Understanding deworming

Preventing worm infestation is a priority for horse owners, but far too many people believe that giving your horse a dewormer once every few months is enough to do the trick. Good worm control, however, should begin in your regular yard practice. It is also important to know the worms you’re dealing with and what potential problems they could cause. Our experts help you to understand deworming.


Know the enemy

These are some of the most common worms that infect horses:

  • Tapeworms

This white, flat and triangular-shaped worm infects your horse when the horse eats its first host, a type of mite, while grazing. The worm is still an egg when hosted by the mite, but develops inside the horse and attaches to the intestinal lining where it absorbs nutrients. Eggs are excreted in the faeces. This worm causes a dull coat, colic and general loss of condition.

  • Roundworms

Inside the horse, roundworms move to the liver, heart or lungs. They are coughed up and swallowed back down again to the stomach where they mature and lay eggs. They can grow up to 30cm long! They cause a rough coat, coughing and general loss of condition.

  • Redworms

While grazing in contaminated pasture, your horse ingests the redworm larvae which then burrows through the wall of the large colon and becomes encysted. They are tiny red worms about 1cm long and are highly resistant to treatment. They can cause diarrhoea, colic, weight loss and general loss of condition.

  • Bloodworms

Eaten as eggs, the bloodworms migrate into the blood vessels. They are red worms between 1 and 3cm long, and can cause weight loss, anaemia and colic. In severe cases they could lead to rupture of intestinal blood vessels, damage to the intestinal wall and even death.


Prevention begins with good yard management. Our experts recommend the following:

Don’t put too many horses in the same paddock as this could increase the volume of worms. Clear manure away regularly. Rest your paddocks between grazing, or allow other animals who are not susceptible to the same worms to graze them (for example cattle or sheep). This will break the lifecycle of the worms. Keep young horses separate from older horses as they are more sensitive to worm infestations. About three days before deworming, move horses to a new paddock. This will ensure a varied population of susceptible and resistant worms.

Early detection

Ask your vet to do a worm egg count to find out which horses are actually carrying worms. In that way, you are able to only treat the horses where the number of worm eggs per gram of faeces is more than 200. This reduces resistance to dewormers.

Organise your worm egg count before every scheduled deworming. Once you have tested the faeces, deworm only half of the horses and then test the dewormed horses’ faeces again after two weeks. If there are no eggs present, deworm the other horses the same way. Screen horses every three months and immediately deworm any new horses and then screen them two weeks later.


Regular deworming is a necessity at most yards. Speak to your veterinarian about which dewormer is best for your horses. There are pellet-based dewormers which can be mixed with feed and paste-based dewormers which you administrate straight into the horse’s mouth. If you have requested a worm egg count, you will be much better informed and will be able to choose the right dewormer for your horses.

Text: Peta Daniel