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Resistance to Wormers

In recent years, resistance to worming products has become more common, particularly in small redworm (cyathostomin) populations. This is now widely regarded as a serious problem because there are no new products to turn to if worms develop resistance to the drugs currently on the market. Resistance is the result of over usage of wormers as part of the traditional 'Interval Dosing' worming programmes.

The best way to slow down the development of resistance is to minimise the use of wormers. This is achieved in two ways: 1) Good pasture management - regular collection of droppings is the safest and most economical way to reduce the worm burden of your horse and therefore reduce the need for worming products. 2) Measuring the worm burden of your horse with worm egg counts and deworming accordingly when required.

Distribution of worms It is normal for all horses to carry a small number of intestinal parasites. However it is a common misconception that all horses grazing the same pasture will have a similar worm burden. Studies have shown that within a herd, 80% of the worms live in 20% of the horses. This means that many horses are being over treated with worming products, contributing to the spread of resistance and costing money where it could be saved.

Therefore its is essential to consider each horse individually when deciding on a worming program and targeting treatment at those that specifically need it. Worm Egg Counts. Worm egg counts can be used to identify the horses that are carrying a high worm burden and contaminating the pasture.

These horses are treated with a worming product and the other horses are left untreated. This reduces the use of worming products and therefore the likelihood of resistance on the yard. It is recommended that worm egg counts are initially done at 3 month intervals. It is important to remember that worm egg counts only detect adult worms and are not sensitive to tapeworm.

A tapeworm product should be used in all grazing horses once per year, regardless of worm egg count results. If you have not been in control of the horse's worm program continuously over a two year period a larvicidal treatment should also be given annually.

Your regular Vet should be able to do worm egg counts for you, and if not then there are many Companies who offered the service by post.

You will advised of the results and from this will be advised whether your horse needs to be wormed or not.

Worm egg counts save you money and reduce the danger of drug resistance.


Remember that the most effective and economical way to minimise the need for wormers is with good management practices.