Horse Worms

Large RoundwormThe planning and selection of your horses’ worm control program is one of the most important annual management decisions you make for your horse.

The importance of controlling gastrointestinal worms in the horse cannot be overstated. Worms are responsible for a wide range of clinical syndromes ranging from

  • Ill thrift
  • Weight loss
  • Anaemia
  • Coughing
  • Diarrhoea
  • Colic, and in extreme cases
  • Death

Frequently, the horse owner is bombarded with information on various horse wormers but very often, important considerations such as rotation of wormer type and selection to eliminate specific worms are overlooked.

Not all wormers are the same!

What is the Enemy?

Different worms based on their life cycles affect different age groups of horses. To some worms, the horse can build up immunity so they may only be seen in young stock. The following chart is a guideline to the common worms found in the different age groups.


Foals Up to Three Years Adults
Large Roundworm Large Roundworms
Large Redworms Large Redworms Large Redworms
Small Redworms Small Redworms Small Redworms
Tapeworm Tapeworm
Hairworm Hairworm
Lungworm Lungworm
Pinworm Pinworm
Bots Bots


What does each of these worms cause?

Lungworm larva.
Adult hairworm.
Strongylus Egg.


Threadworms (Strongyloides westeri) in foals may cause persistent diarrhoea.

Large Roundworms (Parascaris equorum) in young stock cause lack of thrift and in some cases due to their size may cause complete blockage of the small intestine. Coughing may also be evident as the larval intermediate stages of the life cycle migrate through the lungs.

The Large Redworms (Stongylus vulgaris) live in the large intestine and while the levels of this worm have been significantly reduced in recent years, they cause anaemia and illthrift by their blood sucking activity and as the larval stages go through a migratory stage involving the arterial blood supply to the gut, they have been frequently associated with cases of colic due to interruption to the gut blood supply.

The Small Redworms (Cyathostomes) are probably one of the most important worm groups. They have the ability to hibernate in the large intestinal wall over the winter and then come out en masse in the spring, causing profuse diarrhoea. This may be fatal. Usually affecting young stock this group of worms is showing most advanced signs of restistance to wormers.

The Tapeworms (Anoplocephala perfoliata) have been associated with colic involving the caecum. This can become a very serious life threathening colic.

Bots have been implicated in the development of stomach ulcers.

Lungworms (Dictyocaulus arnfieldi) typically affect the lung and cause coughing and in extreme cases pneumonia.

Practical Advice on Worm Control

Worms can become resistant to the wormer being used. So how do we avoid this ?

Basically there are three classes of wormer.

  1. Benzimidazoles eg. fenbendazole
  2. Avermectins eg.ivermectin and Milbemycins eg. moxidectin
  3. Tetrahydropyrimidines eg. pyrantel

In order to reduce the incidence of resistance, it is wise to rotate the routine wormer annually, eg. using a pyrantel based wormer year 1, an avermectin year 2 and then a benzimidazole year 3. However worming is unfortunately more complicated, in that not all wormers kill all worms.

Tapeworms, bots and encysted small redworm larvae pose particular problems.

Tapeworms are susceptible to pyrantel based wormers at double the normal dosage and to praziquantel wormers.

Bots are susceptible to ivermectin and moxidectin wormers.

Encysted small redworm larvae are susceptible to moxidectin and five day courses of fenbendazole.

Most wormers need to be repeated after a period of six to eight weeks. Moxidectin is effective for thirteen weeks.

During the year dosing intervals should be based on these periods and by seeking further advice from your veterinarian in the selection of wormers for tapeworms, bots and small redworms an effective worm control program can be commenced.

Other Control Factors

Horses that are well minded and on a good plane of nutrition are less susceptible to worms.

  • Do not overstock the pastures with horses.
  • Practice paddock rotation.
  • Remove horse manure regularly from the paddocks, at least once weekly.
  • Worm any new horses arriving on the yard and stable for 48 hours to prevent pasture contamination with its possible worm burden.


The control of worms in the horse is a very important subject. Hopefully, this article will have helped you the reader understand better the complexities involved in controlling these parasites.

Please contact your veterinarian for further advice on appropriate wormers and worming programs for your horse.

Comment is closed.