Cattle producers today have many tools for controlling parasites, but must use them properly or the parasites tend to develop resistance.
By Heather Smith Thomas
Dr. Joe Gillespie, DVM, Boehringer Ingelheim Animal Health, says it’s important to understand the risks. “This is the first step in determining how to do a better job of managing, and reducing the chance for parasite resistance.”
The advice he gives producers regarding resistance issues—whether antibiotics, internal parasites or external parasites (flies, lice, etc.)–starts with three things. “No matter what product you use, always use according to label directions. Secondly, apply that product at appropriate dose, since under-dosing (and overdosing in some cases) can increase the risk for parasite resistance. Third, apply at the appropriate time to best manage that type of parasite.” For example, don’t put fly tags in cattle in January. Wait until the flies are well started, for most effective control for that fly season.
“Regarding resistance, we must also understand that if we use the same tool over and over, ultimately there will be some flies or parasites that survive. They will be the main ones reproducing, building a population of resistant pests. Maybe they are not completely resistant, but they are more tolerant of the control method, and we can’t kill them all,” says Gillespie.
Today more people are using combination therapy. “For a product to be labeled for a certain application it must have a certain amount of efficacy. If we have two products that each have successful effectiveness, and use them in combination, we multiply that level of effectiveness; a much lower number of parasites survive and become resistant,” he explains.
This method is used a lot for controlling internal parasites, especially in small ruminants that have serious issues with resistance. “We can take this into consideration when making a plan for controlling parasites in cattle. I tell producers they should have a conversation with their herd health adviser, which is typically their veterinarian. It’s important to consult with someone who understands the implications of using products inappropriately or effectively. We then have lower risk of parasites developing tolerance—and ultimately resistance—to the products we’re using,” he says.
Many people think that if they use X product this year and Y next year and keep alternating them, they will thwart resistance. “But what often happens is that instead of getting resistance in 5 to 10 years to one product, we now have resistance to two products.” This is what happened early on with the use of fly tags.
“However, with a combination of products—which is more widely used in dealing with internal parasites than with flies—we can delay resistance much longer, especially when using correct dose and application. We can maybe eliminate the opportunity for long-term development of resistance,” says Gillespie.
Another tactic is using insect growth regulator in minerals or feed. In this situation the product ends up in the manure where flies are laying their eggs. Even though the eggs hatch, the larvae can’t mature and become flies.
The fewer flies in the population—because we’re limiting their development—is helpful. Using a feed-through product is easier in a feedlot or small pasture, however, where cattle are consuming the mineral or supplement, than when grazing extensive range pastures.
The main thing is to have a good management plan each year. “If you don’t have an action plan for parasite control, you are at the mercy of that year. In a year when there are more flies, you will have more fly issues. In a year with fewer flies, you might get along fine. But if you come into each production year with a plan—to treat at a specific time that is optimal for your season and geography—and use a product labeled for that use, and use it at appropriate application and dosage for weight of the animals (or appropriate number of fly tags), you will have more success.”
You will also have more success in reducing the level of resistance. You will never completely eliminate it, but can reduce it and slow it down, because there are not as many parasites that escape your control method. And if you have issues in the future, consult again with your veterinarian. You may be able to come up with a new plan that might help.