Parasites & Deworming

For a brief overview of Fecal Egg Counts and deworming, please visit our Sample Deworming Protocol page.

For many years, deworming horses using a rotational method was deemed to be the best standard of care for protecting our horses from internal parasites. However, Janssen Veterinary Clinic no longer recommends this method. The reasons are multifactorial, but the biggest two are these:

  • There has been more and more parasite resistance occurring in horses and the industry has no new deworming products in development.
  • Some horses have less susceptibility to parasites than others, and are being dewormed more than is necessary. We recommend caring for the herd by treating each member as an individual.

Emerging research has demonstrated increasing parasite resistance to deworming products, or anthelmintics. Similar to the way bacteria become resistant to overused antibiotics, equine internal parasites have been shown to develop resistance to deworming medications. The anthelmintic kills the weaker parasite, leaving the strong to reproduce and further increase the resistant population.

In order to combat this resistance, rather than using the drug as a preventive in a horse that might not need it, we recommend treating horses with anthelmintics only when necessary to control the parasite population in that specific horse. It has been documented that 80% of all internal parasites are found in only 20% of horses. Utilizing a testing procedure called the Fecal Egg Count (FEC), a fresh fecal sample is tested to determine the number of parasitic eggs shed by that horse. FEC results classify each horse as a low, moderate, or high shedder of parasite eggs. High shedding horses can be targeted with more frequent dewormings, while lower shedding horses (the majority of horses) can be dewormed as little as once or twice a year with no ill-effects.

Parasites in the horse. There are numerous parasites that affect horses. Large strongyles, bots, and pinworms are not unusual, but less problematic than the ones that are our primary concern:

  • Small Strongyles (cyathostomes) are the most common and the most troubling parasite found in the adult horse. The life cycle of these parasites takes place both internally and outside of the horse. Small strongyle eggs are passed in the feces, hatch in grass, and develop into larvae, which are then consumed by the horse during grazing. They then move through the gastrointestinal tract to the large intestine where they develop into adult worms. Some of the larva will encyst in the wall of the large intestine and lie in a state similar to hibernation. Adult small strongyles produce eggs, which are then shed in the feces and contaminate the pastures, starting the cycle over again. After a period of time the encysted larva emerge from the wall of the large intestine and can cause health concerns if the horse is carrying a large burden. A severe case of strongyles robs your horse of nutrients in their diet and can cause weight loss, anemia, unthriftiness and colic.
  • Tapeworms. While small strongyles are of primary concern in adult horses, another significant parasite found in horses is the tapeworm. Tapeworm larvae develop inside the Oribatid mite, which is ingested by the horse during grazing. Developing tapeworms are released inside the horse during digestion. Eggs are released into the manure only to be ingested by the mites in the pasture and continue the cycle. Tapeworms contribute to colic by causing inflammation, ulceration and spasms in the digestive system.
  • Roundworm control is most critical in the young horse. These parasites migrate through the lungs and liver of foals, causing unthriftiness, weight loss, respiratory disease, and colic. Roundworms are resistant to ivermectin products but are susceptible to high doses of Panacur and Strongid. Fecal egg counts are important to monitor a foal’s roundworm counts, but are not as specific as the adult horse. Therefore we recommend deworming foals on this schedule.

How do I get started on the FEC deworming protocol?

  • When initially starting this deworming protocol, a fecal egg count should be performed when a horse has not been dewormed for a minimum of 2 months.
  • Any new horse entering the farm should be tested.
  • The average horse should be retested twice a year.

Which dewormer should I use?  Anthelmintics available for use in horses:

  • Benzimidazoles: Fenbendazole (Panacur), Oxibendazole (Anthelcide)
  • Tetrahydropyrimidines: Pyratnel Pamoate (Strongid)
  • Macrocyclic Lactones: Ivermectin (Equell, Zimectrin), Moxidectin (Quest)
  • Praziquantel (found in combination with macrolytic lactones)

Strongyles have developed resistance to the first two categories, but every farm may have different strains of resistance. To determine an individual farm’s resistance to a particular anthelmintic, we can perform a fecal egg count reduction test (FECRT). To perform FECRT, complete a fecal egg count, and then deworm the horse. A second fecal egg count is to be performed 14 days later. If the parasite load is found to be decreased by more than 90%, the product is considered effective. This test is specific to the anthelmintic used, and only needs to be performed once every few years.

Remember that some small strongyles become encysted in the wall of the large intestine? Moxidectin is the only drug that consistently kills those encysted larvae and should be administered at least once yearly.

Tapeworm eggs do not show up in fecal egg counts, so their presence is more difficult to detect. Because tapeworms are prevalent in the horse population and can still have detrimental effects on your horses’ health, it is recommended that all horses be treated with a product containing praziquantel in the fall every year. Praziquantel is the only product available that is effective against tapeworms and is usually combined with Ivermectin or moxidectin (Equimax, Quest Plus).

When should I give my horse dewormer?  Another important factor in parasite control is timing. It is essential to understand the life cycle of parasites to understand when to deworm your horse. For example, Small strongyle eggs hatch based on temperature, specifically between 46 degrees F and 100 degrees F. Therefore, during the Indiana winter, horses may be shedding strongyle eggs onto pasture but the eggs cannot hatch due to the cold temperature, the infective larva is not consumed by other horses. Similarly, in the hot summer months with temperatures greater than 85° F) the eggs will hatch, but the heat quickly kills the larvae. The optimum temperature for strongyle transmission is 77 degrees F, temperatures commonly found in the spring and fall. Therefore we focus most of our parasite control during these times of the year.

What else can be done to reduce parasites in the horse’s environment? Protecting horses from internal parasites requires a multi-pronged approach. Part of the solutions is reducing the internal parasite burden on the horse by using the appropriate anthelmintic and proper timing of the administration of the dosage. Also, reducing pasture contamination by utilizing good management practices will help control parasites. Avoid spreading manure on pastures that will be used for grazing, as this only increases the consumption of parasite larvae. In addition, do not overcrowd your pastures. Horses naturally defecate in areas separate from where they graze, but overcrowded pastures cause horses to graze on top of manure piles. Again, timing can be important! Ideally, using a chain harrow to drag a pasture should only be done when the temperature is greater than 85°F. The larvae will then be exposed and killed by the high summer heat. Dragging the manure around the pasture when it is less than 85°F only spreads eggs and larvae and will increase the parasite load in your horse.

There is not one perfect deworming protocol for every farm, but you may view an example schedule for adult horses here. Feel free to give us a call to discuss your farm’s deworming protocol today!

For additional detailed information, please refer to the American Association of Equine Practitioners’ website: