Inside Effective Parasite Control


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Delivering the right de-worming dose helps maximize effectiveness and minimize potential for resistance.

Cooperia spp. have become one of the most prevalent parasites in the United States. This is thought to be, in part, due to the widespread use of macrocyclic lactones. A 2012 study by the University of Minnesota showed one species of Cooperia has a significant effect on cattle productivity, both reduced weight gain and decreased feed intake – making this issue one producers can see on their bottom line.1

With no new deworming molecules poised to enter the market soon, it becomes increasingly important to protect the effectiveness of the products available today. Producers and veterinarians can help do just that by rotating the classes of products used on the farm and delivering an effective dose to each animal.


Mark Alley headshot

Mark Alley


Rotate classes

Producers can increase their chance of having an effective deworming program if they include more than one class of dewormer. Combination dewormers – products that include multiple classes of parasiticides in one product – can help make that decision easy.

“One of the things we’ve learned from other industries is that we can delay the onset of resistance. We’re going to have to do that because we don’t have any new ingredient classes in the pipeline,” said Mark Alley, DVM, Diplomate, American College of Animal Welfare and managing veterinarian, Zoetis.

Alley cautioned that some geographies may find that parasite resistance to macrocyclic lactone products has increased. Adding in “white dewormers” to a deworming protocol can help continue to control important parasites like Cooperia spp. 

“White dewormers were typically provided as a drench, but now we have a molecule that will do something similar without the need to drench those animals,” Dr. Alley said. “Valcor® with doramectin and levamisole comes as an injectable solution which helps ensure animals get all the intended dose and delivers two ingredient families in one product. Levamisole fills that role of the white dewormer against Cooperia spp.”

Apply the full dose

Another key component of any deworming program is applying the full dose of product for the weight of the animal, Dr. Alley advised.

“In general, we don’t do a good job of weighing individual animals. When using an average weight of the group, half of the animals are not going to get the proper dose,” he pointed out. “A lot of times we don’t have scales on the operations, or producers don’t want to slow down the processing.”

In addition, pour-on dewormers may not be evenly applied across the animal. Splashing product on the chute or pouring on mud-caked hair can lessen the product’s ability to absorb and make a real deworming difference.

“We are always going to have a population of parasites where we are not going to kill 100% when we use less than the label dose. The longer the parasite is exposed to the dewormer, the next generation of parasites may become resistant. We have to be good stewards of what we have today.”

Understand the herd

A whole-herd parasite management plan should also consider the molecules used to control flies, ticks and lice – plus regular fecal egg count reduction tests (FECRTs) on a subset of animals.

“The diagnostics when it comes to parasites are rudimentary at best, but what we’ve learned is that we can’t look at an animal to determine what level of parasites they have,” Dr. Alley explained. “We don’t have to test every animal, but we need some semblance of a plan. I encourage producers to think of their parasite control program as a holistic plan that considers both internal and external parasite control.”



1 Stromberg BE, Gasbarre LC, Waite A, et al. Cooperia punctata: effect on cattle productivity?. Vet Parasitol. 2012;183(3-4):284-291. doi:10.1016/j.vetpar.2011.07.030.

2 Vercruysse J, Clarerbout E. Resistance to Anthelmintics. Merck Veterinary Manual. Updated: November 2022.


Key Points:

  • The development of significant levels of resistance seems to require successive generations of parasites exposed to the same class of anthelmintic (dewormer).2
  • Evidence suggests that genes for resistance are invariably present, at a low frequency, for any given anthelmintic.2


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