10/04/2025
Equine Deworming
Brian S. Burks DVM
Diplomate, ABVP
Board-Certified in Equine Practice
Everyone knows that their horse should be de-wormed. Parasites remain a leading cause of intestinal disturbance in horses. There is much more to parasite control than simply giving your horse a deworming paste every so often. First, many paste products are the same active ingredient, so that even though products are changed, the same pharmaceutical ingredient is used. (See table) Anthelmintic pastes do not get every stage or even every parasite. Secondly, there should be an understanding of parasite life cycles and which one should be targeted at a given time of the year. Only your veterinarian can help you with the important details of de-worming your horse. It is not nearly as simple as giving a paste in your horse’s mouth.
In decades past, large strongyles were responsible for an estimated 90% of colic due to migration of larvae through the cranial mesenteric artery. With the advent of more efficacious deworming in the 1970’s and 80’s this parasite came under control. Since then, however, the parasites have developed resistance to these chemicals. Today the small strongyles (cyathostomins) are more prevalent, harder to control due to resistance, and harder to diagnose accurately due to their life cycle. The larval stages encyst in the colon wall and ‘hide’ from most anthelmintics. They may stay there for variable lengths of time varying from weeks to years. Slow build up of encysted cyathostome larvae can occur over several years. Only the adult cyathostomins are killed by deworming, which, in turn, decreases egg shedding. Tapeworms have also become more important, especially on certain farms. Large strongyles and bots are also of concern. In foals, weanlings, and yearlings, round worms (Parascaris equorum) are a significant parasite. There are multiple other parasites that will be controlled by most de-worming protocols.
Cyathostomins (small strongyles) may cause intestinal disturbances from subclinical to severe changes in intestinal function. Severe disease may result in weight loss and protein losing enteropathy (protein loss through the inflamed intestinal wall). Other horses may experience more mild symptoms such as mild, self or easily resolved colic symptoms, and peritonitis (inflammation and/or infection in the abdomen). Young horses are also quite susceptible to this parasite.
Tapeworms attach to the mucosa at the ileocecal opening- the junction of the small intestine and cecum, or appendix. They may block the opening resulting in reduction of fluid flow from the small intestine into the cecum. They may also cause weight loss with chronic, low grade infestation.
Round worms are present in all ages of the horse. Young animals, one year of age or less, are the most susceptible to this parasite due to a lack of immunity against parasites. Parascaris equorum may cause small intestinal obstruction due to the large number of adults that are found there. A single female may produce 10,000 or more eggs per day. The life cycle also involves migration of larvae through both the liver and the lungs, which may result in dysfunction of these organs. Pneumonitis and/or pneumonia are common sequelae to migration through the lungs. Even though infection may be present, antibiotics alone are not effective; de-worming with an appropriate anthelmintic is necessary.
Slow rotation programs, whereby the same active ingredient is used for an entire year are concerning due to small strongyle resistance and the fact that certain parasites are not affected by all of the different anthelmintic classes. Other programs include a change in class of de-wormer NOT just a change in product. This may still causes some parasite resistance, as frequent use may allow them to develop ways to avoid the pharmaceutical.
So how do we treat parasites in our horses? First, we need to find out the parasite load. This involves f***l egg counts. These should be done at least once, if not twice per year. Second, the type of worm must be identified. Each parasite has its own life cycle. Third, we must decide on an anthelmintic based on the first two criteria. Fourth, pasture management strategies should be implemented. This may include pasture rest/rotation, vacuuming or f***s removal, stocking density, and harrowing.
F***l egg counts will reduce the need for chemical control of parasites in many horses. Approximately 30% of the horses contain 80% of the worms. Identification of these horses will allow more intensive treatment of those horses that need it, while decreasing unnecessary treatment in other horses. Since parasite resistance is becoming rapidly wide-spread, this will also keep the de-wormers working in the future.
In western Pennsylvania, the critical periods for de-worming are the spring, summer, and fall/early winter. Since most parasites will not be active during the winter, continued treatment is not necessary. Cyathostomins will tend to excyst from the colon and cecal walls in the spring, and will encyst during the winter. This makes spring and early winter ideal for treatment of this parasite. Tapeworms tend to proliferate in the late summer and autumn, making winter a good time to remove these parasites.
Younger animals will need more frequent f***l egg counts and treatment. This is due to their lack of inherent resistance. Infestations in young animals tend to be more severe and to make them ill much more quickly than in adult animals.
In conclusion, parasite resistance is increasing; therefore we need to be proactive in treating those horses that require frequent de-worming, while not treating animals that have low f***l egg counts. Strategic treatment will more efficiently control parasitism, and preserve anthelmintics for future generations of our horses (less parasite resistance). Involving your veterinarian by having him or her run f***l egg counts, you will have a better idea if your horse is a heavy, moderate, or light egg shedder. Your veterinarian is also trained in parasite life cycles, making him/her the best source for your de-worming purchases, when necessary. This will allow a more tailored de-worming program, suited for your horse. It may even save you money in the long run!
Anthelmintic Drug Classes
CLASS PHARMACEUTICAL
NAME BRAND NAME GENERIC BRAND NAME
BENZIMIDAZOLE Fenbendazole
Oxibendazole Panacur
Panacur Powerpack
Anthelcide Safe-Guard
TETRAHYDROPYRIMIDINES
(PYRANTEL SALTS)
CONTINUOUS PYRANTEL PRODUCTS Strongid Paste Rotectin 2
Equi-Cide
Strongid-C Continuex Daily Dewormer
AVERMECTINS/MILBEMYCINS Eqvalan Paste Ivermectin
Iver-Care
Agri-mectin
Bi-Mectin
Zimecterin
Equimectrin
Zimecterin Gold
Equimax
Quest (Moxidectin)
Quest Plus
Fox Run Equine Center
www.foxrunequine.com
(724) 727-3481
