Attention to active ingredients is helpful in effective parasite control

Parasites pose major challenges, but careful management of active ingredients used against them offer promise in battling resistance.

2 Min Read

An operation using parasiticides in cattle without considering the active ingredients in those products is making risky choices — it might help produce short-term gains but could accelerate resistance in parasites over the long term. Using different parasiticide active ingredients can extend the effectiveness of parasite control products and play an important role in the management of healthier cattle.

Parasites must be dealt with, of course, but producers need to find a balance in their management practices. Treating every animal in a herd with the same parasiticide active ingredient could accelerate resistance.   

Here are three common active ingredient families and how they work to control parasites:

• Benzimidazoles — Deplete energy reserves in parasite cells and inhibit the elimination of waste — actions that appear to play an essential role in having a lethal effect on worms because they prolong the time the parasite is exposed to the drug.1

• Macrocyclic lactones — Exposure to macrocyclic lactones leads microscopic worms to experience paralyzed pharynx, body wall and uterine muscle, which leads to death.1 This active ingredient works by binding to receptors that open up parts of the nematode to allow the influx of chemicals that cause the paralysis.

• Imidazothiazoles — This group of parasite control products stimulates muscle contractions in worms that leads to paralysis.1

Overuse or underuse of these active ingredients can accelerate the development of resistance. For example, if an entire herd is treated with one active ingredient, all parasites susceptible to that ingredient will be removed — and only the parasites resistant to it will remain in the herd, effectively creating resistance to a product.

The judicious use of parasiticides with different modes of action will control parasites in a herd and also help keep products effective longer in the industry. Keep these tips in mind when developing a management plan:

• Dosing is based on weight. Underdosing is a contributing factor to parasite resistance.2 Accurate weights help drive accurate dosing, which can improve product efficacy.

• Careful management practices. Resistance is a major problem.3 A growing trend calls for replacing the current practice of repeated dosing of whole groups of animals with a move to targeted selective treatments to animals showing clinical signs or reduced productivity.3

 Use different active ingredients to reduce risk. Most of the commonly used parasiticides are either benzimidazoles, macrocyclic lactones or imidazothiazoles. Thus, resistance to one particular compound may be accompanied by resistance to other members of the same group — also known as side-resistance.3 Theoretically, resistance may be delayed by using products with different modes of action annually between dosing seasons.3

For more information on parasite control products and solutions, visit

1 Mechanisms of Action of Anthelmintics. Merck Veterinary Manual website. Accessed Dec. 27, 2018.
2 Safety of Anthelmintics. Merck Veterinary Manual website. Accessed Dec. 27, 2018.
3 Resistance to Anthelmintics. Merck Veterinary Manual website. Accessed Dec. 27, 2018.
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