What You Maybe Didn’t Know About Anthelmintic Resistance, What You Need to Know and How it Can Pay Off on Your Operation

Let’s say that for the past few years, your deworming efforts have gone smoothly and have seemingly cleaned out the herd.

But more recently, the triumphs appear to have stalled and the treatments aren’t nearly as successful as they once were.

A few rough-looking coats on a few less-than-hefty-looking cows. It isn’t pleasing to the eye, and it surely won’t be pleasing to the wallet come sale time.

This could be anthelmintic resistance at work.

While resistance to antibiotics has been and continues to be a hot topic, only in recent years has resistance to deworming products become a more legitimate concern.

A limited number of dewormers – and active ingredients – are available on the market, which is why it’s important for you to know how each product works and how they should best be utilized to delay or reverse the onset of resistance.

Through education and working with your veterinarian to develop the right plan, we can slow the spread of anthelmintic resistance and ensure our deworming products stay effective as long as possible.

Get educated about resistance

Resistance occurs when a parasite population begins to survive treatment from a drug that at one time was effective.

After treating an animal with any dewormer, resistance can take place at two points in a drug’s journey: at the head and at the tail.1

  • Head selection for resistance: When parasites survive the drug’s initial peak1
  • Tail selection for resistance: When parasites survive because the drug drops below therapeutic levels before it exits the body1

One of the most common causes of head selection is treating to the average weight of the herd. While convenient, this can underdose a significant number of the cattle and diminish the effect of the drug.

For this reason, you should use a scale and treat each animal according to your chosen deworming product’s label.

Tail selection happens when small amounts of the drug’s active ingredient linger too long in the animal’s body and parasites are exposed to the low dosage.1 A long-lasting tail can help stronger parasites survive and breed, passing the resistant genes along to the next generation.

With each successive treatment, the newly resistant parasites grow in numbers and begin to outnumber those still vulnerable to the dewormer, rendering the treatments useless.2

Anthelmintic resistance itself isn’t a new concept. It’s been studied globally for decades, focused heavily on sheep, goats and even horses.

But it is a relatively new fear in the cattle industry. Not to say that it’s not a concerning issue (because it is) or that it doesn’t deserve real attention (because it does).

How can you fight resistance?

Armed with the right data and good management practices, you can actively work to slow or halt resistance on your operation.

Two of the more known and suggested combatants to resistance have garnered positive results for cattle producers: refugia and concurrent therapy.

Curbing resistance with refugia

Maintaining a refugia program means leaving a small percentage of the herd untreated, intentionally allowing some parasites to survive.

While this may seem a bit illogical, since internal and external parasites are considered to be one of the greatest health challenges for cattle producers, this is the same concept that has helped corn producers successfully combat corn rootworm.

Here’s the method to what may appear to be madness.

Because the majority of cattle receive treatment, susceptible parasites can begin to outnumber resistant parasites and help dilute the resistant genes.2 Should the herd become reinfected, it is much more likely that cattle will ingest susceptible parasites, which will be shed back into the pasture.2

If managed successfully, the cycle continues and the dewormer can stay effective in battling resistant parasites.2

Of course, a refugia program can only be successful if the dewormer is highly successful in killing parasites. Because no dewormer exists that can take out every parasite, concurrent therapy may be considered in conjunction with refugia.

Two may be better than one: concurrent therapy and resistance

Concurrent therapy, or combination therapy, is the practice of using two or more dewormers of different classes.

Three classes of anthelmintics are currently approved in the United States – macrocyclic lactones, benzimidazoles and imidazothiazole.

Some parasites are naturally resistant to certain classes, and studies show that deworming with an additional anthelmintic class can help take out the parasites that would survive treatment from a single product.

This is definitely something not to try on your own. No dewormers are presently labeled for combination therapy use. Only your veterinarian can determine if you need a plan for treating cattle with full doses of two or more products of differing classes.

Talk to your veterinarian about resistance

Even producers highly educated about resistance can benefit from working with a knowledgeable veterinarian. A vet can help you create both short- and long-term parasite control goals for your operation while minimizing the risk of driving resistance.

Every producer’s situation is unique, and there’s no cookie-cutter answer when it comes to parasite control. But a number of factors can help your vet determine what product(s) and strategy are right for you, such as:

  • Geography: Different areas of the country experience parasite burdens at distinctive times and at varying degrees.
  • Seasonality: While conventional wisdom says producers should deworm in the spring and fall, parasites can cause problems on your operation all year-round.
  • Type of operation: The end goal is to kill parasites to keep cattle healthy, but specific strategies often differ among cow-calf, stocker and feedlot operations.
  • Amount of time on grass: Some products last longer than others, and you want to treat with a dewormer that best suits your operation’s grazing schedule.

After a comprehensive evaluation of your operation, your veterinarian can help you decide what will most benefit your herd and bottom line while mitigating resistance concerns.

Even if your current deworming protocol is working well, we encourage you to reach out to your vet to discuss resistance. It’s an issue that isn’t going away anytime soon, and an open conversation is your best bet for getting ahead of any possible resistance drivers that could affect you in the future.

1 Sutherland I, Scott I. Gastrointestinal Nematodes of Sheep and Cattle. 2010; 139-144.
2 FDA.gov. Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It. Helpful Information for Veterinarians. Available at: http://www.fda.gov/downloads/AnimalVeterinary/ResourcesforYou/UCM347442.pdf. Accessed May 30, 2014.