Why doesn’t my dewormer work?

Before you quit deworming your herd, understand how parasite control products work and how one active ingredient is in a class of its own.

It’s easy to question the need for deworming. The damaging effects and severity of internal and external parasites are hard to see with the naked eye — especially when compared to other diseases that have obvious clinical signs. What you can’t dispute, though, is the years of research that underline the fact that parasites will rob your herd of profits. 

Don’t assume that  just because you’re deworming cattle with a product labeled to control internal and external parasites that you are 100 percent covered. Dr. Mac Devin, a Professional Services Veterinarian with Boehringer Ingelheim Vetmedica, Inc., says producers need to understand these products aren’t created equal and understanding these differences will help you effectively fight parasites.

Two different families

Endectocide parasite control products fit in the broad category of chemicals known as macrocyclic lactones. Macrocyclic lactones are then broken-down further into two categories: avermectins and milbemycins. 

Both of these groups have their own chemical structure and thus, behave differently when used for internal and external parasite control. “Avermectins include ivermectin, eprinomectin, abamectin and doramectin; milbemycins, a different molecular structure, has moxidectin,” explains Dr. Devin, when discussing the most common active ingredients found in parasite control products.

Classes of commercially-available parasite control products for cattle

Figure 1: Moxidectin, the active ingredient in Cydectin, is in a class of itself.

Dr. Devin says the structural differences of Milbemycins change the distribution of the drug in the animal. “Moxidectin, the active ingredient found in Cydectin^®, has a much greater affinity for fat than other macrocyclic lactones.” “This affinity for fat means that the active ingredient will stay around longer,” says Dr. Devin. He adds, “This longer duration allows Cydectin to have greater persistency against some species of internal parasites.”