Decoding Dewormers: Types, Resistance Concerns, and Use for Horses
Dewormers are a common treatment for internal parasites in horses. It is important that horse owners use the correct type of dewormer at the appropriate dosage to ensure dewormers remain effective on their farm.
A dewormer is an antiparasitic drug known as an anthelmintic given to an animal to destroy or inhibit the development of internal parasites. All domestic horses have some level of internal parasite load and dewormers are given to help manage these loads to avoid undesirable health issues that can accompany large parasite populations. Many dewormer products are available for treatment so it is important that you consider the type of dewormer needed and current resistance to that dewormer when selecting which product you will use. The goal of treatment with dewormer is not to eradicate the parasites which is almost impossible but instead to manage the internal parasite load at a level that your horse will remain healthy.
Types of Dewormers
There are many brand names of dewormer products available for horses but the listed active ingredient is the key component to consider when determining which type to purchase. There are only three main drug classes used for internal parasite treatment in horses
| Drug Class | Parasites Targeted |
|---|---|
| Benzimidazoles (fenbendazole/oxibendazole) | Large strongyles, small strongyles (not encysted), ascarids, and pinworms |
| Pyrimidines (pyrantel) | Large strongyles, small strongyles, ascarids, tapeworms (if given as a double dose), and pinworms |
| Macrocyclic lactones (ivermectin/moxidectin) | Large strongyles, small strongyles (including encysted – moxidectin only), ascarids, pinworms, and bots |
Each of these drug classes targets specific internal parasites to kill them and cause expulsion from the horses body. The drugs do this by preventing the parasites from obtaining nutrients causing them to starve to death.
Because there are only three main drug classes maintaining efficacy ability of the drug to be effective of these drugs is critical to ensure they keep working to reduce parasite burdens. If all three become ineffective on parasites we horse caretakers will have no more options to control these parasites.
Therefore to best manage parasite loads of horses we should be strategic about our deworming practices and use them in combination with other farm management practices such as removing manure from dry lots or rotating pastures to reduce parasite burdens.
Concerns about Resistance to Dewormers
Anthelmintic resistance or the ability of parasites in a population to survive treatment with a dewormer is a growing concern in internal parasite management. Some parasites have become highly resistant to current drug classes making the drugs less effective. This resistance occurs at the farm level rather than the horse level meaning that resistance to particular drug classes varies from farm to farm and even country to country. This is not just a local problem but an international one for the equine industry. Therefore steps to slow the rate of anthelmintic resistance must be implemented on each farm to ensure drugs remain effective.
The rate of anthelmintic resistance in a parasite population is determined by how frequently the parasites are exposed to dewormers selection pressure and how effectively surviving parasites pass their genes to the next generation. In other words the more you use a drug
class on a parasite population the faster resistance develops. This is because parasites that are still susceptible to the drug class are killed and only the resistant parasites are left to reproduce. The previous standard practice of deworming all horses every 8 weeks created strong selection pressure for small strongyles to develop resistance to the drugs.
If all parasites in a population become resistant to a drug class then the drug class will not be able to reduce the parasite burden in a horse. Our goal then is to keep susceptible parasites alive so that when we use the drug class
