Did you know that horses can live happy, healthy lives even if they have some parasites?
Many horse owners hate the idea of their horse carrying a high load of parasites, but it is important to remember that parasites are a natural element for horses. The goal for parasite management in horses is keeping parasites low because complete elimination is almost impossible. We want to keep our parasite numbers low to keep our horses healthy and happy because, in rare cases, high parasites can lead to poor coat quality, weight loss, diarrhea, and colic.
How to Deworm Your Horse
In the past, horse owners routinely dewormed their horses to reduce parasite loads—most often by rotating between anthelmintic pastes every eight weeks. As drug-resistant parasites and the lack of new dewormers become increasing issues, more horse owners, barn managers, and equine practitioners are changing their parasite management practices and turning toward targeted parasite control programs.
Modern horse deworming methods are more targeted, starting with one or two annual treatments for adult horses. These treatments give the basic foundation to target large strongyles and tapeworms. Additional targeted treatments are given to high parasite egg shedders, meaning horses with a high fecal egg count, to bring down the parasite population.
Types of Equine Intestinal Parasites
Small Strongyles
There are a number of different species of small strongyles. Small strongyles are very prolific and highly resistant parasites due to old routine deworming practices. Small strongyle resistance is one of the main reasons deworming methods have shifted in recent years. These parasites burrow into the walls of the large colon and cecum and become encysted. Encysted parasites can remain in the body for up to two years, waiting to mature.
Large Strongyles
Large strongyles are uncommon due to decades-old treatment strategies. Horses eat large strongyle larvae when they graze. These parasites migrate into and damage the arteries of the horse’s abdomen.
Tapeworms
Tapeworms can be difficult to detect with fecal testing. If your veterinarian suspects tapeworms, a blood test or saliva antibody test will be required. These equine parasites attach to the horse’s hindgut and can lead to colic.
Roundworms
Roundworms are most common in young horses and foals. Most adult horses develop an immunity to them. Unlike strongyles and tapeworms, roundworms live on common surfaces like buckets, fences, and other horses rather than grass. This makes it easy for a foal who is not yet grazing to ingest them. Once ingested, roundworms migrate through the circulatory system and enter the lungs. Once in the lungs, horses will cough them up and re-swallow them. Roundworms have developed resistance to most commercial dewormers, which makes management challenging.
Bots and Pinworms
While bots and pinworms can be very irritating to your horse, neither carry a high risk to their health. Bots burrow into the mouth and stomach, while pinworms lay eggs around the anus.
How to Develop a Targeted Parasite Program
Have a Fecal Egg Count Done for Your Horse
The first step to developing a targeted parasite program is having a fecal egg count (FEC) performed for your horse or horses. If you have multiple horses, be sure to check each horse. They will have different parasite loads even when living in the same barn and pasture. The FEC will measure the number of strongyle eggs your horse passes in each gram of manure. Most veterinarians perform FECs in-house, but if they do not, there are several independent laboratories for mailed fecal samples. FECs should be completed in the Spring when parasite transmission increases among the herd.
Assess Your Horse’s Egg-Shedding Level
FEC results can be confusing the first time you read them. You’ll see numbers like 40 eggs per gram (EPG) but not know what that means or what to do with that information. This number will tell you if your horse is a low, medium, or high parasite egg-shedder. You and your veterinarian can use this information to create a targeted deworming plan. Horses with a high shedding count may not show any signs of a high-parasite load but are carrying adult worms that are laying eggs that spread to other horses.
Adult Horse Deworming Schedule
Colorado State University recommends the following protocols for adult horses based on the results of the fecal samples. Remember, these are generic recommendations and do not consider your horse’s environment or specific health issues. Always consult your veterinarian before changing programs for your horse.
