Diagnosteq, the parasite diagnostic service operated by Leahurst, University of Liverpool, receives many queries from vets and horse owners about worm control. There is a great deal of confusion about different worm control programmes and about how to use different worming drugs.

This confusion arises from a variety of sources but is made worse by the reluctance to ask for veterinary advice. Quite correctly, worm control is regarded as an essential part of maintaining a horse’s health but it is also costly and time-consuming. How can horse owners do the best for their horses in the most cost-effective manner?

Recognising the need for further knowledge and advice on the subject of worm control in horses, Diagnosteq commissioned an expert panel meeting. A group of experts was assembled, and given the task of formulating advice on worm control.

The experts were a mixed group including a veterinary parasitologist, vets with particular expertise and experience in stud medicine, equine specialists from general practice and a stud owner/ manager.

Their brief was to provide advice that was scientifically sound, but also practical and cost-effective. These are the guidelines the experts came up with:

  • Seek veterinary advice. There is much confusion and conflicting advice about worm control, and best advice changes according to current knowledge. Your vet is the best source of unbiased advice that is relevant to your situation. Custom-designed advice is essential to address the different problems faced by different set-ups.
  • Pasture hygiene. The regular (weekly) removal of droppings is a highly effective means of controlling the spread of worms between horses.
  • Reduce reliance upon wormers. If pastures are kept clean, horses do not need frequent worming. This saves money on medicines and labour and slows down the onset of resistance.
  • Selective sue of wormers. Different types are good at treating different types of worms. Talk to your vet about which to use when.

    Pasture Hygiene

    A recent survey of stud farms revealed that two thirds of studs responding to a questionnaire used pasture hygiene (collection of droppings) as part of their worm control programme. However, many were still using wormers extensively.

    The panel agreed that if pasture hygiene measures were used effectively, it was safe and economical to reduce the frequency of wormer treatments. This has financial benefits and is scientifically sound, as the risk of worm resistance is reduced if wormers are used less frequently.

    Effective pasture hygiene is any management practice that decreases the likelihood of transmission of worms between horses. Many readers will be using pasture hygiene measures without realising it. For example, the collection of droppings from the pasture prevents the infective stages of worms passing from a fresh pile of droppings on to the surrounding grass, where they can be eaten by other horses.

    What some horse owners might not realise is that it may be safe to reduce the frequency of worming drugs if pasture hygiene is good. This can be done safely by monitoring worm egg counts in droppings while worming drug usage is decreased.

    Other hygiene measures are the rotation of pastures to allow horses to graze “clean” pasture, grazing cattle and sheep on the same pasture (these species eat the worm larvae without becoming infected), and ensuring low density of horses on the pasture.

    The experts were also enthusiastic about diagnostic tests to monitor worm burdens. A simple worm egg count on a droppings sample will give information about the number of adult strongyle worms present. A similar test on droppings or a blood test will reveal the presence of large numbers of tapeworms.

    Using this information, a vet can advise whether treatment is necessary, and if so, which would be the most appropriate to use.

    Worm resistance

    The spectre of widespread resistance to worming treatments is looming. Parasitologists have recognised this phenomenon in the worms of sheep and cattle and they expect it to become more of a problem in horse parasites.

    At present, many populations of worms are resistant to the benzimidazole class of medicines. Scientific advice, based on computer modelling and experience wit other species, suggests that the best way to slow down the onset of resistance is to minimise the use of wormers.

    Use them selectively on the horses that really need them and minimise the need for wormers by good pasture management. The treatments we have available at the moment are excellent; let’s preserve their efficacy for as long as possible.

    Whatever worm control programme you use, whether it relies heavily on anthelmintic (wormer) drugs or pasture hygiene measures, periodic testing is useful. Hopefully, this will confirm that your parasite control programme is effective and that your horses are not at risk from high levels of infection.

    If worryingly high levels are detected, it is clear that the worm control programme is not working and needs to be altered. It will also give you time to implement these changes before they lead to colic or other disease.