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Health Feature on Worms
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Most horse owners devote considerable amounts of time and money to controlling intestinal worms. In spite of this, vets regularly have to treat horses with colic, weight loss or diarrhoea caused by worms! The fact is that the worms are very clever at evading our attempts to control them. This article highlights some of the pitfalls of worm control and suggests some solutions to the problems.

Why do we need to control worms in horses?

Because the consequence of not doing so might mean severe disease for your horse. Each parasitic worm of the horse causes a different type of problem. Tapeworms can cause colic, small redworms can cause diarrhoea or weight-loss. What is important to remember is that low levels of infection do not cause problems. It is horses with heavy worm burdens that are most likely to suffer from disease.

Problems with worm control
  • The worming drugs currently available for use in horses are extremely effective. However, horse owners should realise that individual drugs only treat certain worms and that some drugs only treat some of the life-cycle stages of certain worms. A further complicating factor is that some worms have developed resistance to certain worming drugs. The more frequently worms are exposed to the drug, the greater the chance of resistance developing. The bottom line: It is not as simple as giving a dose of wormer and all the worms are killed!
  • Not all horses are equal! Some horses are predisposed to worm infection and are likely to become repeatedly infected with heavy worm burdens. Many horses however, are relatively resistant to developing heavy burdens. With such a variation in individual susceptibility it makes no sense to treat all horses the same.
  • Most horse owners have struggled with the question of which worming regime is best for their horse. Because every horse is different, and because the conditions under which every horse is kept are different, there is no single worming regimen that suits every horse. Your vet is the person best placed to advise on your particular situation.
  • Environmental issues are never far from the news headlines. Did you realise that some worming drugs have a marked impact on the environment? Residual drug activity is present in droppings when they fall onto pasture. This will lead to the death of insects and earthworms that usually break up the pile of droppings. This does not mean that worming drugs should not be used. It does mean that they should be used sparingly and only when necessary.

Methods of worm control
Tape Worm Image

Pasture hygiene

Scientific studies have demonstrated the effectiveness of regular removal of droppings from horse pasture. If this is done twice a week, there is minimal transmission of worms between horses. Other management techniques that can be used are rotation of pasture and grazing sheep or cattle on the same pasture. These species will eat the eggs of the horse worms without becoming infected, thereby acting as "biological vacuum cleaners!

Targeted dosing with worming drugs

By using diagnostic tests, heavily infected horses can be identified and treated. Non-infected horses do not need treating thus saving money on expensive drugs, reducing the tendency for resistance to develop and leading to less impact on the environment. Periodic diagnostic testing is essential for this method of control. Not only does this allow targeting of worming drugs it allows horse owners to check that their worm control programme really is working.

Interval treatment with worming drugs

This technique involves the repeated use of worming drugs at specified intervals. At each treatment, more than 90% of the worms are killed (depending upon the drug used) and the period between treatments is defined by the speed at which the horse develops an adult population of worms in its intestine once again. This regime inevitably involves the treatment of many horses that have only low levels of infection. However, under certain circumstances, it is the only safe approach to worm control.

Diagnostic tests

Until recently, horse owners have been happy to assume that all their horses are infected with worms and to use interval dosing with worming drugs. There is an increasing awareness that diagnostic tests, interpreted by veterinary surgeons, can help identify heavily infected horses and help to target treatment.

  • Do you want to spend less on worming drugs?
  • Are you interested in a "greener" approach to worm control?
  • Are you worried about resistance to worming drugs?

If you answer "yes" to any of these questions then a targeted worming regime may be for you!

A simple worm egg count on a sample of droppings gives an indication of the number of adult redworms a horse is carrying. It will also reveal large roundworm infection. Tapeworm infection has traditionally been more difficult to diagnose. Research work at Liverpool University Veterinary School has lead to the recent development of a blood test for tapeworm infection. Both of these tests are now available to veterinary surgeons through "Diagnosteq," a specialist parasite diagnostic service run by the University of Liverpool.

Benefits of testing for worms
By targeting treatment with worming drugs at only those horses with significant levels of infection, horses and their owners can benefit in a number of ways:

  • Reduced expenditure on worming drugs.
  • Reduced pressure on the worms to develop drug resistance.
  • Reduced environmental impact due to unnecessary use of wormers.

These benefits arise at the same time as owners knowing that worms are controlled in their horses. They know this because they have test results to prove it!

This approach to worm control is not just scientific theory. We know that it works. An increasing number of horse owners and vets are using diagnostic tests to target worming drugs. Most owners who try this method find it to be cost effective and are pleased with the "peace of mind" that it brings them. Diagnostic tests have also been used to help reduce the incidence of colic in yards with worm problems.

Article kindly supplied by:- Diagnosteq, "Solutions to worm control",  Equine Division, Leahurst, Neston, Wirral CH64 7TE 

Tel/Fax: 0151 794 6184 email:

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