Worm control

Every horse owner has heard about the danger of worms but without some knowledge it is hard to understand the full impact of a worm infestation (or burden). Intestinal worms can seriously damage your horse and in some circumstances can kill it, even if the burden is not life-threatening your horse may lose condition. Implementing an effective worm control programme is one the greatest responsibilities of a horse owner.

Anthelmintics (wormers)

Dosing your horse with anthelmintics has two beneficial effects: adult worms in the intestine are removed (some wormers also control larvae to varying degrees) and the risk of re-infection is reduced because the number of eggs in the faeces and so infective larvae on the grass are reduced.
Dosing your horse with anthelmintics does have a major disadvantage – worms can develop resistance to the wormers. Just as you have heard about the resistant bacteria that people can catch in hospital, some horse worms have already developed resistance to some of the wormers that are available for you to use so that if you use that drug it will not remove the worms from your horse.
You should ask your vet to test to see if your horses already carry worms that are resistant to a common anthelmintic. This will require taking faeces before treatment and carrying out a faecal egg count, then treating the horses with the wormer and then collecting faeces for a second faecal egg count 7-10 days later. This is called a faecal egg count reduction test, the degree of reduction demonstrating whether on not the worms are resistant to or partially resistant to the wormer. Your vet will be able to advise on whether or not to use that wormer at all.
Because of resistance you should use the wormers as little as possible to reduce the likelihood of the worms becoming resistant to them. Each time you worm your horse you remove all the susceptible worms but if you have worms resistant to the wormer in your horses these will not be removed. Thereafter only the resistant worms produce eggs onto the grass so that your horses are eating more and more resistant worms with the grass until they have so many that your horse is ill and it will be difficult to get rid of the worms.

Ways to use anthelmintics (wormers)

  • Worm only the wormy horses: This will reduce the amount of wormer you use and so reduce the proportion of horses producing resistant worms. This works because some horses always carry large numbers of worms while other horses only carry very few. These horses that have few worms will produce few eggs and so can be recognised by faecal egg counts and these horses don’t have to be treated.Your vet will tell you when to begin, usually in spring. All the horses have a faecal egg count performed. The egg count above which a horse must be treated will be determined by your vet. The high egg counts will be treated and re-examined to see if they need treatment 4-12 weeks later depending on the drug used. The low count horses won’t be treated, but will have an egg count done again 2 weeks to a month or more later, when again they may not need to be treated.
    Some studies have shown that the money saved by not worming some horses pays for the faecal egg counts and with time the savings become greater.
    In addition to being cost-effective this programme has another advantage. While horses that have been wormed will pass eggs produced by wormer resistant eggs in their faeces, horse that have not been wormed will have wormer susceptible eggs in their faeces. When both develop and are eaten by horses they resistant parasites and susceptble parasites will breed together and dilute out the resistance worms.
  • Worm all the horses: Depending on the circumstances at your stables your vet may recommend that all the horses are wormed at regular intervals. He/she will advise you on the required interval between doses. This will vary from year to year as the interval depends on the drug being used in that particular year.

Faecal egg count monitoring

Faecal egg counts are not only used to diagnose parasite infections, they are also an extremely useful and cost-effective tool in monitoring the effectiveness of picking up the faeces or worming programme. Your vet will advise you as to the best time of year to check your horses’ counts. It is good practice to have a faecal egg count performed 2-3 or more weeks after worming your horse. Effective wormers remove adult worms (and some drugs remove some larvae also) and so should reduce the faecal egg count to close to zero for up to 4-12  weeks depending on the drug used.

Effective parasite control is achieved by one or more of the following steps: pasture management, faecal egg count monitoring, worming treatments and biosecurity of your stables. Your secret weapon in the attack against internal parasite is your vet. He/she can advise you on the most effective programme for your horse.
Remember that a small number of worms cause very little harm and indeed act almost as a natural “vaccine” building up an immune resistance in your horse – “a little bit of dirt is good for you”! This will help protect your horse should it later suddenly encounter a large number of worms. It is the large number of worms that is bad for your horse. The control programme you use will depend in part on the management and circumstances at your stable.

The aim of this is to break the cycle of infection by removing the infective eggs and larvae from the pasture.

