Rotating Wormers in Horses

Aug 30, 2011
Chris Thomas

Rotating Wormers can Combat Resistance in Horses - C. Thomas

Rotating the active ingredient in a wormer will assist in keeping parasite levels in horses to a minimum. Rotating wormers is therefore important.

As little as 5% of parasites are found in the horse’s body. This leaves around 95% to be found in the external environment. However, this seemingly small percentage can cause a lot of damage to the horse’s body condition. Areas affected can include the:

Liver and lungs
Digestive tract
Eyes and coat

Parasite Resistance and Rotating Wormers

Resistance to particular types of wormers (their active ingredient) has been proven in cattle and sheep. The rotation of the main active ingredient in a wormer is believed therefore to be a worthwhile process to extend the viability of wormers in a horse and prolong the possibility of resistance building up in horses.

Worming pastes often fit into one of two categories: the ‘azoles or BZs’ or ‘mectins’, depending on the active ingredient. Rather than switching the active ingredient each time the horse is wormed; the idea is to worm the horse for one full year on a product that is mectin based, having the active ingredient of ivermectin, moxidectin or another ‘mectin’ in it.

Following that, the product should be switched to a benzimidazole for the 12 weeks of summer, changing to a mectin halfway through autumn, followed by a benzimidazole then mectin in winter and ending the year with doses of BZs. This two year process can than be repeated. One such program is displayed in Virbac’s rotational program guide.

‘Overdosing’ the Horse with a Wormer

Other than not changing active ingredients, the amount of worming paste administered to the horse can do more harm than good if not given in correct amounts. Wormers are administered based on the horse’s weight. This weight is often a guess or estimation.

If underestimating a horse’s weight, the result may be that the owner gives their equine a smaller dose than would be recommended. This can result in parasites being exposed to the active ingredient, but not necessarily killed.

This can then lead to parasites building up a resistance to that active ingredient. Suddenly the act of worming the horse becomes null and void. Because of this, it is recommended by many manufacturers of worming pastes to err on the side of caution by giving a little more than recommended, rather than a bit less. Weight tapes or a scale can assist with determining general or specific weights.

The Other Ninety-Five Percent

If only 5% of parasites are found in the horse, this places an increased importance on paddock management or the external environment the horse lives in. Practices such as manure removal, harrowing and cross grazing can all be beneficial.

The rotation of wormers is good management practice for horse owners to undertake. This combined with care of the environment the horse lives in will help to decrease worm numbers and the detrimental effects they can be responsible for.