Strategies to Manage Internal Parasites in your Sheep Flock Back »

Written collaboratively by Dave Ollila and Larry Holler.


One of the most costly problems for sheep producers is internal parasites. It is important to develop a consistent and deliberate program to manage internal parasites in your flock. Your parasite management program includes your type of production system, use of available land resources and the effectiveness of anti-parasitic drugs.

In many sheep production systems, grazing tame pastures or native rangelands is a major component of providing feed for the flock. Expect the flock to be exposed to internal parasites through the grazing season. The degree of exposure will depend on the flock stocking rates and the amount of rest-recovery each paddock receives before being grazed again.

Internal parasite larvae are ingested along with blades of grass as sheep graze a paddock. When sheep graze areas where fresh feces is present they are continually being exposed to parasite larvae because the feces still contains the parasite eggs. Understanding the environmental conditions needed for the internal parasite cycle will help you to create a management plan that reduces internal parasites in your flock. Begin your management plan at lambing time to reduce parasite exposure of both ewes and lambs.

During winter, inhibited larvae of the nematode parasite reside inside the gestating ewe in a state of dormancy until climate conditions are favorable for them to emerge. These dormant larvae are much more difficult to kill with any treatment. Often, internal parasite emergence coincides with lambing time and is an opportunity to lessen internal parasites in a dry lot situation before turning the flock out to pastures. Part of your parasite management plan might be to monitor internal parasites during lambing by doing fecal sampling. The longer you can wait post lambing to treat, the more likely it is the inhibited larva have developed into adults, and therefore, be more susceptible to an anthelmintic treatment. In order to reduce re-exposure to internal parasites, especially those that have a direct lifecycle, producers should minimize feeding on the ground and keep feed bunks, troughs and water sources clean. Producers who lamb in pastures should rotate pastures or paddocks frequently to lessen internal exposure to parasite larvae at an early age. The lambs once infected will produce 10 fold more contaminating eggs than a mature ewe.

Resistant parasite populations, especially Haemonchus contortus (H. contortus) has become a management and treatment challenge. Sheep producers should select an anti-parasitic drug that performs with a minimum of 98% effectiveness. Unfortunately, parasite resistance to all the major classes of anti-parasitic drugs has increased over the years and producers need to determine the level of control the drug they are using provides. This can be accomplished by identifying (with ear tags, ear notches or scourable paint) at least 15 ewes within a flock as a representative sample. A fecal sample is taken from each ewe and labeled. The drug is then given and the flock held for 48 hours on the same ground or moved to temporary lots to allow any unaffected eggs in the gastro-intestinal tract to be shed. The flock is then moved out for grazing on fresh pasture. The sample group needs to be fecal sampled again 10-14 days after treatment to determine the percent effectiveness of the drug. If the drug is not found to be 98% effective the producer should consider a different family of anti-parasitic drugs to use for the next treatment. If a drug is effective it should be continued to be used until the percent effectiveness falls below the minimum of 98% effective.

Producers must understand that parasite resistance will continue to be a challenge and best management practices will need to be implemented to prevent further parasite associated losses.

These strategies include rotational grazing, strategic testing for parasiticide resistance and management of new purchases to ensure that you are not introducing resistant parasites to your flock. Immediate practices that should be conducted even while you are developing a parasite management program include the following:

  1. Develop a quarantine protocol when introducing new sheep to the flock. Knowing the parasite program used by the previous owner and treating the new sheep accordingly will reduce the introduction of resistant parasites.
  2. A grazing plan designed to make the best use of the available forage while minimizing exposure to parasite larva is essential. Parasite larva can be found on a leaf blade of grass within 6 days of the fecal material being deposited. Producers in areas with more moisture and frequent dew must move flocks regularly to break the infestation cycles. Arid regions of the country require larger pastures to support the same number of sheep as compared to wetter climates so producers can extend the number of days that an area is grazed. The larger grazing area reduces sheep’s exposure of defecated material and the drier environment lessens the chances for the larva to place itself on a leaf blade to be grazed.
  3. Alternating species of livestock on pastures yearly provides opportunity to break a host specific parasite’s life cycle. Allow cattle to graze a pasture one year followed by sheep utilizing the same pasture the next year. Monitoring internal parasite loads should continue as there are several internal parasites species that can be found in both sheep and cattle. . Fortunately, H. contortus does not effectively overwinter in the upper plain states.
  4. Locating parasite free land resources. Seek opportunities to place ewes and lambs on tame pastures or cropland that has not had sheep or cattle and should be free of internal parasites. Cooperating with farmers to plant cover crops into crop residue serves “soil health initiative” goals while providing a grazing opportunity. These scenarios can provide an opportunity to break parasite life cycles in the pastures you currently use as well as providing a nutritious forage for fast growing lambs or flushing of ewes.
  5. If parasite resistance is detected in your flock, the concept of “refugia” will need to be considered. Refugia is a technique by which a few animals within the flock are not routinely treated, and allowed to produce and maintain a population of worms that are still sensitive to common dewormers. These more sheep friendly parasites will compete with resistant populations, and hopefully keep the less friendly parasites at bay.

Contact your local veterinarian to assist you in conducting fecal egg counts, determining types of internal parasites present and determining which anti-parasitic drug to use for control.

Sample Submission

SD Animal Disease Research & Diagnostic Laboratory | Brookings, SD

Sheep producers who would like to participate in collecting data regarding internal parasites can send fecal samples to the South Dakota Animal Disease Research and Diagnostic Laboratory:

South Dakota State University
Attn: Dr. Larry Holler
119 Animal Disease Research (SAR), Box 2175
Brookings, SD 57007
Office: 605.688.5798 Email Website

 

Please contact Dr. Larry Holler at 605.688.5798 prior to sending samples.

More Information

For further details or assistance in setting up an internal parasite management plan, contact Dave Ollila, SDSU Extension Sheep Field Specialist | 605.394.1722

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