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Horse Disease Focus - Worms

Updated on March 6, 2013

Horses and Worms

Horses get worms. In fact, horses get a lot of worms...there are almost 150 species of internal parasite that affect horses.

Wild horses range over large areas. This reduces the chance of a horse grazing close to its own or somebody else's dung and picking up worms. Domestic horses, confined to smaller pastures or stalls, often pick up a significant parasite load. Severe infections cause weight loss, anemia and colic.

Because of this, worm prevention is an important part of managing a horse.

Controlling Worms

Controlling worms is done by using a wormer.

Horses should be wormed every six weeks or so. Wormer is administered orally, either by the use of a powder mixed with a grain feed or by using a syringe to squirt paste into the mouth. In general, the syringe method is preferable. Dosage is more accurate and some horses will refuse to eat feed that has wormer in it. (Presumably, it doesn't taste that good).

Foals and yearlings should be given wormer specially designed for young horses.

Many people recommend rotating types of wormer. The most common active ingredient is ivermectin, but if you only use ivermectin the worms may become resistant. The best practice is to rotate through four or five different active ingredients. Always buy wormer from a reputable vendor and be aware that once a syringe has been opened, the unused wormer often goes bad fairly quickly. Dosage is based off of weight and wormer comes with charts - invest in a weight tape to give a quick and reasonably accurate estimate of your horse's weight. The exact schedule depends on your location, climate and the level of risk of specific parasite species.

In addition to using wormer, picking up dung from stalls and small paddocks regularly helps control worm infection. Harrowing in mid summer can also can expose and kill worm larvae.

Any animals that are turned out with your horse should also be wormed. Cattle, sheep and goats can be infected by the small stomach worm, which also affects horses. Donkeys and mules get lungworm.

Avoid over grazing pasture and if possible rotate pasture. (This is not always feasible, especially at urban barns).


Most equine worms are 'stomach worms' (in fact, they generally live in the gut). Donkeys, however, commonly get lungworm.

A donkey with lungworm will have no symptoms even from a fairly heavy infection. Horses, however, will suffer extreme respiratory distress and can die from the infection. Lungworm is only normally found in horses pastured with donkeys or mules, as the worm cannot breed in horses. A broad spectrum wormer will normally kill it, but mysterious respiratory complaints in horses that are kept with donkeys or mules could be a sign of resistant lungworm and indicate that a change of wormer is necessary.

Treating a heavy infestation

A neglected horse may have a very heavy worm infestation. The temptation is to give them a massive dose of wormer to kill it.

Do not do it! The wormer will kill the worms...and then the dead worms will give the horse severe colic. Many horses have been rescued from bad situations by well meaning people only to be killed by an excessive dose of wormer.

If you have a horse that is in low body condition and has been neglected, work with your veterinarian. The vet may suggest a specific worming program with increasing doses or may give the horse corticosteroids to help reduce the risk of colic.

Things That Are Not Worms

Ringworm is a fungal infection of the skin. It is not a worm, but be careful - ringworm also infects humans.

Some people also think bots are a kind of worm, as they are a common stomach parasite. They're not. They're a kind of maggot, the larvae of the horse bot fly. However, they are vulnerable to ivermectin-based wormers. Botfly eggs are laid on the neck and shoulders of the horse and can often be found and removed when grooming.


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    • jenniferrpovey profile image

      jenniferrpovey 5 years ago

      Good additional points from Ann...particular for those of us who have drafts.

    • Ann1Az2 profile image

      Ann1Az2 5 years ago from Orange, Texas

      Good information. A good idea (at least on the Gulf Coast of Texas) is to get horses wormed by the Vet at least once or twice a year, and then worm them yourself in between. That way, they are apt to become less resistant to wormers, and they get the proper dosing. Importunately, wormers seldom go over 1200 lbs, so if you have a horse that weighs 1400 lbs, you have to buy more than one dose and split one. You also have a problem sometimes with minimum doses. Most wormers start at 500 or 600 lbs. If you have a pony that weighs less than that, you have to be careful not to overdose. Voted up.