Does my horse have worms?

Almost every horse will have worms at some point in it’s life. In fact, most horses have a low level of internal parasites (worms) throughout their entire lives. A small number of worms will not harm a horse, but regular treatment with deworming medications is important to keep the worm numbers low and your horse healthy.



Dead roundworms that were in the small intestine of a horse with a severe infection. Image from Diagnostic Imaging Atlas.

Roundworms primarily affect horses less than 3 years old. As part of their life cycle, roundworms travel through the blood to the lungs, where they are coughed up and swallowed. Then they grow to adults in the horse’s small intestine. The worms live off the food that the horse eats, so they can interfere with your horse’s absorption of nutrients. Horses with severe roundworm infections may cough or have runny noses, they also may either lose weight or have trouble gaining weight. Foals should be treated with a deworming medication every three months until they are 3 years old to avoid a build up of a large number of adult worms.  If a horse that has a high number of worms is dewormed with a regular dose of deworming medication, it can kill all the worms at once and cause an impaction colic.



A large number of bots attached to the lining of a horse’s stomach at autopsy. Image from Diagnostic Imaging Atlas.

Bots are the larvae (babies) of a type of fly. The adult fly lays eggs on the hair on a horse’s chest and legs. The horse licks itself, and swallows the eggs. Once in the stomach, the eggs hatch into larvae. The larvae attach themselves to the lining of the stomach and continue to mature. Eventually, they let go of the stomach and are passed out in the horse’s manure. After they are back out in the environment, they grow into adult flies and start the cycle over. Bots typically do not cause many problems for a horse. If there is a very large number, they can cause irritation of the stomach, and potentially mild colic. Bots are easy to treat, and only require one treatment with a deworming medication in the late fall or early winter to keep populations under control.



A single tapeworm attached to the lining of a horse’s cecum. Image from Diagnostic Imaging Atlas.

Cats and dogs need to eat infected fleas to get tapeworms. Horses need to eat infected mites to get their kind of tapeworms. The tapeworms live in a very specific part of the horse’s intestinal tract – at the junction between the large colon and the cecum (the cecum is similar to a very big appendix). Tapeworm infections are not common, and most horses that do have tapeworms only have a few parasites. However, if a horse has many tapeworms they can cause irritation of the lining of the colon and cecum, and can cause colic. Tapeworms are also relatively easy to treat, and only require a  treatment with deworming medication once a year in the late fall or early winter. However, just like tapeworms in dogs and cats, not all deworming medications will kill tapeworms. Make sure you are working with your veterinarian to design a deworming plan for your horse that will help keep him as parasite-free as possible.


There are two types of strongyles that horses can get – large strongyles and small strongyles. Large strongyles used to be a major parasite of horses, but with good deworming protocols these are much less of a concern now. Small strongyles, on the other hand, continue to be a problem in many horses. The small strongyles will burrow into the lining of the large intestine and form cysts (like scar tissue). With a high number of these cysts, they can cause inflammation and interfere with nutrient and water absorption from the large intestine. Under times of stress, the small strongyles may come out of their cysts. When a large number come out at the same time, they can cause significant damage to the walls of the large intestine and can cause diarrhea and colic in your horse. Horses should be treated with a deworming medication at least twice a year to control both large and small strongyles (typically in the spring and the fall).

Monitoring for Parasites

A test called a “Fecal Egg Count” and a “Fecal Egg Count Reduction” can be performed before and two weeks after giving your horse a deworming medication. The first test will tell your veterinarian which parasites your horse has, and will give a rough idea of how many parasites are living inside your horse. The second test will tell your veterinarian if the deworming medication is working. Many parasites have developed resistance to some of the common deworming medications, and will live through a treatment. It is important to identify these, and to find a deworming medication that will kill the parasites that are infecting your horse.

Some horses are naturally more resistant to parasites than others. If the results of the Fecal Egg Count show that your horse has no or very low numbers of parasite eggs, your veterinarian might recommend that you not deworm that horse at this time. This prevents you from spending unnecessary money on a treatment that your horse does not need. It also helps to prevent the development of further resistance to deworming medications.

Environmental Control

Frequently picking up manure from your horse’s stall and pasture will help to decrease the numbers of parasite eggs and larvae that your horse is exposed to. The manure should be composted, and this will also help to kill any parasite eggs or larvae present in the manure. Most parasite eggs and larvae cannot survive during very cold weather or during very hot, dry weather. Leaving a pasture out of rotation (not using it for grazing or turnout for any animals) during these times of year may also help to decrease the numbers of parasite eggs and larvae that your horse is exposed to.

When is the last time your horse had a physical examination, a fecal exam, or was treated with a deworming medication? Call us to schedule an appointment to have our veterinarian come out to give your horse a full examination, and make some personalized recommendations for a deworming protocol for all the horses on your farm!

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