Worming drugs explained

Here are the four classes of worming drug explained:

Worming.jpg

1. Benzimidazole

In horses this drug is called fenbendazole and there’s now widespread resistance to it. Approximately 75% of cyathostomins (that’s small red worms to you and I) are resistant to it. What’s more, I’m interested to learn resistance to this drug is worse in the south of the country where race yards and studs have over-wormed on a large scale, for a long time. In rural Scotland, resistance is less of a problem.

Treats: Encysted small redworm larvae, large and small redworm and large roundworm. 

2. Tetrahydropyrimidine 

Rolls right off the tongue this one doesn’t it? The equine version of this drug is pyrantel and the good news is that in the UK, resistance to it isn’t too bad. In the US however, a bizarre move saw this drug introduced in a daily in-feed wormer, leading to complete resistance. This would be like us taking a little penicillin every day to ensure we never got an infection – a surefire way to cause resistance.

Treats: Adult redworm, large roundworm and a double dose will treat tapeworm.

3. Avermectins

In horses these include ivermectin and moxidectin. The latter tackles encysted worms (a job that was once shared with fenbendazole, but resistance to this drug means it’s no longer useful for this).

Treats: Redworm and roundworm (ivermectin), encysted redworm larvae and roundworm (moxidectin). 

4. PRAZIQUANTEL

This drug tackles active tapeworms in horses. 

Treats: Tapeworms only. 


If you’re anything like us you’ll probably be thinking that worming our horses would be much easier if these drugs were easier to remember. With all the dines, trins and zoles it’s no wonder we get in a muddle!

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