What do I worm my horse with to overcome wormer resistance?

Before the first worming treatments were available, many horses suffered serious and sometimes fatal disease from parasite infections. In the 1960’s the first horse worming treatments came onto the market; first fenbendazole (marketed as Panacur), followed by pyrantel (the first version known as Strongid P).

Suddenly we had cheap and effective drugs to prevent worm damage in our horses. Administering them regularly had a radical impact on the health of our horses. Ivermectin, moxidectin and praziquantel licences followed through the eighties and nineties and for a short time, equine health was blessed with a complete and effective armoury to protect them against parasites.

Could you be giving unecessary wormers

Fast forward sixty years and we’re in a very different place now. Regular exposure to these five chemicals has helped the worms evolve to become more and more resistant to the drugs. This means they aren’t susceptible to the medication and we can no longer give a wormer and expect it to work. No further wormers have been licenced for horses and there are no more on the horizon as they are costly to licence and horses aren’t a priority.

The table below shows the resistance status of these five main wormers. 

Resistance status of the five main wormers

Click on the image above to open a bigger version as a .pdf

This means we have to be extremely careful with how we handle parasite burdens.

– Not trying to eradicate all worms as this is impossible, rather aiming to manage them at acceptable levels that aren’t going to have a damaging effect on the horse.
– This means testing before treatment to target wormers only where they’re needed to reduce exposure and slow down the development of chemical resistance.
– If we do need to treat, choosing the drug carefully by being aware of which parasites the chosen wormer targets and the time of year.
- Conducting reduction tests to ensure that treatment has been effective.

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