Resistance to wormers is are large potential problem. It is the worms themselves that become resistant to the wormer and not the horse.
How does resistance develop?
Within a population of worms there will be a variety of genetic differences as there are within any population. Mutations can happen and this may lead to a worm having an advantage as it may not be killed by the wormer given to the horse. Mutations are random and many of them may have no effect on the worm or may make it not survive as well, however, some mutations may give the worm an advantage. This is effectively evolution and they are increasing their ability to survive in the environment. When a wormer is given to the horse it will kill most of the worms, if there are worms present with an advantageous mutation then they will survive to reproduce and may pass on this genetic advantage to their offspring. Therefore the next time the horse is wormed a larger percentage of these worms will not be killed.
Types of wormers
Different types of wormers work in different ways to kill the worm. Therefore if a worm becomes resistant to one type of wormer a different one can be effectively used. However, there are only 3 families of wormers used in horses and problems arise when the worms become resistant to all of these types. It also takes a very long time for new drugs to be developed for use which increases the problem.
Resistance is already developing
In some livestock species there have been reports of worms that are resistant to all types of wormers. Resistance in the horse is not as common yet in the horse but there is evidence that it is increasing. For example, resistance has been found to ivermectin on some farms in Italy from Parascaris equorum.
The test they use to test for resistance is a faecal egg count reduction. They will carry out a faecal egg count, worm the animal then carry out another test to see how much the worm burden has decreased. However, there is a lack of a definition for resistance in horses.
How can we help prevent resistance developing?
Worming horses with the correct dosage will help to prevent resistance developing. This is due to the worms not coming into contact with a small dosage of the wormer but not enough to kill them which may lead to them developing resistance.
In Denmark there are restrictions on the use of wormers and they have found little evidence that resistance is happening in horses there. Therefore restricting the use of wormers may also help to prevent resistance developing. It is best to worm horses only when they need to be wormed and not on a scheduled programme where they are wormed on a certain date whether they need it or not. However, this may be hard to put into place on large livery yards.