SALEM, Mo. – While the use of commercial dewormers has helped improve animal health and productivity in beef herds, their use has also resulted in resistance among parasite populations, according to livestock experts.

“As dewormers lose effectiveness, producers need to develop a broader strategy for managing internal parasites in their livestock,” said Eric Meusch, University of Missouri Extension livestock specialist.

A recent study funded by Merck Animal Health and led by Eric Bailey, a University of Missouri associate professor of animal science, conducted fecal egg count reduction tests in beef herds across the state, finding resistance to macrocyclic lactone (widely used in veterinary medicine as antiparasitic agents, particularly for treating ticks and mites) dewormers.

Moreover, the benzimidazole (anti-worm drugs in both veterinary and human medicine class of dewormer) still showed efficacy above the threshold for effective deworming, which is 90 percent.

Meusch said the dewormers losing efficacy are products commonly used in pour-on and injectable forms, and have become popular due to cost and ease of administration: “The popularity and frequency of use of these products has likely led to resistance.”

Jeff Lehmkuhler, University of Kentucky associate professor of animal and food science, said spring is always the time when both internal and external parasites become more prevalent.

“Many beef operations will apply some level of management to control parasites that can rob nutrients from the cattle,” he said. “This begs the question, ‘Are you controlling what you think you are controlling?’

“During the spring and fall of 2023, University of Kentucky (UK) Cooperative Extension agriculture and natural resource county agriculture and natural resources agents, Kentucky Beef Network facilitators, as well as Dr. (Michelle) Arnold (UK large ruminant extension veterinarian) and I, set out to assess the prevalence of internal parasites in Kentucky beef herds,” he added.

He said, “Additionally, many of the deworming products have been on the market for decades (e.g.,1960s for levamisole and 1980s for ivermectin), so we wanted to evaluate the efficacy of products being utilized by Kentucky beef herds.”

He added that a total of 180 fecal samples were collected from both mature cows and growing calves, and beef producers chose their own dewormers, which were categorized as macrocyclic lactones (like ivermectin, used to treat several parasitic infections in humans and animals), benzimidazoles (white pastes), or combinations.

He said stomach worm eggs were found in 60 percent of mature cattle and 78 percent of younger cattle. Cooperia, another common internal parasite, appeared in 22 percent of mature and 74 percent of young cattle.

To manage parasites, Meusch said it is important to understand their life cycle: “Stomach worms must complete several life-cycle stages, both inside the gastrointestinal tract of cattle, and on the pasture. The cows can only be infected by consuming grass from infected pastures, and pastures can only be infected by cows carrying adult stomach worms.

“Worms that are resistant to a dewormer (less than 90 percent are killed by the treatment) survive and lay eggs that are deposited in the pasture,” he added. “Over time, the percentage of resistant larvae increases in the pasture until the animals are infected with worms that can no longer be controlled with the product.”

Grant Dewell, Iowa State University beef extension veterinarian, told Farm World, “Strategic or targeted deworming has been advised to more appropriately use anthelmintics (a group of anti-parasitic drugs that expel parasitic worms).”

He said, “Here in the Midwest where parasites are less of a problem, compared to the Southeast, we try to focus deworming on populations that are most at risk: young calves and two- to three-year old females. Generally, mature cows in the Midwest do not need to be dewormed regularly.

“For fecal testing, producers can collect manure samples from subset of cows (usually five to 10), and have their veterinarian determine the number of parasite eggs in manure,” he added. “This can give you some idea of the herd’s parasite load.

“This is somewhat subjective, depending on the activity of the worms, decreased egg production in winter, daily fluctuations of egg production, and manure consistency,” he added. “If most of the samples have high fecal egg counts, then it may be beneficial to deworm them. Again, it’s best to focus on the young animals the most.”

Meusch said better grazing and pasture management can limit the exposure of cows to parasites on pasture: “Rotational grazing can allow cows to avoid times when the most larvae are emerging on the grass. Because the larvae remain lower in the grass surface layer to keep from drying out, leaving a higher residual when grazing can help avoid exposure to the larvae as well.”

He said when temperatures drop to 28 degrees Fahrenheit a few times, typically in the late fall, it can be a good time to deworm because it isn’t likely that cattle will be reinfected from the pasture after that point.

He said this allows them to go through the winter without stomach worms, ensuring they benefit the most from supplemental feeding, and ensuring cattle aren’t depositing more eggs on the pasture over the winter.

“The parasites infecting pastures will become active again in the spring when temperatures regularly exceed 50 degrees,” he said. “As the eggs emerge into larvae and advance to the third stage (when the larva transforms into its adult form), the cows will be infected again. The next strategic time to deworm is about six weeks after the springtime temperatures exceed 50 degrees.”

He said it isn’t always possible to visually identify which cows are carrying the heaviest worm loads, but culling cows that chronically show symptoms of a heavy parasite infection can also benefit overall herd health.

“By managing grazing and strategically deworming cattle to avoid further pasture contamination, producers can keep their animals healthy and productive, as well as help ensure the long-term efficacy of deworming products,” he said.