Horse Tack Review

Submit your reviews! We will be giving away a pair of the HandsOn Grooming Gloves for the best review posted from now until November 31st. Please read the November 1, 2016 newsletter for additional information on how to enter.

Rotation Makes for More Effective Deworming


Internal parasites are a serious concern for horse owners across the country. Protecting horses from the severe damage these pests can cause, however, means much more than administering a dewormer once every few months. Owners must select deworming products in order to rotate between chemical classes throughout the year. A carefully designed rotational deworming program is the best way to assure effective deworming by maximizing the best attributes of each product.

Equine veterinarians agree that this rotation between chemical classes--not just between brand names of dewormers--is a critical element in the success of any deworming program. Unfortunately, effective rotation is often easier said than done for horse owners. Store shelves are packed with an astounding number of deworming options and it can be difficult to choose the best products to use throughout the year.

Armed with a complete understanding of what rotation is and how to complete it successfully, owners will be more easily able to provide high-quality parasite protection for all of their horses.

Rotating between Chemical Classes

All of the many brand names of dewormers on the market today can be arranged into just a few major chemical classes. Making sure to utilize the different classes effectively serves to reduce the chances of developing resistance to parasites, and maximizes the best attributes of each different chemical compound.

"There are really only three major chemical classes of dewormers available to horse owners today," said John Donecker, VMD, MS, senior veterinarian at Pfizer Animal Health. "All of the products within each chemical class will affect parasites in much the same way. Owners can find the chemical compound name for every brand of dewormer by looking carefully on the packaging. Strategically using a dewormer from each chemical class, spread throughout the year, is the best way to get the most parasite protection."

A major classification of dewormers is known as the benzimidazole class. This group includes dewormers such as oxibendazole and fenbendazole, which kill parasites quickly and offer broad spectrum protection against parasites including large and small strongyles and ascarids.

A second major chemical class is called the pyrantels. Including pyrantel pamoate and pyrantel tartrate, these dewormers kill parasites more slowly by causing paralysis in a wide variety of dangerous worms.

The last and perhaps most well-known class of dewormers is the macrocyclic lactone class. This group includes the ivermectin and moxidectin dewormers and works by causing paralysis in parasites. This class not only kills nematodes, like the small strongyle and the ascarid, but also attacks benzimidazole-resistant small strongyles and bots.

More recently, manufacturers have combined macrocyclic lactones (like ivermectin) with a second deworming chemical called praziquantel. These combined products help to attack the same broad range of parasites as ivermectin alone, but have the added advantage of praziquantel's ability to kill equine tapeworms (Anoplocephala perfoliata).

Rotation and Daily Dewormers

Many deworming products come in paste form and owners administer them about once every other month depending upon geographic location and the advice of their veterinarian. These dewormers (also called purge dewormers) work by killing off primarily the adult parasites in the gastro-intestinal system at that moment. Within one to two days, however, the purge dewormer has left the body and the horse is again vulnerable to parasite infection. Pastures and other locations can harbor hundreds of thousands of immature parasites, which--if ingested by the horse--will have almost two months to migrate around the horse's body and cause damage before the next dose of a purge dewormer is given.

By contrast, dewormers administered daily are top-dressed on the horse's feed and provide a daily dose of pyrantel tartrate continuously. Daily deworming protects horses from damage caused by some infective stage parasites as well as adults. This situation is important because, although the damage is not outwardly visible, these tiny infective parasites will migrate throughout the horse and can be a serious threat to the health and performance of affected horses. For example, migrating immature ascarids move through the liver and lungs, scarring these important organs as they go. While the liver can regenerate itself eventually, damage caused to the lungs is permanent--the horse will have less viable lung tissue throughout its life.

Creating an effective rotation with a daily anthelmintic is easy: simply feed the recommended dose of pyrantel tartrate every day and administer an ivermectin/praziquantel combination dewormer twice a year. This method is a rotational program and supplements the action of pyrantel tartrate by attacking bots and tapeworms with a macrocyclic lactone in addition to the broad spectrum of parasites affected by the daily dose.

"When daily deworming is combined with twice-yearly use of an ivermectin/praziquantel combination it creates a deworming program that is clinically effective against both clinical and subclinical levels of parasitism," said Donecker.

Pfizer's Deworming Options

Pfizer Animal Health offers numerous deworming products to make critical rotation decisions simple for horse owners. For additional information on Pfizer's portfolio of equine products, visit