Farm Ireland

Friday 19 January 2018

What drives parasitic resistance to horse wormers?

Resistance occurs when a parasite population that was previously controlled by a drug is no longer susceptible to it.

Maintaining the effectiveness of the wormers we currently use is the basis behind the 'manage, test, plan and dose' approach to worming.

Resistance develops due to artificial selection by horse wormers. The active ingredient of the horse wormer kills the sensitive parasites in the population. However, those parasites not affected go on to create new generations of resistant parasites.

When this resistant population of parasites reaches a certain level, there is said to be 'resistance' to that active ingredient.

What drives the development of resistance?

There are a number of factors that may be involved in the development of resistance. The two most important factors are:

1. The level of exposure to a specific wormer. The more often a wormer is used, or the shorter the intervals between dosing, the more parasite generations are exposed to the drug. This exerts greater selection pressure for resistant worms.

2. The correct use of a wormer. Incorrect use of a wormer can lead to under-dosing.

Also Read

Parasites that are exposed to a wormer at an insufficient dosage to kill them may survive and pass their "immunity" to subsequent generations.

Correct use of the right wormer at the right time will help maintain the health of your horse and the effectiveness of the wormers that are available.

For more information and advice on worm control, check out

Irish Independent