Horses are primarily exposed to worms or internal parasites by grazing on contaminated pastures. Horses shed parasite eggs in their stool. Depending on the parasite, these eggs infect other horses as they graze and ingest the eggs. Some eggs will “hatch” into an infective larval form, which is also ingested by grazing horses, continuing the process of parasite infection.

Some parasites—especially ascarids or roundworms in young horses—pose an infection risk in any environment due to their high numbers, their resilience and the inquisitive licking and nibbling behaviors of foals.

In many adult horses there may be no overt external signs of parasites, which is why performing regular fecal egg counts on your horses is so important.

The “classic” clinical signs that horses have worms are a potbelly, a rough haircoat and generally being unthrifty. These signs are especially evident in foals the first 2-7 months of age with heavy roundworm infections.

Short answer: yes, all horses have worms. But there can be significant variation in the number of parasite eggs that an individual horse sheds into the environment. That’s one important reason for performing fecal egg counts on horses. Based on the results, your veterinarian can then tailor a specific parasite control program for that horse or group of horses.

The important thing to remember is that not all horses need to or should be treated the same for parasites. Most adult horses need a basic parasite control program, while others may need a more intensive program depending on fecal egg counts and other risk factors as determined by your veterinarian. Foals require a different parasite control program because of their age and the specific parasites that, for the most part, aren’t found in healthy adult horses.

Your veterinarian is the best person to help design parasite control programs for all your horses, regardless of age.

The parasites that pose the greatest health risks to your adult horses are small strongyles (and their encysted stages) and tapeworms. In foals, the primary parasite of concern is roundworms (or ascarids).

Other parasites can also be living in your horse, but they don’t pose a significant health risk and are generally controlled with the same products that are used for small strongyles and roundworms. A good parasite control program will address all your horses’ needs.

You may find out what kind of worms your horses have by asking your veterinarian to perform a fecal egg count. This has a twofold benefit: first, it measures how many eggs your horse is shedding into the environment; second, it identifies the types of parasites your horse has. 

There’s one important parasite exception to this testing, and that’s tapeworms. Tapeworm eggs are not consistently found in fecal egg counts. However, studies show that most horses are exposed to tapeworms.¹ For this reason, it’s recommended that all adult horses be dewormed at least once per year for tapeworms. Deworming for tapeworms is done at the end of the grazing season, which typically translates into late fall or early winter. 

Only certain deworming products are effective against tapeworms, so it’s important that you discuss them with your veterinarian.

It depends. There isn’t one deworming product that’s best for all horses in all circumstances. Considerations include the horse’s age and geographic region. Other risk factors can also determine which deworming product is best for your horse. Another important issue is the presence of parasite resistance to certain deworming ingredients.

But in general, for adult horses, most deworming products contain either moxidectin or ivermectin. Another ingredient (added to the moxidectin or ivermectin) called praziquantel specifically targets tapeworm parasites. Currently, the American Association of Equine Practitioners’ Internal Parasite Control Guidelines indicate that moxidectin is the treatment of choice for adult and encysted stages of small strongyles.¹

In foals, where there’s most concern about roundworms, pyrantel or benzimidazole dewormers are the products of choice.

It’s important to consult with your veterinarian about which products are best for your horse or equine operation.

Again, this depends on several factors, including fecal egg count results, age, stocking density, manure management, feeding practices and whether horses are on pastures or kept in stalls.

In general, for adult horses that are shedding low levels of parasite eggs into the environment, one to two treatments per year should be adequate. Treatments should be administered at the beginning and end of the grazing season. The end-of-grazing-season treatment should contain ingredients which are effective against the encysted stages of small strongyles, tapeworms and bots. Adult horses that shed higher levels of eggs into the environment may require one or two additional treatments during the year.

Foals need more frequent deworming during the first year of life because their parasite risks are different than in adult horses. For these youngsters, deworming is recommended four times in their first year. The first two or three treatments target roundworms, while the third or fourth treatments address small strongyles as well as tapeworms as foals age and their parasite risks change.

To understand when and where to use deworming products, it’s important to consult with your veterinarian and work together to set up an effective program for the horses under your care.