In this video, Dr. Martin Nielsen, helps us understand what parasite resistance in horses means to us and our animals. Nielsen is a DVM, PhD (parasitology), DVSc (doctor of veterinary science), DACVM (American specialist in parasitology), and DEVPC (European specialist in veterinary parasitology). He is the Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky’s Gluck Equine Research Center.

Basically, we need to understand whether the dewormer we are giving our horses to combat a specific internal parasite and stage of development still works effectively.

(Editor’s note: The content of this video is the expert’s approach to the topic. Please consult with your practitioner if you have questions.)

Click on the image above to play the video.

Parasite Resistance

Nielsen clarified by explaining when veterinarians and researchers talk about parasite resistance, they are talking about drug resistance within the parasite. In other words, the drugs meant to kill the parasites are not working well or at all in some “worms.”

He said that by definition, a dewormer that once worked doesn’t work any more. “There has to be previous data on this drug,” Nielsen said. “It has to have been approved for this particular parasite and stage of the parasite. And with a labeled efficacy level.”

With that foundational information, researchers then can determine whether those claims are still true.

Classes of Dewormers

While there seems to be many different types of equine dewormers on the market, there are only a few “classes” of drugs. “When they belong to the same class, they have the same mode of action,” said Nielsen.

There are only four classes of dewormers available for horses, he said. Some of them only have one drug “dewormer” in that class.

“The most important class in many ways are called the macrocyclic lactones,” noted Nielsen. “That is the class where we have ivermectin—the most-used anthelmintic (dewormer) ever in history. And we have moxidectin, it’s ‘cousin’ in that drug class.”

Another class of dewormer for horses are the pyrimidines. “That is the class where we have the pyrantel dewormers,” noted Nielsen. “There is just one active ingredient in that class. It has been around since the 1970s.

The other class of equine dewormer is benzimidazoles. In the U.S. there are two different drugs in that class—fenbendazole and oxibendazole. Nielsen noted that in other countries, there might be other members of this class of drug.

Benzimidazoles have been around since the 1960s. “That’s also the class we see the most drug resistance,” said Nielsen.

The fourth class is one that has an equine tapeworm drug. Praziquantel is often found in combination with some of the dewormers mentioned above.

Global Dewormers and Parasite Resistance

Equine dewormers are used differently in countries around the world, said Nielsen. The same drugs are available in most countries. This is because there are generally the same types of parasites in horses no matter their locations.

However, the use of equine deworming products is regulated very differently based on the country, noted Nielsen.

In the United States, horse owners can go choose dewormers off the shelf or buy them online. “In other countries, they are prescription only,” explained Nielsen. “Only the veterinarian can prescribe the product. That means there has to be a diagnosis for the problem.”

Nielsen said in the U.K., for example, in addition to the veterinarian, there are other “certified” individuals who can prescribe equine dewormers.

This causes horses to be dewormed less frequently in those countries, explained Nielsen.

Further Content

  • Kimberly S. Brown
    Kimberly S. Brown is an award-winning writer and publisher. She is the Editorial Director for My Senior Horse. Brown spent 10 years at Equine Network, parent company of My Senior Horse. Prior to that she worked for three years in equine nutrition after she retired from nearly 30 years working at The Blood-Horse. Brown spent the last 15 years of her time at that organization creating and developing The Horse and TheHorse.com.
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