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Reducing Resistance to Horse Wormers

Unfortunately, resistance of equine worms to chemical wormers is becoming more common. Resistance means that the chemicals contained in certain horse wormers no longer effectively control the worm population.

Resistance develops due to the small genetic variations that occur within a worm population which lead to some worms naturally being more resistant to chemical wormers than others. Therefore, when a horse is treated with a wormer all of the worms which haven’t evolved with this advantage (susceptible worms) are killed, leaving only the worms which are resistant. These resistant worms then pass on their resistance, creating a wormer resistant population. This is an example of natural selection.

Resistance may be increased when too low a dosage of a wormer is used for the weight of the horse, or if the wormer is used too frequently. To help reduce the incidence of resistance we should:


  • Use faecal worm egg counts to determine whether a wormer is needed and which one to use.
  • Weigh (or weigh tape) our horse/s before dosing in order to avoid under dosing.
  • Rotate the active ingredient/class of wormer used for each grazing season.
  • Target specific worms with an effective product at the correct time of year.
  • Ensure good pasture management- poo pick fields twice weekly.
  • Cross graze pasture with other species, for example sheep.
  • Avoid over-crowding of fields
To reduce the incidence of resistance good pasture management is essential.
To reduce the incidence of resistance good pasture management is essential.


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