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To Worm or Not to Worm?

Historically most horse owners have wormed their horses every few months, (so called ‘interval dosing’), without checking the worm burden of the horse first.

However evidence is increasing all the time that we should change our policy:

  1. Worms are becoming ever more resistant to all the groups of wormers, primarily due to the overuse of wormers. There are no new wormers on the horizon, so we need to use existing wormers as efficiently as possible to minimize the development of resistance.
  2. Horses do not need to have a zero worm burden. A low worm burden helps stimulate immunity and is probably beneficial. A faecal worm egg count (FWEC) of 100-200 eggs per gram (epg) is considered acceptable by most clinicians.

In several recent surveys 85-90% of faecal samples had an egg count below this threshold and therefore worming was unnecessary.

Targeted Dosing

A better way of worming is to do regular FWECs and only worm horses with a significant worm burden (‘targeted dosing’). Even allowing for the extra costs of egg counts this works out cheaper for most horses as it reduces the use of wormers. It also slows the development of resistance and is better for the horse population as a whole.

Our findings agree with recent surveys that showed that 80% of worms are carried by only 20% of horses i.e. in a group of horses all kept under the same conditions only one may have a high worm burden. FWECs can identify these horses which need to be tested more regularly. They have a lower immunity to parasites and will probably need to be wormed more frequently than other in-contact horses.

How do you implement a targeted dosing programme for your horse?

Programmes vary slightly depending on individual circumstances. Initially FWECs should be performed at 3month intervals. In small yards with good pasture management and a low worm burden the interval can increase to every 6 months. Larger yards may need to continue on more frequent testing. Fresh faecal samples (only 5 grams, not a whole rose bed full!) need to be dropped off or posted to the practice.

What are the limitations of targeted dosing?

FWECs do not quantify a horse’s tapeworm burden although segments appear intermittently in faeces. Tapeworm burden can only be assessed using a blood test so in practical terms it is easier to interval dose with a tape wormer once or possibly twice per year. Horses pick up tapeworms during the grazing season, so the autumn is the best time to treat them. An untreated tapeworm burden may cause colic.

Egg counts do not detect immature, encysted worm larvae which are not producing eggs. If the horse’s worm history is unknown, especially young horses which are more susceptible to encystment, initial worming is recommended.

If the horse does not get wormed because it has a low egg count you will not remove bot larvae that live in the horse’s stomach over the winter. Bots may not be pathogenic but most owners prefer to remove them.

So what year round worming policy do we recommend?

  1. All new horses and horses with an unknown worming history should receive a combined round/tape wormer e.g.  Equest Pramox , Eqvalan Duo.
  2. Thereafter horses should have regular FWECs. The interval can increase in yards with a track record of low burdens. N.B. All horses in a yard should be tested , as they may have widely varying worm burdens, due to differing levels of immunity to worms.
  3. All horses should be wormed in the late autumn with a combined round/tape wormer to remove bots, tapeworms and any adult roundworms.
  4. 4. For many horses once a year worming will be sufficient. Horses identified with higher worm burdens will need to be wormed more frequently.

Please contact us if you have any questions about worming!

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Pete has looked after our event horses over the past few years and has played a crucial role in helping me to achieve my goal of competing at the London 2012 Olympics with ‘Happy Times’. The Dorset Equine Team are highly professional and a pleasure to work with.

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