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Dewormer Debunking: Not all cattle dewormers are created equal

Using a drench dewormer can ensure that cattle are receiving the appropriate dose. Photo by Dr. Erica Koller.
Dewormer

Pour-on dewormers are a popular choice within the ranching industry as the ease of use is rather high, but does the effectiveness match the efficiency? Upon digging, it’s discovered that the answer is no. Pour-on dewormers often miss the mark when it comes to clearing the guts of parasites. 

While external parasites are a pest, naturally, internal worms are a far greater concern to producers for the effects they cause, such as poor feed efficiency, leading to poor breed-back rates and milk production in cows and minimal weight gain in calves. 

“Sometimes you don’t see the effects of that, the weight loss happening, because they could chalk it up to a bad year or drought year,” said Dr. Erica Koller with Cheyenne River Animal Hospital in Edgemont, South Dakota. 

There are four common methods of deworming cattle: the widely-used pour-on, an injectable format, an extended release bolus, and a drench. Three drug classes also exist: Imidazothiazoles, such as levamisole, Benzimidazoles, such as albendazole or fenbendazole, and Macrocyclic lactones or avermectins, such as ivermectin or moxidectin. By switching drugs for fear of resistance, but using a dewormer in the same drug class, true resistant parasites won’t be affected. 

“Any of your pour-ons that I know of on the market right now are all part of the same drug class,” Dr. Koller said. “So we’re worried about resistance because of the actual drugs being able to combat the parasite, then we’re using the same drugs over and over, even if we’re switching companies.” 

Often, misapplication is the greater concern with pour-on dewormer than actual resistance, whereas, other methods of application ensure that cattle are receiving the proper dosage. Upon applying pour-on, it is possible that the medicine isn’t seeping under the hair properly, the medicine isn’t applied along the entire backline, the temperature is too low for the medicine to be effective, or it could wash off in weather. Generic dewormers have been found to be as effective as saline, said Dr. Koller’s co-worker Dr. Stephanie Stephens, improper medicine application can cause a re-infestation. 

With all dewormers, the expensive portion of the drug isn’t the deworming agent itself, but rather the carrier that gets the medicine to the gut, Dr. Koller said. 

“The longer the product is on the shelf, even if it isn’t past the expiration date, the carrier availability diminishes,” she said. “Pour-ons that are past expiration dates, close to expiring, or have been exposed to the elements are going to be less effective than if you get it the day it was made.” 

There is no bad time to deworm, Dr. Koller said, but there are better times to deworm more effectively. She recommends deworming for internal parasites four to six weeks after putting cattle out on fresh grass in the spring, and treating external parasites in the fall, generally around Thanksgiving. 

Similarly Dr. Berit Bangoura, an assistant professor of Veterinary Parasitology with the University of Wyoming, recommends deworming cows before they calve, treating calves a couple weeks after birth, and deworming cows again after giving birth. 

“In this part of the country, people use pastures and don’t have cattle in all the time, so they can’t deworm monthly, or even every three months,” Dr. Koller said. “In confined areas, we would probably have to do that, but it would be more readily available.” 

Worms harbor in the ground over the winter, and in spring, larvae crawl to the top of grass blades where they can be picked up by cows and other livestock, where they remain all summer long, Dr. Koller said. 

She recommends deworming for internal parasites in the spring and fall for a few years, if cattle haven’t been dewormed regularly in some time, and switching what dewormer is used in the spring and fall, using an injectable one time then a “white dewormer,” or drench, the other time. 

Injectable dewormer must be kept warm, unlike vaccinations, while administering. 

One brand of drench dewormer, SafeGuard, can be given in high doses several days in a row to animals who have extensive worm issues, ensuring the cow gets what she needs and is still deemed safe. The downfall to drenching is that the animal must be caught by the head, but the upside of drenching is the appropriate amount of medicine is reaching the animal. 

Both drenching and injectable dewormers can be administered at the same time if an animal is exhibiting signs of a true resistance, Dr. Koller said. Some recommend keeping 10 percent of the worms in a cow’s gut to prevent resistance. 

“I am a little bit hesitant to promote doing that,” Dr. Koller said. “I don’t do enough fecal egg counts to prove we have a resistance problem around here. Usually when I go places, I see more of a misapplication than resistance. The theory behind saving 10 percent is you’re saving a small culture of the worms that are not resistant to reinfest for next year, so that instead of the big super bugs reinfesting, the non-resistant bugs are there.” 

There is no limitation on the amount of parasites that can be present, she said. No “no vacancy” sign, so to speak. Pasture rotation can help avoiding re-infestation and resistance as well. 

“Pasture rotation is always a very good thing if people have an opportunity to do so,” Dr. Bangoura said. “When animals are treated, I would move them to a new pasture one to two days later so they can excrete worm burden on the old pasture. It can provide the best effect to combine pasture rotation with deworming.” 


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