Low Shedders (<200 EPG – eggs per gram of manure)
- Fecal egg count performed prior to deworming in spring (ideally spring and fall)
- Spring (March) – ivermectin (Equell®, Zimectrin®, Rotectin®, IverCare®), moxidectin (Quest®)
- Fall (October) – ivermectin with praziquantel (Equimax®, Zimectrin Gold®) or moxidectin with praziquantel (Quest Plus®)
Moderate Shedders (200-500 EPG)
- Fecal egg count performed prior to deworming in spring (ideally spring and fall)
- Spring (March) – Ivermectin (Equell®, Zimectrin®, Rotectin®, IverCare, etc), moxidectin (Quest®)
- Late Summer (July) – ivermectin
- Fall (October) – ivermectin with praziquantel (Equimax®, Zimectrin Gold®) or moxidectin with praziquantel (Quest Plus®)
High Shedders (>500 EPG)
- Fecal egg count was performed prior to deworming in spring and fall to monitor for signs of resistance
- Spring (March) – ivermectin (Equell®, Zimectrin®, Rotectin®, IverCare®), moxidectin (Quest®)
- Summer (June) – ivermectin
- Early fall (September) – ivermectin with praziquantel (Equimax®, Zimectrin Gold®)
- Late fall (November) – moxidectin
Foal Deworming Schedule
Foals and young horses are more prone to high-parasite loads and resulting health issues because of their suppressed immune system. Here are the Colorado State University recommendations for a foal deworming program:
- Two months of age – fenbendazole (Panacur) or oxibendazole (Anthelcide)
- 4-5 months – Fecal egg count to track the occurrence of ascarids versus strongyles. Treat for ascarids with fenbendazole (Panacur) or oxibendazole (Anthelcide).
- Treat with ivermectin for strongyles at approximately five months
- Treat with ivermectin plus praziquantel before the end of the calendar year
- As short yearlings, check fecal egg count for the presence of roundworms and treat those with fenbendazole or oxibendazole if present
- During the yearling year, treat for strongyles approximately three times with ivermectin, followed by one treatment with moxidectin plus praziquantel by the end of the grazing season.
What if your Horse has a Negative Fecal Egg Count?
If your horse’s FEC results come back negative, your horse likely still has parasites. Horses are grazers and pick things up every time they graze. When an FEC comes back negative, the internal parasites aren’t shedding enough eggs to be detected by the test.
Different Types of Dewormers
Once you have the FEC results, speak with your veterinarian to determine the correct dewormer for your horse. There are a wide variety of equine dewormers, each targeting different parasites.
| Ingredient | S. Strongyle | L. Strongyle | Roundworm | Tapeworms | Bots | Pinworms |
| Fenbendazole | Resistant | x | Resistant | x | ||
| Oxibendazole | Resistant | x | Resistant | x | ||
| Pyrantel Pamoate | Resistant | x | x | x (double dose) | x | |
| Ivermectin | Resistant | x | Resistant | x | Resistant | |
| Moxidectin | Resistant | x | Resistant | x | Resistant |
Creating a Targeted Deworming Schedule
The standard deworming schedule is no longer effective for managing parasites in horses. There are many factors that play a role in your horses’ parasite control program and deworming schedule, including:
- Parasitic shedding level
- Geographic location and climate
- Individual horse’s age
- Number of horses in the pasture
- Manure management practices
Low egg-shedding adult horses may only need annual or bi-annual deworming at the beginning and end of the grazing season. Depending on your location, medium to high shedders may need additional deworming based on their FEC during the summer and fall months. But deworming isn’t the only element of a good parasite control program.
Some of the important points and recommendations are:
Pasture Management
- Rotate pastures
- Remove manure frequently
- Drag pastures during summer months – exposing larvae and eggs to the sun will kill them
- Rest pastures when possible
- Avoid large numbers of horses in one pasture
- Don’t use the same pastures every year for pregnant mares and foals
Manure Management
- Remove manure from stalls and small turnouts daily
- Keep manure pile away from barns, pastures, and turnouts
Measuring Success – Fecal Egg Count Reduction Test
It’s important to measure the success of your new parasite control program to avoid parasite resistance and high levels of equine parasites in your herd. Parasite counts should decrease 14 days after deworming. You can check the effectiveness of your treatment by doing a fecal egg count reduction test 10-14 days after deworming. If you have a herd of horses, retest at least six from the same group. Parasite resistance is becoming increasingly common, as you probably noticed from the deworming chart, so it’s important to ensure you reduce the number of eggs after deworming.
Starting Your Parasite Control Program
While creating a targeted plan for individual horses may seem time-consuming and difficult, it will save you money on unnecessary dewormers and be more effective. Discuss the best approach for your horse with your veterinarian and begin a targeted approach to protecting your horse, their equine friends, and the remaining effective dewormers.
