The drugs don't work

Wormers are rapidly losing their effectiveness, with dire consequences ahead if we don't change our habits. Tim Mair FRCVS explains what every horse owner should know about anthelmintic resistance

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CONTROLLING worms in horses used to be so easy we would regularly give our horses a wormer (anthelmintic) every few months, changing the type of anthelmintic each time in the misguided belief that this would eradicate the worms. Sadly, this wasn't the case and each treatment merely provided selection pressure for a sub-group of worms that were more resistant to the effects of the wormer.

As with antibiotics, horse worm populations have adapted over time, and we now are left in the bleak situation that there is widespread anthelmintic resistance among the common worms that affect horses with no new medications on the horizon. This is a global problem the tsunami of resistance is recognised worldwide.

Every horse that grazes pasture will have worms. This is inevitable, and also completely normal. It is likely that maintaining a low level of worm infestation is beneficial and helps the horse sustain a healthy balance of "good" bacteria in its intestine. Since we started worming horses in the 1960s, the large redworm (Strongylus vulgaris), previously a common cause of life-threatening colic, has become rare in regions where modern wormers are used regularly.

However, the most common type of worm that mature horses harbour is the small redworm (cyathostomins), and anthelmintic resistance is common in these parasites. Large numbers of small redworms in the intestine can result in poor performance, diarrhoea, weight loss and colic, especially in younger horses up to around five years of age.