The right way to worm

Dr Rosie Naylor (BVet MVetMed DipACVIM MRCVS), RCVS, specialist in equine internal medicine and Equine Technical Product Manager at Virbac UK, discusses healthy worming techniques.

Correct worming protocols are a vital part of keeping sports horses in good health and top condition throughout the eventing season.

The intensive use of wormer treatments over previous years has led to the development of resistance to many types of wormer medicines. It is now imperative that equine worming regimes are designed to identify and target only the horses that specifically need worming and use the most appropriate product when treating them, at the correct time of the year. 


The number of different types of worms that affect horses and the many different types of wormers available for horses can be confusing. A good worming programme for adult horses will include the following, but remember to speak to your vet or a suitably qualified person (SQP) for advice.

During the spring and autumn months worming for tapeworm is recommended. Tapeworm infestation will not be picked up by routine faecal egg counts. However, there are new tests available that can detect antibodies to tapeworm, in blood or saliva. If testing is not performed then a wormer that is active against tapeworm should be given at these times of the year.

Faecal worm egg counts (FEC) should be used every two to three months throughout the summer to assess the roundworm burden. Your vet or SQP will be able to advise you on whether your horse requires worming. Usually, horses with an FEC of more than 200 eggs per gram are treated.

The annual treatment for encysted small redworm larvae is usually given over winter. As there are only two effective treatments for encysted redworm (moxidectin and fenbendazole) resistance is a big concern. Many experts recommend that the use of moxidectin should be reserved for a single annual treatment at this time. Winter treatment should also cover bots, which are the larval stages of Gasterophilus flies. 

The need to administer drugs for worm control can be drastically reduced if sufficient attention is given to pasture management. In particular:

  • Avoid overstocking pasture.
  • Poo pick at least three times per week.
  • Co-graze horses with sheep or cattle.
  • Rotate pastures and rest them annually.
  • Graze young horses separately from older horses. 


Where worming is deemed necessary, it is vital to ensure horses receive the correct amount of wormer for their weight. Unfortunately, visual interpretation of a horse’s weight is very unreliable. Studies have shown that horse owners and handlers tend to under rather than overestimate their horse’s bodyweight. Most people underestimate weight by approximately 20 per cent. This means that many horses may be unintentionally under-dosed with wormer. When a weighbridge is not available, a weight tape can be used.


It is imperative that a horse receives the full dose of wormer to be administered. Any ‘spit-out’ will result in under-dosing. This has several consequences; first, the product will not work as it should, and second, it can contribute to the rapid development of resistant worms. Some worming products are also highly toxic to other pets, so be very careful when disposing of used syringes.

If your horse is difficult to worm with a syringe, some wormers are available in a tablet form, which can be added to the feed. These are also handy for dosing small or larger horses.

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