How to deal effectively with worms in horses

Posted by Robson and Prescott Vets on May 4, 11 12:19 PM in Bloggers

Emma Pearson and Karmen Watson share their tips on how to deal with worms in horses.

The war on worms ... how can we avoid resistance and worm horses effectively?

Endoparasites, i.e. worms, come in a variety of species. All have specific lifecycles. Like with any warfare, it is important to know your enemy so that you can launch the most effective defence program.

You must strike at the right time or risk wasting your product.

In general it is necessary to worm for tapeworm twice yearly in spring and autumn.

Roundworms must be tackled at regular intervals - follow veterinary and product sheet advice for how often - from June to November.

Small redworm larvae hide in the gut wall over winter, waiting for hatch time in spring.
Specific larvacidal products should be used in late Autumn - eg November - and just prior to spring - eg February - to remove them.

Bots (larvae spread by flies) need tackling in late autumn also.

Most owners find it easier to use a broad-spectrum wormer with several chemicals. Unfortunately, this untargeted approach has meant that there are now a significant proportion of worms out there that are resistant to certain wormers.

The sheep industry is currently struggling with multi-drug resistance and the horse industry may well be in this position if products are not use intelligently. Resistance is when worms are no longer sensitive to drugs that previously would have killed them.

It is possible to encourage resistance by dosing too often but under-dosing and by using a product that does not kill all life-stages of the worm.

Dosing too often selects for resistant worms; under-dosing kills only the weakest worms off, leaving the stronger, resistant worms to thrive; and killing off only the adult worms mean that resistant larvae will hatch out later.

Resistance development must be picked up early to avoid detrimental effects to your horse. The best way of detecting is for your vet to conduct a worm egg count pre- and post treatment.

Your vet will collect a sample of faeces and work out the number of eggs per gram, ensuring that the number of eggs decreases after worming.

You can avoid resistance by following these points:

* Use an effective wormer with no resistance and always dose to the animal's bodyweight. Your horse's weight in kilograms can be calculated by:
1. Measure your horse's girth (around its withers to its sternum) and square this number.
2. Measure your horse's length from point of shoulder to point of buttocks and multiply this by the square of your horse's girth as obtained in step 1.
3. Divide the value obtained in step 2 by 11,877

* Use wormers less often. Different wormers have different lengths of efficacy. Choose a product such as moxidectin that has a 13 week dosing interval. (Consult your veterinarian first to ensure the product is safe for your particular horse).

* Use the principle of 'in refugia'. You are unlikely to be able to eradicate every worm from your pasture so it is better that you settle for a reduced population of worms that are susceptible to the wormers you are using. These worms are said to be 'in refugia' and should not affect the overall performance or health of your horse if kept at acceptable numbers. Worm your horse after turning out onto pasture so that your pasture is exposed to a controllable number of susceptible worms. If you worm before turn-out you are more likely to infect your pasture with resistant worms that will soon become uncontrollable.

* Remove dung from your pasture at least twice weekly so that eggs do not get a chance to hatch or infect your horse.

* Do not overstock your pasture. One to two horses per acre is best. High stocking densities will mean that those horses lower in the pecking order are likely to graze nearer faeces and become infected more easily.

* Graze pregnant mares and foals in large paddocks separate from other adult horses as young and pregnant animals have lower immunity to worms and excrete high numbers of eggs.

* Rest your pasture or alternate with sheep/cattle. Worms are species specific so sheep will eat eggs passed by horses without being infected: 'the biological vacuum cleaning effect'. Some eggs and larvae survive for years on infected pasture so rest alone is not enough.

* Worm any new horses arriving at the yard with a drug with no or very little resistance ask your vet for advice) and keep stabled for 48hrs to allow the worms brought from other yards to be passed and not affect your yard.

* Target your treatment. Only worm when necessary. Get your vet to do a faecal egg count so that you only have to worm those animals with a high worm burden. You can also get your vet to blood test for some species of worm. This will help with the 'in refugia' principle described above.

These tips are only useful if you can get your yard on a synchronised scheme where owners work together. With co-operation, however, you will spend less time and money on wormers, avoid resistance, and have healthier horses. It may be worth asking your vet to come and talk at your yard so that you can be on the winning side in the war on worms.

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