The sheep industry is using everything in the tool box to defeat Haemonchus contortus.
Better known as the barberpole worm, H. contortus is the scourge of sheep producers. Adult worms attach to the lining of the abomasum or true stomach and feed on the blood, causing anemia and potentially death. Females can lay over 10,000 eggs a day; those are passed out in the animal’s feces, and the larvae climb up on grass blades to be re-consumed, continuing the cycle.
The worm has gained varying degrees of resistance to anthelmintics. Dr. Charlotte Clifford-Rathert, state small ruminant specialist for Lincoln University Cooperative Extension in Jefferson City, Mo., said that’s from using dewormers too frequently, rotating them too frequently and underdosing.
“That results in the increased selection of ‘superworms’ that are resistant to all drugs,” she told Ozarks Farm & Neighbor. “It’s genetically controlled and once established, it is set in the population.”
The worm has developed higher resistance to some chemistries (Ivermectin, Albendazol, Fenbendezol) than to others (Levamisole, Moxidectin, Amprolium). Clifford-Rathert said it’s best to use common sense when managing for the parasite. Since dry grass forces the larvae to stay at the base of the plants, delay grazing until dew has lifted, and don’t distribute feed on the ground.
The FAMACHA system is used to determine how severely an animal is infected; the redness or whiteness of the conjunctiva of the eye is measured against a chart to determine the extent of the anemia. It’s now been extended to a five-point system that also assesses the back for the body condition score, the tail for the soiling of breech or Dag score, the hair coat for roughness, and the jaw for the accumulation of fluid or “bottle jaw.”
But Dr. David Fernandez, University of Arkansas Extension livestock specialist at Pine Bluff, said producers are going to have to stop relying upon ineffective dewormers.
“This really bothers a lot of producers,” he told OFN. “Producers would like to have a clean animal, and they don’t want to have any parasites in the animal. But the animals live outside, so they’re going to be ingesting parasites on a regular basis. The question is really how many can they carry, and still not be adversely affected? We’re going to set a threshold where, above a certain limit, we’re going to have to treat, but below that we’re just going to tolerate that worm.”
He called for an integrated pest management approach that includes keeping grass more than 3 inches tall, because the larvae won’t climb that high; grazing cows with the sheep, because cows are a terminal host for the barberpole worm and won’t excrete the eggs; and genetic selection. “If you find that you have to treat that animal multiple times, that’s an animal that should be culled,” Fernandez said. “About 20 percent of the animals in your flock or herd are producing 80 percent of the eggs that are being put out on the pasture, so eliminating that 20 percent is almost as good as deworming your entire flock or herd.”
Control can also get an assist from high tannin feeds like sericea lespedeza or chicory, in combination with a slow-release copper oxide wire particle bolus (COWP) a couple of times a year. If all of that fails, Fernandez said, that’s when you would want to move to the chemical dewormer. “There’s still a place for using them,” he said, “but what we’re trying to do is reduce their use to the point where we lower the resistance of the worms to that chemical dewormer, so it will stay effective for as long as it possibly can.”
Treated sheep should be released into the pasture with the rest of the flock. The idea is that the only worms left in that animal will be the ones resistant to that particular anthelmintic; their hatchling larvae will mate with the other worms in the field, some of which will still be susceptible to that chemistry, so the offspring will not be uniformly resistant. It’s just one more tool, and producers need them all because, Fernandez said, “if we can’t really get a good handle on them, it’s going to put the smaller producers in the country out of business.”
