by Sally Colby

Cattlemen who expect a dewormer to work this year, next year and five years from now should develop a management program that includes judicious use of anthelmintics.

Dr. Bill Epperson, veterinarian and professor of pathobiology and population medicine at Mississippi State University, said it isn’t a matter of whether there are parasites present, but to what degree. The two major stomach worms of economic consequence in cattle are Ostertagia (brown stomach worm) and Haemonchus (barberpole worm).

Epperson said that for many years, producers expected to eliminate all worms with a single anthelmintic, but successfully managing internal parasites is more complicated. “In 1981, the introduction of ivermectin made it easy to control parasites because it controlled them all,” he said, “but that isn’t the case anymore.”

There’s no single deworming program that suits every farm. Considerations for a successful deworming program include animal age, production phase, season of the year, pasture and production goals and limitations.

“We don’t want to lose any of the anthelmintics we have,” said Epperson. “In sheep and goats, resistance is rampant. We’re not at that stage with cattle, but there is clear evidence that the effectiveness of dewormers has diminished.”

In pasture conditions, animals will graze where moisture is highest, and that’s also where worms will be highest. Epperson noted that during a drought, worms aren’t dead – they’re just waiting for moisture. When that happens, they reproduce quickly.

Regarding various administration methods, pour-on products result in the most variable and least consistent results. If not all animals get the correct dose, or there are some misses and product ends up in the chute or is licked off, the application is close to worthless.

“The first thing is using the right product at an appropriate time at a dose and frequency that gives us high efficacy,” said Epperson. “Think about anthelmintic programs as something that requires more planning than simply pulling a product from the shelf. Consider the animals, the environment they’re in and the fecal egg count.”

Rotational grazing maximizes beef per acre, and is a good management practice. But several factors must be weighed, including forage height, frequency of rotation and whether the pasture is being hayed between cattle. Age of cattle, grouping and stage of production can influence the parasite load. “Young, highly naïve animals coming onto a very contaminated pasture are going to transport nematodes like wild,” Epperson said.

Biosecurity is another consideration in managing parasites. “A great way to spread resistant genotypes is to purchase them,” said Epperson. “Most disease problems are brought in by the producer – how are you managing that? Animals coming into the herd should be isolated and dewormed with a program that works prior to comingling.” He added that producers should consider cattle groups – which are susceptible, which aren’t and how can they be managed on grass?

Veterinarian Dr. Harold Newcomb, Merck Animal Health, said parasites are estimated to cost the U.S. cattle industry $2 billion annually. “Parasites do three things,” he said. “They reduce feed intake, cause a drop in production and reduce the animal’s ability to respond to other diseases.”

Parasite lifecycles vary depending on the worm species, but the larvae infect and develop in the intestinal tract. “The lesions they produce will be a consequence of the interaction of larvae and/or adult with the host,” said Newcomb. “The lesions lead to a reduction in feed intake and reduction in digestive efficiency – and those are the production loss areas.”

Newcomb suggested working with a vet to obtain fecal egg counts, which provide a more accurate picture of the parasite burden. He also stressed the importance of understanding the three classes of deworming products: macrocyclic lactones (ivermectin), benzimidazoles (white wormers such as Safeguard) and imidazothiazoles (levamisole). All dewormers within a class act the same way, and the most effective deworming practice involves using a combination of classes.

Another issue with deworming is that producers dose animals based on average weight rather than actual weight. Newcomb said this means about half the animals will be underdosed, which leads to resistance. The length of drug activity is important – the longer-acting products lead to the selection process and products eventually become useless.

“Using two different classes concurrently can delay resistance to one or both compounds for up to 20 years,” said Newcomb. “It’s important we do that, but do it correctly. Each class of dewormer has its own strengths and weaknesses, and if we know which parasites we’re going after, we can pair the classes to fight the parasites we have.”

Administering dewormer should be based on cattle weight, but not all operations have scales. Newcomb said scales are a good investment long-term, not just for dosing anthelmintics but for obtaining weights at key periods throughout growth. The best way to learn to estimate weights fairly accurately is to track weights of sold cattle. Newcomb said producers tend to underestimate weights, and that not all cows weigh 1,000 pounds, which means many are underdosed.

Understanding the concept of refugia is important in managing parasites. Refugia are the parasites left untreated in the animal or on pasture. These parasites are those susceptible to anthelmintics. “The trick to keeping everything working is to keep a certain percentage of parasites susceptible,” said Newcomb, adding that the most critical group in managing refugia is cow-calf operations. “You can treat the cows one time, the calves the next time and the heifers the next time,” he said. “If we’re leaving one group untreated at all times, we’re maintaining some refugia.”

Pasture management plays a role in parasite management. Consider the height of the forage and make sure it isn’t being overgrazed. Parasite larvae can only travel upward three to four inches, so if grass is maintained at that height, the parasite load can be managed.

Newcomb advised cattlemen to figure out problems via diagnostics, then determine a plan. A parasite management built solely on chemicals is doomed to fail. Using a combination of diagnostics, selecting the appropriate anthelmintic products and managing pasture results in a sustainable anthelmintic program.