Worming Resistance

How many times have you heard: “The wormers don’t work-I’ve wormed regularly every 6 weeks as I was told to and there are still worm eggs in the dropping samples.”

This is now commonplace, resistance to anthelmintics is with us. It is not too late for veterinary surgeons, horse owners, educators and pharmaceutical companies to take responsibility for this issue and safeguard the health of our horses.

Many of the activities that have been commonplace may contribute to anthelmintic resistance; poor pasture management, underdosing, repetitive unnecessary interval dosing with various different anthelmintics.

Worm resistance

The interval dosing programme that was originally advised 40 years ago increases the chances of anthelmintic resistance, thus increasing the numbers of eggs shed onto pasture despite dosing with wormers.

The majority of wormers will only kill the adult stages of the worm and not the larval stages that are responsible for clinical disease.

Horses are very variable in their susceptibility to parasite infection resulting in extreme differences in parasite burdens among horses.

The main parasitic pathogens affecting mature horses now are the cyathostomin (small strongyle) and the tapeworm.

We can all take several steps to help reduce the worm population and therefore improve our horse’s health.

  • Know what worms are important in your situation
  • Understand the life cycle of the worms that you are dealing with
  • Know at what stage of the lifecycle you should worm your horse
  • What wormers to use
  • Biosecurity for new horses coming onto the farm
  • Pasture related tactics to reduce the pasture contamination
  • Don’t opt to reduce the worm population to zero

What can we do?

  • Worm egg counts are our most valuable ally
  • Worm egg count will indicate whether you need to worm and which worm species is involved
  • Worm egg reduction counts after worming will indicate the level of resistance in your population
  • Worm egg reappearance counts will help to formulate a plan and can help in detecting early signs of resistance
  • By adopting a strategy of pasture management, testing and timing the worming regime we can help to reduce the amount of resistance that is developing in our horse population.
  • Pasture management includes removing the droppings, cross grazing, resting the ground and taking hay off the paddock
  • Testing involves the use of worm egg counts and although they are only a snapshot of the actual state, and may well have inaccuracies, they will indicate horses that do have a worm burden and these horses can then be treated, thus reducing the number and cost of treatments. Remember that the worm egg count does not detect the encysted larval stage.
  • Structured worming to take into account the time of the year, the age of the horse, pregnant mares or horses recovering from an illness.
  • Over time by adopting a more structured approach we can reduce the treatments given to horses relying more on management of individual cases and a knowledge of the local factors on the farm. This approach to worm management requires advice and support from vets, suppliers and land management experts.
  • This will lead to economy with less use of wormers that are more effective and increased food utilisation.

If you recognise that worming your horse does not appear to be giving the results expected then it is wise to check that the worm egg count is still high and that there are no other reasons for the signs that you are seeing.

If there appears to be a resistance problem contact your vet or retailer to discuss the recent wormer history and get advice on what wormers appear to be working best in your situation and area.

Wormer resistance varies between horse, yard and area of the country and advice as to what strategy to use must be tailored to your horses’ specific requirements.

Blindly worming with several different wormers will only increase the resistance in the local worm population.

The inclusion of refuge horses that have a population of worms that are not exposed to wormers is important.

  • Dose correctly for body weight
  • Age range of population
  • Mobility of population
  • Pasture hygiene
  • Non-uniform dispersal of parasite population (80/20)
  • Immunity never 100%
  • Most (80%) adult horses will have low level infection
  • Variable parasite susceptibility
  • Target treatment of animals based on FWEC
  • Actively look for AR with FECRT
  • Select products carefully and “rotate” annually
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