Equine Worming

WORMS - general principles to control internal parasites in your horse

The most important worms to be aware of are:

  • Small Redworms (Cyathostomins)
  • Large Stongyles
  • Tapeworms

Of lesser importance:

  • Bots
  • Pinworms

The main wormers available:

  • Benzimidazoles (BZs) e.g. Fenbendazole
  • Macrocylic lactones include: Ivermectin; Moxidectin
  • Tetrahydropyrimidines such as Pyrantel 
  • Pyrazino-isoquinolines e.g. Praziquantel 

Resistance to wormers (anthelmintics) has been a problem since the 1970s and it is vital that horses are wormed appropriately. Small redworms are the most important as they are the most common and can ‘hide’ in the intestinal wall thus making treatment difficult. No resistance in bots or tapeworms have yet been reported. Large strongyles are relatively rare in well-maintained animals. Pinworms are common but easily managed when treating strongyles. Here are some guidelines to slow the development of resistance:

Faecal Worm Egg Counts and Testing for Resistance

  • A small handful of fresh faeces from each horse should be brought to the surgery (yesterday's droppings are too old!) We will examine and offer recommendations (it may not be necessary to treat). Treated horses can be re-sampled 14 days later to check the effectiveness of the wormer. If resistance has developed, another type of treatment should be used.
  • Initial microscopic examination should be carried out. Horses should only be treated when faecal egg counts exceed 200 eggs per gram. Our in-house faecal egg count costs £8.50 plus VAT for the first horse, then £7.20 plus VAT for additional animals and for future tests in the same season.
  • Yards should try to establish whether they have resistance on their premises by doing an egg count immediately before treatment and then again 14 days later. We would recommend testing a group of 5 or 6 horses on the same date.

Benefits of Testing for Resistance:

  • Fewer wormers are used hopefully delaying the development of resistance. 
  • Initial set up costs may be higher than the traditional system but there should eventually be a cost saving on the purchase of wormers.
  • Fewer drugs are administered making this a greener system of managing your horse.
  • Monitoring faecal egg counts confirms the worm control strategy is working well.

Anthelmintic dosing:

  • Ensure correct dose by weighing or using girth tapes. Under dosing contributes to resistance, while overdosing can cause colic
  • Some horses do not need to be wormed as frequently as others, and it is important to encourage natural immunity where possible
  • Despite the foregoing, it is important to clear the tapeworm burden at least annually (sometimes twice a year depending on circumstance)
  • Generally, young horses and foals require worming more frequently than mature ponies and horses

Pasture Management:

  • Avoid high stocking density and over grazing.
  • Pick up equine faeces from the field twice weekly in the summer and once weekly in the winter.
  • Rotate the pastures grazed, particularly by the foals.
  • Mixed grazing with sheep and cattle is beneficial as they can ‘hoover up’ the eggs without harming themselves.
  • Ideally, avoid mixed age groups of horses in the same field where possible.


The most important parasites in foals are small redworms, P equorum and Strongyloides westeri. Most studs tend to administer anthelmintics monthly to foals from 6 weeks of age until 6 months of age and then 6 weekly until the foal is a yearling. This is because foals have no immunity to gastro-intestinal parasites. So check the worm status closely until 3 years of age. In practice the need for routine treatment of foals will depend upon management, the treatment of the mare and any previous parasite problems on the yard. It is a good idea to treat the mare just before foaling to lower her faecal worm egg count, reducing spread to the foal and shedding of Strongyloides larvae in the mare’s milk. Additionally, try to avoid breeding from wormy horses.

There are no new wormers currently being developed for use in horses and therefore we need to use the drugs responsibly.

If you would like further information please do not hesitate to contact us on 01291 672637.

Our Main Vet Centre in Usk is supported by surgeries in Caerleon and Pontypool

Which enable us to provide veterinary services throughout Monmouthshire, Newport and Torfaen including Abergavenny, Gilwern, Goytre, Cwmbran, Chepstow, Blaenavon, Blackwood, Newbridge, Risca, Magor, Caldicot, Tintern and Trelleck.