and employing sound pasture management is the key to control. Waiting until anemia, scours, lack of thrift and growth are present before treatment is ineffective in solving the real problem. Preventing pasture contamination is the most cost effective method of parasite control.