SALEM, Mo. – The use of commercial livestock dewormers has paid off in terms of animal health and productivity, but it has also resulted in resistance among parasite populations.

“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.

Resistance to deworming products

A recent study funded by Merck Animal Health and led by Eric Bailey of the University of Missouri conducted fecal egg count reduction tests in beef herds across the state, finding resistance to macrocyclic lactone dewormers (avermectin-type dewormers). The benzimidazole class of dewormer (“white” dewormer) still showed efficacy above the threshold for effective deworming, which is 90%. The results of the Missouri study are consistent with a larger data set compiled by Merck Animal Health.

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,” said Meusch.

Understanding the enemy

To manage parasites, it is important to understand their life cycle, said Meusch. 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% are killed by the treatment) survive and lay eggs that are deposited in the pasture. 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.

Manage the problem strategically

Better grazing and pasture management can limit the exposure of cows to parasites on pasture, Meusch said. 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.

Strategic deworming is another concept that can help avoid exposure to parasite larvae on pasture. This is basically timing deworming treatments to take advantage of when the worms on the pasture are dormant and not reinfecting animals. 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. This allows them to go through the winter without stomach worms, ensuring they benefit the most from supplemental feeding. It also ensures 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. As the eggs emerge into larvae and advance to the third stage, the cows will be infected again. The next strategic time to deworm is about six weeks after the springtime temperatures exceed 50 degrees. At this time, the cows will have been reinfected but are not yet depositing eggs in the pasture. This extends the time that eggs aren’t being deposited and helps reduce the parasite burden on the pasture.

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,” said Meusch.

For more information about deworming strategies or assessing the worm load in your cow herd, contact your local MU Extension center, or email Eric Meusch at meusche@missouri.edu.