Deworming cattle is easy, often providing significant economic return for relatively little cost.

Deworming cattle effectively and sustainably, though, is more complex than many believe. That has plenty to do with the apparent growing resistance of common internal cattle parasites to dewormers (anthelmintics).

Common parasites include Ostertagia ostertagi, which impact both young and mature animals. Cooperia sp. and Haemonchus placei are common calf parasites.

The three main classes of dewormer are macrocyclic lactones, benzimidazoles and imidazothiazoles.

There is no national statistic for anthelmintic resistance in beef cattle in the United States. However, various analyses, such as fecal worm egg count reduction trials, suggest growing parasite resistance to common anthelmintic classes. Globally, resistance is recognized as a key threat to grazing livestock.

“Anthelmintic resistance has been a recognized problem in small ruminants for decades. Now, there is increasing concern about resistance of cattle parasites to dewormers and the ability of cattlemen to continue to have cost-effective parasite control,” explains Christine Navarre, DVM, Extension veterinarian at Louisiana State University.

This graphic shows how refugia, or drug-sensitive parasites, can be maintained in a herd. The presence of some drug-sensitive parasites decreases (dilutes) the proportion of resistant parasites within the parasite population on a ranch or farm. Graphic courtesy of FDA Center for Veterinary Medicine.

“Worldwide, there is documented anthelmintic resistance to all commercially available products in all of the important livestock parasites. But, the extent of the resistance in cattle varies from country to country and from ranch to ranch.”

Test for resistance, and Navarre bets most cow-calf producers will find it in their herds; it’s just a matter of how much. She’s referring to the fecal egg count reduction test (FECRT), which helps determine efficacy for recent deworming treatments and provides the ability to estimate resistance.

“For $100, give or take, you can get some really good information,” Navarre says. “It helps you to see where you are and how much of a resistance issue you have.

Plan your attack

“You’ve got to have this information to arm yourself in developing a plan … The key to parasite control is to deworm often enough, and at the right time, to minimize the economic impacts, while at the same time preventing the development of anthelmintic resistance,” she says.

That’s where it gets complicated, and why developing and executing a parasite control plan should be done with a producer’s veterinarian.

Every operation and set of cattle is different. Effective plans will differ based on such things as cattle age, purpose and type.

“Many factors, including rainfall, environmental temperatures, pasture type, grazing management, age and immune status of animals, previous product use and anthelmintic resistance patterns all determine the severity of parasite problems on an individual ranch in a given year,” Navarre says. “A unique parasite control program must be developed for each.”

Never mind nutrition and overall herd immunity. As Navarre emphasizes, parasite control is just one part of a comprehensive herd health plan.

Even with testing, she cautions that diagnosing anthelmintic resistance can be difficult.

“This is a very complex and serious issue, and claims of resistance should not be made in haste or with very little evidence to support them. Conversely, if resistance is occurring, it needs to be documented,” Navarre says.

“Resistance is often suspected when poor performance or clinical signs of parasitism don’t improve following deworming. This can be caused by several things and should not be immediately interpreted as a failure of the anthelmintic product,” she adds.

Simple things like guessing the weight of cattle for dosage, improper application, or figuring that stuff gathering dust on the back shelf is still good can all derail the intended outcome. In some cases, such misuse also contributes to resistance.

Then there’s the parasite population left alone by deworming — referred to as refugia. Navarre says refugia are a key contributor to slowing down resistance, but likely for the exact opposite reason logic might suggest.

Leave some parasites to fight

“Refugia is the portion of the parasite population that is not selected by drug treatment; worms in refugia have a genetic makeup that make them susceptible to an anthelmintic,” Navarre says. “The more refugia in a population, the more the resistance genes in a population are diluted, and the more effective the anthelmintic will be.”

The notion of refugia is relatively new for some. In simpler and inexact terms, she says it’s akin to leaving enough non-resistant parasites to breed with the resistant ones, thus slowing the progress of resistance.

Refugia can be on pastures. So, pasture type and management, as well as treatment timing, affect refugia.

“Historically, we have exploited those times of extreme heat or cold when Ostertagia larva cannot survive on pasture, and most of the total parasite burden is in the animal as hypobiotic larvae,” Navarre explains.

“Deworming at this strategic time with products effective against inhibited larval stages can greatly decrease the overall parasite burden in a herd. However, it is a very effective way of eliminating refugia and may lead to resistance problems long term,” she adds.

Similarly, Navarre notes that pasture rotation can control parasites, at least theoretically. But the timing of rotations is variable, depending on stocking density, age of cattle, time of year, recent rainfall and all of the rest. “Rotate pastures to maximize nutrition and pasture use, not to control parasites,” she says.

Refugia can be maintained in the animals, too, by leaving some untreated. There is no cookie cutter for deciding which ones. It’s ranch-specific and depends on myriad factors, including cattle age and purpose and results of the FECRT.

If it’s a cow herd in the Gulf States, for example, Navarre says one approach to maintaining refugia is to leave the adult cattle untreated. Cattle up to 2 years old are commonly most susceptible to parasitic infection.

Conversely, the answer will be different for replacement heifers grazing permanent pasture.

Again, it goes back to developing a plan based on data provided by testing — at least for cattle remaining in the same operation. There’s little to be gained from testing for a stocker operation bringing in calves from multiple sources and grazing them across the same pastures.

In that case, Navarre suggests a starting point is leaving 10% untreated. Based on limited economic research, she says the heaviest calves are the logical choice for the untreated group.

“The quickest way to get widespread anthelmintic resistance is to deworm an entire group of cattle and then put them on a clean pasture,” Navarre says. “We have no refugia left on pasture, and we eliminate refugia in the animals. The only parasites left in the animals are resistant.

“When they reproduce, they will contaminate the pasture with an almost pure population of resistant parasites. There are no refugia on pasture to dilute the resistant worms,” she explains.

One more thing when deworming: Navarre says two different classes of dewormer should be used at the same time. Also known as concurrent deworming, it boosts the odds of killing more resistant parasites, slowing resistance.

“But this must be in combination with a refugia program or you will be selecting for pure populations of multidrug resistant ‘superworms,’ ” she says.

“Cattlemen should work with their veterinarian to find a balance between keeping overall parasite levels low enough to prevent economic losses, while at the same time retaining some refugia to slow the progression of anthelmintic resistance,” Navarre emphasizes.