by Sally Colby

With beef cattle parasites becoming increasingly resistant to available deworming treatments, what’s next for managing internal parasites? A new dewormer would be helpful, but Dr. Ray Kaplan, parasitologist and professor at the University of Georgia College of Veterinary Medicine, said it’s unrealistic to expect that any time soon.

“It’s been more than 40 years since ivermectin (a macrocyclic lactone, or ML) and we still don’t have new dewormers,” said Kaplan. “There’s a good reason for it: the process of developing dewormers is very difficult and very expensive. Any new drug is going to be more expensive. But even if we get a new drug, and there’s nothing coming soon, resistance will continue to outpace any new drug that’s introduced.” Until a new drug becomes available, which Kaplan said could be at least 10 years away, cattlemen should concentrate on how to use existing products most effectively, and in some cases, differently than in the past.

Some widely used dewormers are administered as pour-ons, which while convenient, easy and safe can also lead to resistance. “The problem with pour-ons is that they were developed when worms were highly susceptible to the MLs,” said Kaplan. “Almost any drug thrown on them would work, but now the worms are more resistant. There’s a lot of variability between animals in drug concentrations due to differences in absorption, and it’s also easier to misapply a pour-on.”

Pour-on products are meant to be poured along the entire back from the withers to the tail, not sprayed in one spot then allowed to drip off the side of the cow. “You don’t want to put a pour-on on top of mud or on long, matted hair,” he said. “The drug is not going to get to the skin where it needs to be. Poor application technique is common when animals are running through a chute quickly and someone is shooting the stuff onto a cow. So not only is pour-on a poor method from a pharmacology standpoint, but misapplication on top of that means you won’t get the results you want. If you aren’t applying the proper dose, that also promotes resistance.”

With resistance becoming a reality, the only way to achieve adequate efficacy is by using drug combinations.

It’s important to understand the various drug classes and select deworming products from two different classes. Although rotating deworming products among the classes has been a common practice, Kaplan cautioned producers to avoid this practice this because rotation speeds up resistance.

“If you use two drugs together, the only worms left are worms that are resistant to both products,” he said. “More worms are killed. Some people think by using drug combinations we’re going to end up with super-resistant worms, but that isn’t the case – you actually get fewer resistant worms because the worm has no other resistant worms to mate with except for worms that are already just as resistant.” Kaplan further explained that as reinfection occurs, worms will mate with more susceptible worms, whereas with individual dewormers, double resistant worms are now mating with single resistant worms, which results in more double resistant worms.

“Combinations help us get to the high efficacy levels required to knock back worms and to take full advantage of refugia,” said Kaplan. “If you use combinations without managing refugia, you can get into big problems with multiple drug resistance. But if you use two drugs along with refugia, it’s actually going to slow resistance.”

Computer modeling demonstrates how using two drugs in combination works. “Combinations kill more of the resistant worms,” said Kaplan. “Any worm that survives drug A is killed by drug B. It’s eliminating the resistant worms from the population; they can’t reproduce and it’s more sustainable. But to get the benefits of refugia, you have to maximize efficacy, and if you maximize efficacy, you also need refugia to prevent multiple drug resistance.”

Kaplan said farmers won’t likely notice any immediate difference with combination treatments and may not think they need to use a combination because their program seems to be working fine. “If you have 98% efficacy, 2% of the worms are surviving,” he explained. “If you use two drugs, both at 98%, then you’re up to 99.96%. Now you have only 0.04% surviving, which means 50 times fewer resistant worms surviving. That’s going to greatly decrease the evolution of drug resistance. If you continue using the drug with 98%, it’s going to rapidly fall to 95% to 90% to 80% to 60% within just a few years. But using two drugs in combination and getting 50-fold dilution, you will probably have high efficacy for decades before resistance becomes a problem. The difference between the percent of worms killed is not that great – only about 2% more – but the percent of resistant worms surviving is hugely different. This is how refugia along with high efficacy with combinations works.”

Whether or not they’re aware of a problem, Kaplan said most farms already have resistance to at least one drug. In that case, studies show that combinations are still effective. “If we use one drug alone with low refugia, resistance happens quickly,” he said. “If we add a drug that only added 50% to that drug, if we use it by itself it won’t last long. If combined with a drug that has only 50% efficacy, resistance evolves slowly – more years of using that combination before resistance is a problem.”

Kaplan said if today’s strategies had been initiated a decade ago, we’d be in a lot better shape, but it isn’t too late. “If we get into these habits now, it will help prevent Ostertagia resistance from exploding,” he said. “Also, when we finally get new drugs, we’ll be in the habit of using them differently. When we do get a new drug, it’s going to be a long time before we get another new drug and we don’t want to burn it out quickly.”

All the evidence shows that a combination of strategies will slow resistance. But these strategies are more complicated than routine deworming and there’s no one size fits all. Cattle producers should work with their herd veterinarian, and vets need to be informed so they can provide the best advice.

“Ultimately, these strategies should result in healthier, better-managed cattle,” said Kaplan, “which will increase profits and consumer confidence that drugs are being used responsibly.”