Removal of faeces

This has been shown to be a highly effective method. The most effective is to remove and dispose of faeces from the pasture every day which is obviously time consuming. In larger stables mechnical sweepers can used. If you cannot achieve this daily, removing the faeces on a regular basis (at least twice a week) is still effective in reducing the numbers of larvae on the pasture.

Pasture rotation

Pasture rotation can also be an effective way of reducing the number of larvae on the grass. The pasture should be rested for at least 6 months to successfully reduce the numbers of larvae. Allowing other livestock, such as sheep, cattle and goats, to graze on the pasture can help to break the lifecycles of the equine worms. The other livestock will eat the horse larvae and eggs, greatly reducing their numbers. There are one or two parasites shared between these livestock and horses so discuss this measure with your vet.
If you transfer your horses directly onto the fresh rested paddock they may contaminate it with worms. Preferably ask your vet to carry out faecal egg counts on the horses before transfer, then you can worm the horses that have worms 48 hours before they are moved on to the fresh, rested paddock. Ask your vet for advice as to which wormer will be most effective. Worming them will prevent immediate contamination of the pasture with the worms they are already carrying.
Other important steps in pasture management include:

  • Keeping the number of horses per acre to a minimum to reduce the amount of faecal contamination and so numbers of infective stages on pasture.
  • Turning out horses of similar age together to reduce exposure to certain parasites and to increase the effectiveness of your worming programme.
  • Supplying hay or grain in a rack rather than feeding from the ground where it could become contaminated with sticky eggs or larvae.

 Management measures for bots

  • Regular grooming – when you spot the small yellow bot eggs take care to remove them completely (cut them off or pluck out the hair).
  • This should be done daily or at least twice a week in the warm weather.
  • This is the most effective measure and method of choice for the control of bots since no drugs are required.

Biosecurity, namely preventing horses bringing worms, particularly worms resistant to wormers, onto your property, is becoming more and more important. You need to discuss this with your vet who will know which worms in your areas are developing resistance to wormers; this will determine the appropriate wormer that should be used. It may be that your vet will recommend using two wormers, one given shortly after the other.
All incoming horses should be stabled, wormed and remain stabled for at least 3 days – all faeces produced in the stable and when the horse is exercised should be removed and destroyed – after this time, the horses can then graze the pastures. Preferably, a faecal egg count should be carried out before, and 7-10 days after worming, keeping the horse in at all times. If the second count is not zero the procedure should be repeated.

Worming treatments can become ineffective over a period of time because the parasites build up resistance to them. To reduce the chance of this happening, it is extremely important to take measures now. If we do not use the available wormers with care the same will happen in horse stables as has occurred on sheep farms in various areas of the world, including the UK. On these sheep farms the worms have become resistant to all the different wormers available – the farmers can no longer prevent the worms causing severe disease and have had to remove the sheep from the land and give up sheep farming!!
To reduce the chance of resistant worms developing you could use a faeces removal programme and so use no drugs. Worming only the wormy horses would help reduce drug use. When rotating wormers it is vital that you not only change the brand of the worming treatment but that you use a product with a different ‘active ingredient’.
It is also important to remember that all wormers offer different parasite control, so you may need to use combinations of products. Ask your vet for advice on treatments as this will also depend on your location, the time of year and knowledge of the drugs to which worms have become resistant recently.

The dosing frequency of worming treatments varies according to the age of your horse, its management, climatic conditions and season. When using a worming programme you should expect to worm your horse every 4-13 weeks. The interval depends on the drug being used. Your vet is the best person to draw up the most effective worming schedule tailored for your horse.
Some useful guidelines…

  • Keep written records detailing the date or worming and the type of product used for each of your horses.
  • Worm all horses that share the same grazing at the same time with the same product.
  • New arrivals should be stabled and isolated for 48-72 hours and wormed according to your vet’s instructions.
  • Mares should be wormed immediately on return from according to your vet’s instructions.
  • Foals can be wormed from 4-6 weeks of age.
  • Always give the correct dose based on the weight of your horse.

Wormers in the form of oral paste or gel syringes or feed additives are very popular because they are so convenient to use. However, some horses find these unpalatable and spit them out. For the wormer to be effective it is essential that the horse swallows the full dose based on its weight. If you find it difficult to worm your horse in this way, your vet will need to administer the wormer via a tube placed directly into the stomach (nasogastric tube).