Amesbury Animal Hospital

277 Elm Street
Amesbury, MA 01913


 Intestinal Parasites and Deworming Recommendations:

Intestinal parasites plague all of our horses. Even seemingly healthy individuals can carry a heavy and potentially dangerous parasite load, so effective parasite control is an essential component of successful management. New recommendations on parasite control and deworming strategies have changed how we handle intestinal parasites in horses. Please read on for more information.

There are over 150 species of internal parasites that can infect horses. The most common types are large strongyles (bloodworms or redworms), small strongyles, roundworms (ascarids), tapeworms, lungworms, pinworms, bots, and threadworms. The first four types are the most common and most important in terms of health risks. 
Here is a heavily parasitized horse.
Signs of parasitism include:

 - Dull, rough haircoat
 - Lethargy (decreased energy) or depression 
 - Decreased stamina 
 - Unthriftiness or loss of condition 
 - Slowed growth in young horses
 - Pot belly (especially in young horses)
 - Colic 
 - Diarrhea

Parasite lifecycles:

There are a variety of different parasite lifecycles, but most involve three life stages - eggs, larvae (immature worms) and adults (mature worms). Eggs or larvae are shed in the manure and infect the ground, paddock or pastures. Horses swallow these eggs or larvae while grazing. The larvae mature in the horse's gastrointestinal tract and the cycle begins again. Some larvae migrate out of the intestines and into other tissues or organs before returning to the intestines to fully mature. Some parasites even lay dormant or "encysted" in the intestinal wall while maturing and can cause extensive damage when they emerge or "hatch". These parasites are immune to dewormers while encysted. Because different parasites have different lifecycles, there are different periods of time when they are susceptible to dewormers. Knowing what types of parasites are infecting your horse will help you determine what type of deworming protocol you need.
This is a type of roundworm or ascarid.
Fecal examination:

The first step is having your veterinarian run a fecal examination with a fresh fecal sample. This is a microscopic examination of fresh manure for parasite eggs. We perform quantitative fecal egg counts here, which are currently the gold standard at determining your horse's parasite burden. Fecal egg counts (FEC) are used to classify horses as light, moderate or heavy shedders. FEC allow us to identify the horses in a barn in need of the most intensive control measures as well as assist with stable-wide deworming decisions and determining the efficacy of the products and deworming strategy used. Resistance to dewormers is a major concern of the horse industry. The
historical "deworm every eight weeks" schedule used by so many barn is now considered outdated. Deworming recommendations should be individualized to each horse as some horses shed more than others and some horses develop more natural resistance to parasites than others. No single dewormer (or anthelmetic) will kill every parasite every time. Simply rotating dewormers without understanding your horse's parasite load only serves to breed resistance to these anthelmetics. After performing a FEC to assess your horse's status, we can make recommendations about deworming and monitoring their parasite load.

Please check out these links for more information about current deworming recommendations:

Equine Parasites: 6 Tips for Learning to Live with Worms

Diagnosis: Examining the Evidence (Parasite Primer Part 6)


There are a number of different dewormers or anthelmintics available. Most are broad-spectrum, meaning they are effective against several different types of parasites. We generally recommend using a broad-spectrum dewormer as the basis of your deworming protocol. There are generally three types of deworming programs:

1. Continuous - Feeding a daily dewormer (like Strongid C) year-round or throughout the grazing season.
2. Interval - Deworming at regular intervals or 1, 2 or 3 months, depending on the product and management system (generally not recommended as this program increases resistance to anthelmetics)
3. Strategic - Deworming only at certain times of the year, depending on your horse's parasite burden (**recommended)

You can also have a combination of these programs and say for example, strategically deworm for bots if you are using a continuous deworming. The important point to understand is that there is no single deworming protocol that suits all horses or management situations. We can help you design one that is specific to your management needs. 

Good pasture and manure management:

Because parasites are spread through manure, another equally important part of an effective deworming protocol is good manure management. We recommend that you:

- Pick up and dispose of manure regularly
- Do not spread manure on fields. Instead compost it away from the pasture.
- Keep the number of horses per acre of pasture to a minimum to reduce overgrazing and minimize pasture contamination with eggs and larvae
- Keep foals and weanlings separate from older horses to minimize the risk of infection to the more susceptible population
- Use a feeder for hay and grain rather than feeding on the ground
- Consult with your veterinarian to evaluate the effectiveness of your deworming protocol

If you have any questions or concerns about intestinal parasites and deworming, please contact us at the number below.