Your Informative Guide to Deworming Horses

Strongyles (Red Worms or Blood Worms)

Strongyles infect horses after a horse eats the larvae. They develop into adult parasites while they make their trek through the intestines, causing colic, enteritis, anemia, weakness, emaciation, or diarrhea.

Tapeworms

Nearly all horses will be exposed to Tapeworms at some point, typically young or older horses. These infestations don’t often show any outwardly clinical signs but can cause damage to the digestive tract. They do not appear on Fecal Egg Count tests. Products such as Zimecterin Gold and EquiMax will also kill off tapeworms, making those dewormers a great option to do at least once a year, in the spring and/or fall.

Ascarids (Roundworms)

Many young horses suffer from Roundworms as their bodies don’t always have the immunity needed to fend them off in their early life. These worms begin their life in the intestines but go exploring and find their way into the liver, lungs, and throat. Heavy loads of roundworms can cause a pot-bellied look, poor coat, weight loss, colic symptoms, and even stunt a developing horse’s growth.

Bots

You have likely seen these during the summer months and may not have even realized what they were. Bot eggs start out as tiny yellow dots sprinkled across your horse’s legs, chest, and neck after an adult fly lays them there. These eggs will then get ingested from a horse grooming or biting at themselves during fly season, where they eventually make their way into the stomach lining. They can stay there for 8-10 months before the larvae will pass through into manure and start the circle of life all over again.

Physical Symptoms

It can be difficult to determine if your horse has worms, as some horses may not exhibit any noticeable symptoms. However, it’s important to keep an eye out for any physical signs that may indicate a worm issue. Some common symptoms of worms in horses include weight loss, poor coat condition, a pot-bellied appearance, diarrhea, lethargy, and colic. Additionally, if you notice your horse rubbing its tail or hindquarters against a wall or fence post, this could also be a sign of worms.

Behavioral Changes

In addition to physical symptoms, horses with worms may also display behavioral changes. Some horses may become irritable, while others may seem uninterested in eating or drinking. A horse with worms may also be more prone to colic, as well as having a decreased appetite and lethargy. Because behavioral symptoms can be subtle and harder to recognize, it’s essential that horse owners are familiar with their horse’s typical behavior. If you notice any unusual changes that may be indicative of worms, be sure to consult your veterinarian immediately.

Strategic Deworming vs. Rotational Deworming

There are two main ways to schedule deworming: rotational and strategic.

Strategic Deworming

A strategic dosing program considers seasonal differences and when parasites are most likely to reproduce and shed into a horse's environment. Treatments are primarily administered during the active grazing season when parasites reproduce due to favorable weather. However, these treatments are performed without testing, regardless of the horse's age, health, and environment.

Rotational Deworming

Rotational deworming programs, which are also referred to as interval‐dosing or blind deworming programs, use a one‐size‐fits‐all deworming that treats all horses the same regardless of the type and number of parasites, age of horses, stocking densities, or seasonal variation. A rotational program's objective is to inhibit the spread of parasites by treating a horse with a different deworming product every couple of months based on that product’s Egg Reappearance Period.

Types of dewormers and when to use them

The main type of dewormers that remove worms in horses are Febedazole, Oxibendazole, Ivermectin, Moxidectin, Pyrantel Pamoate, Pyrantel Tatrate, and Praziquantel.

There are many popular dewormers for horses on the market, while most of them come in paste form, making it easy to measure the correct dose based on the weight of your horse. After discussing with your veterinarian, you can determine which product would work best either based on Fecal Egg Count test results or the rotational schedule you choose.

Here is a breakdown of what types of dewormers work on different parasites in horses:

How to administer your dewormer

Follow these three steps to properly administer your dewormer:

1. Spin the plunger to the horse's estimated weight and remove the cap. Note that most dewormers are rated for 1,200 pounds, so larger horses may need part of a second syringe.
2. Hold the horse's halter with one hand. Gently slide the syringe tip into the corner of the horse's lips, between the incisors and molars.
3. Inject the paste directly onto the tongue, squeezing out a small amount at a time. Slide the syringe back between the cheek and molars, squirting it toward the back corner of the horse's mouth. Avoid bumping the horse's teeth.

How to manage resistance to dewormers

Resistance to dewormers is common but generally occurs at the barn level, at about six or more horses in the barn. This also means that resistance to particular drug classes varies from barn to barn. So, how to prevent this scenario? The best thing to do is to increase refugia, which is the population of parasites not exposed to the dewormer. This dilutes the resistant parasites, making the dewormers more effective. This is accomplished by deworming some horses less than others and allowing them to shed a small number of susceptible eggs back into the environment. This, of course, only works for horses in good health. Do not attempt this with horses that are elderly or immunocompromised. The horses chosen are determined by fecal egg counts. Consistently monitoring the efficacy of dewormers used in your barn through a fecal egg count reduction test every three years can also help you determine which dewormers will work best for your barn.

What’s the role of the Veterinarian in deworming

As we have stressed throughout this blog, your veterinarian is the best person to talk to about a deworming schedule. They know to let you know which dewormer and what dosage will work for your horse, depending on their health and age. Veterinarians can also do fecal egg counts and the important fecal egg count reduction test every few years to ensure your horses have not built up resistance to a dewormer and can monitor efficacy. Routine conversations about dewormers are important during well visits to maintain communication and proper medical files.