Market Hall Vets

Old Market Surgery St Clears Carmarthenshire SA33 4DY

T: 01994 230 451


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Worm infestation is a very common problem for horse owners. Gastro-intestinal worms can lead to loss of condition, colic and in severe cases death. For these reasons it is vital that your horse is routinely wormed.

The main worms that affect horses are:

1) Large and small redworms (strongyles and cyathostomin)

  • Worm eggs contaminate the grass via the horse’s dung.
  • The eggs develop throughout the year into larvae.
  • The horse then becomes infected by grazing on grass containing larvae.
  • The larvae migrate through the gut and other internal organs such as the liver. This causes diseases such as colic which can result in death.

2) Large roundworms (ascarids)

  • Similar to redworms but mainly affect young horses and foals.

3) Tapeworms (Anoplocephala spp.)

  • Horse is infected by eating eggs on the grass.
  • The larvae grow and attach to the horse’s large intestine
  • This can lead to severe colic, reduced gut movement and intussusceptions (one piece of gut stuck inside another piece of gut).

4) Bots (Gasterophilus spp.)

  • Eggs laid by flies on the horse’s legs and chest during summer.
  • Horse becomes infected by grooming itself.
  • Can cause irritation in the gut, but not a major problem.

5) Pinworms (Oxyuris equi)

  • Can affect horses of any age.
  • Horses become infected by eating eggs from pasture or by contacting horses affected with the worms.
  • The pinworms live in the intestinal tract and migrate out of the rectum. When they emerge they deposit a sticky substance around the horse’s anus and tail base.
  • This doesn’t cause a major problem, but is an annoyance as causes irritation and itchiness.

Testing for worms

Faecal Egg Count

  • Useful method of estimating the worm burden by analysing the horse’s dung under a microscope – this is done by your vet.
  • Provides information to indicate whether the horse needs to be wormed.
  • Should be undertaken at least twice a year.

Blood Testing

  • Helps detect whether the horse is infected with larvae. Provides information to indicate whether the horse needs to be wormed.

Worming Control

Pasture Management

Managing the pasture which your horse grazes on is a fundamental part of worm control. Faeces should be removed at least once a week in order to significantly reduce the number of worm eggs and larvae on the pasture. Rotating the pasture on which your horse grazes will reduce the number of larvae/eggs on the grass. This can also be successfully achieved by allowing other animals such as sheep and cattle to graze on the pasture before horses. Undertaking a faecal egg count before your horse is transferred to fresh pasture will provide information on whether your horse needs to be wormed or not. This can be done by your vet.

Anthelmentic Treatment

Worming your horse with the correct worming medication (anthelmentic) is the best way at killing intestinal worms. However it is vital that the correct wormer is used and that only horses with a large worm burden are treated. An inappropriate worming regime will breed resistant worms.

The yearly worming protocol should begin in spring. Before worming your horse you should take a faecal egg count to determine the worm burden. There is no need to worm your horse if the burden is very low - “Only worm high burden horses.” 2-4 weeks after treatment the faecal egg count should be repeated. If the egg count is reduced then the anthelmentic is working. If there is no reduction then there is evidence of resistance. If this is the case then a different anthelmentic is justified.

All new horses introduced to a yard should be quarantined for at least 7 days and wormed. Their faeces should be removed. The faecal egg count should be zero before they are mixed with the other horses on the yard.

Which Wormer to Use?

The three main wormers that are used are:

  • Ivermectins (i.e. Equest® - Pfizer, Bimectin® - Bimeda)
  • Fenbendazole (i.e. Panacur® - Intervet)
  • Praziquantel (i.e. Equest Pramox® - Pfizer) – specifically for tapeworm.

Adult Horses

Routine treatment can be given throughout the year, however the most important periods are spring (February) and autumn (October/November) when there is mass emergence of encysted redworms and tapeworms. It is vital that faecal egg counts are used in conjunction with worming treatment!

Fenbendazole (Panacur®) can be administered as a single dose treatment every 8 weeks throughout the year. A 5 day course of fenbendazole should be undertaken in spring and autumn to prevent mass emergence of encysted small redworm. Ivermectins (Moxidectin®) can be given as a single dose treatment every 13 weeks throughout the year. It can also be used as a single dose treatment in spring and autumn (instead of a 5 day course like fenbendazole). It is vital that praziquantel (Equest Pramox) is also administered in spring and autumn to treat against tapeworm.


Young foals can safely be treated with fenbendazole (Panacur). A single dose of fenbendazole can be administered to treat 2-3 week old suckling foals, suffering from diarrhoea caused by strongyloides westeri. It is vital that foals and youngsters are wormed at regular intervals until two years of age. This is in order to prevent against parascaris equorum infection, which can cause severe colic resulting in death. This worming regime can be performed with fenbendazole treatment at 8 weekly intervals. Alternatively ivermectins can be used at 13 weekly intervals, however these can only be administered after 6 ½ months of age. You CANNOT use moxidectin in foals under 6 ½ months old!

Pregnant Mares

Pregnant mares may be safely treated with a 5 day course of fenbendazole. You CANNOT use moxidectin in pregnant or lactating mares!

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