Livestock production: Dealing with worms in your horses

Worms can cause extensive damage, without you even realising your horse is infected with these parasites.

Many horses may appear healthy despite having significant internal damage due to worms. Worms rob the horse of nutrients and cause irritation of the gut and loss of condition. They may lead to debilitation, decreased performance, colic, intestinal rupture and death!

Weanlings and yearlings have little resistance to worms and often have far higher worm burdens and excrete more eggs onto pastures. Round worms and thread worms are important in this group of horses. The main source of infection to the foal is his/her infected dam.

This shows the importance of deworming within 12 hours after foaling. Foals and weanlings are much more susceptible to clinical disease caused by worms than adult horses.

Adult horses are usually more resistant to worms, but certain individuals have higher worm loads than others – 80% of the worms can be found in 20% of the stud. Large and small strongyles, tapeworms and bots are important in adult horses.


Small strongyles (cyathostomes) are the most important disease causing parasites in horses. These worms often occur in frequently dewormed horses and they can form a resistant “cyst” in the gut wall that is resistant to most dewormers!

  • Most modern worm control and deworming strategies are now based on combating this particular worm.
  • One female cyathostome can lay thousands of eggs a day, meaning a single infected horse can shed millions of eggs a day onto the pasture.
  • After being swallowed while grazing, immature larvae enter the gut wall and either develop further; or form “cysts”.
  • They may accumulate in massive numbers in the gut wall.

If these larvae develop and emerge simultaneously in winter or early spring, they cause severe damage resulting in symptoms such as weight loss, oedema, diarrhoea and death!

Dewormers containing Moxidectin or Fenbendazole used during late winter in warm climates are effective in removing encysted larvae in the gut wall.

Female large strongyles produce massive amount of eggs and their larvae can survive up to a year on pastures. These worms have a long life cycle and can cause anaemia (loss of blood cells), liver damage and even damage to blood vessel walls.

Luckily, most modern dewormers control this worm and these should be used twice a year to target this worm.


  • Tapeworms are flatworms consisting of proglottids or segments attached to each other almost like carriages on a train.
  • Tape worms have an indirect life cycle, needing mites in the soil to complete their life cycle. These worms attach to the gut wall, causing gut damage, impactions and colic.
  • These worms are difficult to pick up on faecal examination.
  • Praziquantel containing dewormers in autumn or spring help to control these worms.


  • Threadworms (strongyloides) are especially a nuisance in foals.
  • High worm loads could cause diarrhoea and weight loss.
  • The mare could pass this worm on to her foals through her milk.
  • Horses tend to gain immunity as they get older.
  • Both fly larvae and worms can affect the horse’s stomach.
  • Sticky yellow botfly eggs can also attach to horses’ coats by the hundreds.
  • Larvae attach to the stomach, robbing the horse of nutrients, can even cause ulcers.
  • Fly control is an important strategy to prevent this from happening.
  • Dewormers containing the Ivermectin group (active ingredient) kill the larvae in autumn and winter.
  • Lungworms are able to migrate through a horse’s lungs and cause “summer coughs”. Donkeys may be the source of contamination.
  • Pinworm eggs laid around the tail cause intense itching and rubbing. Luckily, most dewormers control this worm.


Establishing an effective parasite control programme should be a management priority – especially in the light of the alarming increase in resistance to dewormers.

Managing worms should be a 2-pronged attack, aimed at preventing the worm from completing its life cycle through pasture management and deworming.

Only 10 % of the total worm population occurs in the horse; 90% is on the pasture!

  • If you regularly bring in faecal (stool) samples for a faecal worm egg count your veterinarian will be able to draw up a programme specific to your farm.
  • A faecal worm egg count gives a good idea of the type of worm and the worm burden/level in the herd.
  • This information helps us to determine when to deworm (if necessary) and with which product.
  • If you deworm when levels are acceptable, you will only increase the resistance problem.
  • Regular egg counts save money on expensive dewormers and help to reduce the resistance problem.
  • They can also be done two weeks after deworming to check how effective the deworming strategy was.


  • Deworm all new arrivals and keep them away from other horses for at least 48 hours.
  • Divide horses into specific groups (adults, yearlings, foals) and treat each group according to their requirements.
  • Conduct regular worm egg counts on the various groups.
  • The main source of infection to the young foal is his/her infected dam. Deworm the mare within 12 hours of foaling.
  • Use young/new pastures for weanlings and foals as these should have lower egg counts than older pastures.
  • Regular (even weekly) collection of droppings significantly reduces the worm load on pastures.
  • Breaking up pats in summer helps dry out and kill infective worm larvae. Use the old rotary hay rake, chains, and tyres.
  • Keep the stocking density or number of horses to a paddock to a minimum.
  • Use annual crops rather than perennial irrigated pastures.
  • Rotate and rest pastures; you could also rotate with cattle.
  • Weigh or estimate your horse’s weight with a weight band to ensure accurate dosing.
  • Underdosing will not work adequately and it could lead to the development of resistance.
  • Overdosing on the other hand may be toxic to young horses, in the case of specific dewormer groups.
  • Keep a record of when all horses were dewormed and the product used.
  • Using a longer dosing interval will help to reduce the likelihood of resistance developing.
  • Consult your veterinarian for a worm programme tailored to the specific requirements of your farm.

Also read:
How to buy a horse

How to feed your horse
Being on top of a horse emergency
Treating tendon injuries in horses

  • This article was written by dr. Marc Walton and first appeared in Farming SA.

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