scattered clumps of sheep grazing in a green pasture at the base of mountains

Managing the Barber’s Pole Worm in the Intermountain West

At this time of year I typically receive many inquiries regarding internal parasite issues in lambs and ewes, especially those managed on irrigated pastures. If grazing irrigated pastures, it’s not if, but when, you will need to implement a management plan for internal parasites (especially the barber’s pole worm). The following are some helpful reminders as you put together a plan that works for you.

scattered clumps of sheep grazing in a green pasture at the base of mountains

  • In the summer months, when minimum daily temperatures stay above 50°F, the barber’s pole worm (H. contortus) is the most aggressive internal parasite species in sheep. This parasite is transmitted orally from contaminated pasture through a complex 3-week life cycle. Recent work published in our region found that across 25 sheep flocks grazing irrigated pastures, this was the most prevalent worm species (70% of worm burden) during the June to August sampling period.
  • A clinical burden of adult barber’s pole worms inside the abomasum of a sheep can result in the loss of up to 1 cup of blood per day. The damage it causes to this critical compartment of the ruminant digestive system is a significant setback for affected animals. In addition to the typical fluid swelling beneath the jaw (bottle jaw), decreased weight gain and milk production are all production losses from this parasite.
  • Best management practices now recommend a selective treatment program that focuses treatment of the sheep showing clinical signs, rather than mass treatment of the entire flock. A good rule of thumb is 20% of the sheep are 80% of the problem with this parasite. This principle of selective treatment, or “refugia,” has been proven to maintain a population of internal parasites that have never had exposure to the dewormer, which ultimately keeps it working longer in the flock. Using the FAMACHA system allows you to estimate clinical illness by looking at degree of anemia in the lower eyelid, yet a more simplistic approach entails only treating those “poor doing” animals in the flock. Rotating de-wormers is not recommended until you’re sure a product is no longer working. With a limited arsenal of deworming products available on the market indiscriminate rotation accelerates worm resistance.
  • Barber’s pole worm resistance to common classes of dewormers has been well documented internationally and especially in the eastern region of the U.S. for 20 years. We recently published data that found widespread barber’s pole worm resistance in Wyoming and Montana sheep flocks grazing irrigated pastures. The barber’s pole worm was resistant to benzimidazoles on 92% of ranches, Ivermectins on 50% of ranches, and moxidectins on only 8% of ranches. Is the dewormer you’re using still effective?
  • Combination deworming is providing a full dose of two different classes of dewormer at the same time (do not mix) and has been shown to be effective at killing resistant worms.
  • Lambs and ewes will re-ingest larvae when grazing unless they’re grazing a “clean” pasture that hasn’t been grazed in 30 to 40 days. Some producers have experienced success moving sheep to a fresh paddock every 3 days, and not returning to a previously grazed paddock for 30 days as an approach to avoiding reinfection. Drying out of pastures can kill unhatched larvae in the fecal pellet; shaded areas allow larvae to survive longer.
  • Fecal egg counts are one method to determine the quantity and type of internal parasite burden you’re dealing with. These can be pulled individually from the animal or freshly picked up off the ground but can provide an important baseline so you can implement a treatment strategy.
  • Barber’s pole worm larvae need water to survive and move up the leaf to be ingested. If you don’t graze irrigated acres in the warmest summer months, then you likely don’t have a barber’s pole worm issue. Fecal egg counts were highest from sheep grazing flood- and sprinkler-irrigated pastures: these pastures had 2,656 eggs per gram compared to 433 eggs per gram from those flocks grazing sub-irrigated pastures. The range flocks sampled had little to no barber’s pole worm eggs in the feces.
  • You can purchase the problem of resistant worms residing in the digestive tract of replacement rams and ewes. If purchasing breeding stock from a flock with known parasite issues, an effective quarantine period and fecal egg count from your veterinarian or veterinary diagnostic lab can help you determine if an aggressive worming regime is needed before turning out into the flock.

The field of internal parasite management in sheep is ever evolving in response to these ever-changing worm species.  A good resource for late-breaking information is

To read more regarding the recent study conducted in our region, visit

If interested in learning more about the amount and type of internal parasites in your sheep, email regarding eligibility for free or reduced parasite analysis as part of multi-year study.

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