A new way forward

Worm control in horses is vital, but established methods will not remain effective for much longer. Tim Mair FRCVS explains why and how we need to change

- Tim Mair

THE recommendations for worm control in horses are changing. After years of relying on wormers (anthelmintics) to control the common equine worms in mature horses (small redworms and tapeworms), it has become apparent that the “worms are turning” and resistance to the available wormers is widespread, with reports growing in number at an alarming rate.

There are three main classes of wormers that can be used to control small redworms:

• benzimidazoles (fenbendazole)

• tetrahydropyrimidines (pyrantel salts)

• macrocyclic lactones (ivermectin, moxidectin)

Resistance to all three classes has been observed in redworms in the UK.

Resistance can potentially cross between drugs in a single class so resistance to ivermectin could result in resistance to moxidectin, and vice versa. In addition, resistance to the fourth wormer class (praziquantel), used to treat tapeworms, has also recently been reported.

Resistance in equine worms is caused by widespread overuse of wormers – every time we give a horse a wormer, we increase the risk of resistance developing, and once a worm population is resistant to a certain medication, it is likely to remain resistant.

There is also growing concern about the environmental damage that wormers cause – residues of these drugs pass out in the droppings of treated horses and can be lethal to insects on pasture (including dung beetles, who break down dung and so help maintain healthy pasture by returning nutrients to the soil).