Horse Worming

There seem to be so many different horse wormers on the market that it can be difficult to decide which wormer to use when. Well intentioned but sometimes incorrect advice from friends on the livery yard can often add to the confusion. In order to help our clients choose the correct wormer at each time of the year we have produced the following guide.

This parasite control programme aims to:

  • Maintain your horse’s health by targeting the correct parasites at the correct time of year.
  • Give you tips on how to keep pasture as clean as possible to reduce the parasite challenge to your horse
  • Advise you on the correct rotation of wormers to reduce the chances of resistant strains of parasite developing.
  • Give you some general rules to help you develop your own worming routine.

There are many different types of worms ( and fly larvae) that can threaten your horse’s health.

These include:

Small Redworms (Small strongyles/Cyasthostomes)

These are now the most common worms found in horses. They are found in the large intestine and can migrate to the gut wall where they will encyst. Mass emergence of previously encysted small redworms causes a disease known as larval cyasthostomosis which can potentially kill a horse. Signs include sudden onset diarrhoea, weight loss, colic, oedema (“filling”) of the hind legs and sheath, anorexia and depression.

Large Redworms (Large strongyles)

Adult worms are found in the large intestine but larval stages migrate in a horse’s blood vessels. If the blood supply to a horse’s intestine is compromised, colic (sometimes fatal) can occur. Damage caused by large redworms is now much less common than it used to be due to the availability of effective wormers.

Tapeworms (Anoplocephala species)

Tapeworms attach at the junction of the small and large intestine. Research implicates tapeworms as a cause of potentially severe colic including fatal perforation of the intestine. Highest infection levels are found in horses under two years old and over fifteen years old. Tapeworm eggs are shed in a horses faeces and eaten by forage mites. If your horse swallows an infected forage mite, adult tapeworms may be found in your horses intestine within one or two months.

Bots (Gasterophilus)

These are actually flies, not worms, and often give owners cause for concern over the summer. Bot fly eggs are laid on horse’s legs. The larvae which hatch either crawl into the horse’s mouth or are transferred by licking. They eventually arrive in the stomach where they remain for the next 10 -12 months. Bots can cause damage to the stomach lining. Physical removal of bot fly eggs from your horse’s coat is an effective way of controlling bots. Administration of an ivermectin or moxidectin wormer such as Eqvalan, Equimax or Equset in the winter is also effective.

Keeping Pasture Clean

Where there is extensive grazing and limited horse numbers under the control of a single owner, pasture management can be very effective at reducing levels of parasitism. Unfortunately most of our clients have limited access to grazing, or share livery yards with a changing population of horses and owners. Although the following precautions will help to control parasite burdens on pasture we must emphasise that they are NOT a substitute for an effective worming strategy.

  • Try to remove horse droppings from pasture regularly – twice weekly if possible.
  • Divide paddocks into smaller areas to allow rotational grazing and make it easier to pick up droppings.
  • Try not to over-graze paddocks – ideally no more than 1 – 2 horses per acre.
  • Grazing pasture with sheep or cattle will reduce the level of horse worm larvae. Horses will still be exposed to some cattle or sheep larvae but these are of limited risk to horses and are easily treated.
  • Harrowing to allow droppings to break up and dry out (thus killing the larvae) is not usually successful in our climate!