Horse Dewormers: Uses, Types, and Schedule

A horse dewormer is one of the most important preventative care measures for horses to help combat parasitic worms.

Parasitic worms naturally live in the digestive tract (most commonly the small intestines) of horses and ponies because they spend most of their lives grazing outside.

Parasitic worms only become a medical issue when the number of worms increases to the point where they become burdensome to the horse's digestive tract.

Dewormers work in a variety of methods, most of which aim to either paralyze or inhibit nutrient absorption by parasites which leads to their death and passing through the digestive tract. By implementing an effective horse deworming protocol, worms can be kept at a minimal level and can be well-tolerated within the horse.

High or chronic parasite burdens may lead to the possible medical issues/emergencies: 

• Diarrhea

• Colic

• Gastric ulcers

• Thin/unable to gain weight

• Lack of energy

• Poor performance

• Rough hair coat

• “Pot belly” appearance—especially in young horses

• Itchy rear end/tail

• Skin problems

How Often To Deworm a Horse

Developing a horse deworming schedule should be based on your horse’s fecal egg count, lifestyle, and geographic location. Your veterinarian will recommend a yearly fecal egg count test in order to develop the best horse deworming schedule.

Fecal egg testing is a simple diagnostic procedure that looks at the number of eggs in a fecal sample. It’s calculated by the number of eggs per gram (EPG) of manure.

Younger horses (below 5 years of age) and older horses (above 15 years of age) are at greater risk for having a larger worm burden due to their weaker or immature immune systems. Veterinarians typically recommend beginning a proper deworming schedule and evaluation beginning at 4–6 months of age.

Depending on the number of eggs that are seen under the microscope, the horse will be classified as a low, moderate, or high shedder. Fecal egg counts should be performed at least once yearly, preferably before deworming in the spring. 

How To Collect a Fecal Sample

1. Collect a fecal sample in an airtight Ziploc® bag or container

2. Fresh samples (less than 12 hours old) are best to use

3. Refrigerate as soon as you collect the sample, then submit to your veterinarian for fecal testing

4. Fecal samples should be tested within seven days of collection

It is important to note that diarrhea is not an acceptable fecal sample for testing for a fecal egg count. Frozen samples are also inaccurate, as freezing will damage the eggs and falsely decrease egg counts. 

Horse Deworming Schedule Example

What Is the Best Horse Dewormer?

All horse dewormers can be effective, but it’s important to remember that no antiparasitic will eliminate all life stages present. Dewormers vary by their active ingredients, with each category of active ingredients treating a different type of worm.

In recent years, resistance to utilizing only one type of deworming medication has been observed, therefore it is important to rotate which deworming medication you use to ensure efficacy, based on your horse’s FEC, location, and history.

The best way to protect your horse from worms and choose the most effective dewormer is to discuss your unique horse's fecal egg count (FEC) results with your primary veterinarian.  

Things to consider when you are picking out a horse dewormer include:

• Season/time of year 

• Parasite you are wanting to target

• What dewormer product you used the last time last deworming schedule

• Geographic location 

• Your horse's unique fecal egg count results

Generally, in the fall it’s recommended to deworm with a dewormer that has the ingredient praziquantel added to it (Zimectrin® Gold, Quest® Plus, or Equimax®) in order to deworm against tapeworms.

How Do You Administer a Horse Dewormer?

Deworming horses starts with reading all the manufacturer’s instructions and following these instructions carefully. To administer a horse dewormer, take these steps:

1. Look at the dewormer plunger, you will notice there are numbers indicating pounds. 

2. Spin the stopper to your horse's estimated weight and remove the cap.

3. Place the dewormer in your dominant hand, with your thumb on the plunger. 

4. Stand on the left side of your horse's head and slip the open end of the dewormer in the corner of the mouth.

5. Once the dewormer is in the horse's mouth, you can push the plunger until it hits the stopper.

6. Once the dewormer has been given, you can remove the tube. Your horse will likely lick and chew at the paste or gel in his/her mouth. 

If your horse doesn’t like the dewormer going in his mouth, you may need to take some time to desensitize them to the deworming tube. Practice putting a little molasses or honey on the end of the plunger to let your horse get a taste of something sweet associated with the deworming tube.

Uncommonly, side effects can occur after deworming that may include diarrhea or decreased appetite, or neurologic symptoms such as ataxia (wobbliness) or drooling. These are most common after an overdose or if your horse’s parasite burden is extremely high.

If your horse develops any mild colic signs (decreased appetite, discomfort, etc.), becomes overly sweaty or lethargic, or if you notice any other abnormal symptoms after deworming, it is recommended to call your veterinarian straight away.

Repeat the steps during your horse’s next scheduled deworming, and always reach out to your equine vet with any questions.