by: Christine B. Navarre
DVM, MS, DACVIM, Extension Veterinarian, LSU AgCenter, Professor, School of Animal Sciences

Controlling internal parasites in cattle is challenging these days. Best management practices have been discussed in the past and are available at More background information can be found in the article Best Management Practices: Internal Parasite Control in Louisiana Beef Cattle.

Now, resistance to cattle parasites is at a critical tipping point. Resistance to Cooperia has been around for years and is widespread. Fortunately this parasite usually only impacts weight gain in young animals, although in very wet years in Louisiana we can see some poor doing calves. What is alarming is the rise in resistance to Ostertagia. Ostertagia is highly pathogenic, even in adult cattle. Those that have been in the cattle business less than 25 years may have never seen the effects of this parasite. I personally don't want to go back to seeing cases. We need to take action now to prevent this situation from getting worse.

What To Do?

We will never stop the resistance trend completely as long as we continue to use dewormers, but we can significantly slow the process. This may mean cutting back on deworming entire groups and using multiple classes at the same time. We may have to give up a little production in the short term to sustain these products in the long term. There are a few new classes of dewormer worldwide, but no indication that they will be marketed in the US for livestock in the near future. Realistically it will be 5-10 years if at all. And they will likely be relatively more expensive than the products we have currently.

The fact sheet referenced above covers some of the basics of the refugia principle and parasite control. Managing refugia is the most important thing that we can do so understanding the principles is critical. A few things that are not covered are targeted selective treatment and using dewormer classes in combination.

First let's cover using combinations. This is giving products from two or three classes at the same time. It's different from "rotation" and actually significantly slows the development of resistance, whereas rotation leads to resistance to all classes in the rotation. It has been proven to work in other countries that have had combination products available for years. There will obviously be a cost to this. But the cost of not doing this is losing all efficacy and having nothing available. Resistance is worse to the macro cyclic lactone class (ivermectin, moxidectin, dormectin, eprinomectin). Simply switching to the benzimidazole class alone (fenbendazole, albendazole, oxfendazole), which many are doing, will quickly lead to resistance in this class too. There is a third class that includes levamisole. While less popular, this dewormer should also be considered an option in combination.

Targeted selective treatment means leaving some animals untreated to provide refugia. In sheep and goats we use FAMACHA to select who to treat and in horses we use fecal egg counts. Neither of these work in cattle. Right now in cow calf herds, I am recommending only treating calves, bulls, and females up until they wean their first calf (the exception to this is for fluke control in adult cows in the fall). If we start to see clinical Ostertagia, we may have to revise this. If you have a group of young cattle (replacement heifers for example, leave the heaviest untreated). The percentage of the group to leave untreated is based on fecal egg count reduction test (FECRT) results. If your combination of choice (a macro cyclic lactone and benzimidazolemost likely) is 99 percent effective, then don't treat 10 percent of the animals. If your efficacy is less than that with a combination, we should probably get some expert help to decide on how many to leave untreated. Pay attention to pasture refugia also.

Bottom Line:

No matter what, each time you deworm, use at least two classes. This applies for treating flukes also, as both products effective for flukes also impact Ostertagia.

Use selective treatment based on fecal tests. Leave at least 10 percent untreated. Especially if they are going onto a clean pasture with no refugia.

Contact your veterinarian for help with diagnostics and for advice about which products to use at the correct time. These are very general recommendations and need to be tailored to each farm or ranch.

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