How can you tell which deworming program is right for you? ollowing a traditional plan may mean that you are worming your horse too often. Does that matter? Well, apart from spending more than you need to, you also run the risk of the worms becoming resistant to the wormers.

Anthelmintic resistance is being recognised as an increasing problem. How common a problem is it? The short answer is that we don't know. But the number of reports of resistance has been growing.

Most significant is the problem of resistance in the cyathostomins, or small redworms. Resistance among these worms to the benzimidazole group of wormers is widespread. Resistance to pyrantel is common in the USA, but less so elsewhere. And now there are signs of resistance emerging to the third major group of anthelmintics, the macrocyclic lactones (ivermectin and moxidectin).

But it is not only the small redworms that are developing resistance. Large redworms resistant to pyrantel have been recorded. And recently, large roundworms (Parascaris equorum) resistant to ivermectin have been identified in foals and young horses.

Unless we change our worming practices now, we risk serious problems of anthelmintic resistance. The traditional worming programs that involve dosing at set intervals are now seen as not the best way to control worms. By treating horses when it may not be necessary such regimes encourage the development of resistance. They are also expensive.

So what can we do? What is the best way to prevent problems due to heavy worm infections whilst limiting the development of resistance?

Concentrate treatment on wormy horses.

In a group of horses often only one or two individuals are responsible for contaminating the pasture. Some horses are naturally "wormy". Others pass only a few worms eggs in their droppings, even if they are not being treated.

Experts recommend regularly assessing the level of infection. The most practical way to do this is to use faecal worm egg counts. By identifying the wormy horses, treatment can be concentrated on the ones that need it the most. Only treat horses with more than 200 eggs per gram of faeces.

Not treating the lightly infected horses ensures that the eggs deposited on the pasture come from untreated worms. This should slow the development of resistance.

Find out if your horse carries resistant worms.

At least every two or three years, check that the anthelmintics used are still effective. This can be done using faecal egg count reduction tests, in which the egg count is measured before and after treatment.

Use alternative methods of worm control wherever possible.

Picking the droppings twice a week in summer, or once a week in winter, will reduce pasture contamination. Use a low stocking density. Don't have too many horses on a small area.

Consider mixed grazing with cattle or sheep. They can "vacuum up " the equine worm larvae. None of the important horse worms can survive in other species - so if ingested by cattle or sheep they are effectively removed from the pasture and can no longer infect horses.

An ideal worm control program depends on how you manage your horses. Ask your veterinarian for advice. You may well be able to save money. At the same time you can slow the development of resistance and prolong the useful life of the currently available wormers.