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  What active ingredient controls what
  When to worm
with what
  Pregnant mares, foals and new horses
  Rotation
  Resistance

TO ROTATE OR NOT TO ROTATE?

The idea, each grazing season, to rotate onto a horse wormer with a different active ingredient to delay the onset of resistance actually derived from sheep farming!

  

(Coles G et al, 1999) and large roundworm resistance to ivermectin (Stancham S et al, 2006), you may want to seek guidance from your vet prior to their use.

The only routine wormers without confirmed resistance in horses in the UK are moxidectin-based wormers. As moxidectin belongs to the same chemical family as ivermectin (macrocyclic lactones) once resistance develops to one of these drugs, side-resistance may develop to the other. However, it has been found that of the macrocyclic lactones, moxidectin selects more slowly for resistance than ivermectin (Ranjan S et al, 2002). Therefore, moxidectin-based wormers should be your first choice when using a macrocyclic lactone.
An alternative strategy to rotation:

An alternative strategy is to use a highly effective wormer with no resistance in horses, such as a moxidectin-based wormer. And, by using moxidectin, you can worm your horse as infrequently as possible (13-week dosing interval), while still protecting its health.

In addition pasture management and faecal worm egg counts (FWEC) can also play a role in delaying the onset of resistance.

• Pasture management with the removal of dung    from the pasture helps to remove most sources    of re-infection, including resistant worms    (Wolstenholme A et al, 2004) and so breaking    their life cycle.

• FWEC through extending dosing intervals, only    treating animals when they are producing eggs    above a specific level. It is important to note,    however, that FWEC do not give an indication of    larval burden and more specifically would not    indicate whether your horse is harbouring an    unknown burden of encysted small redworm    larvae.
Situation in sheep farming:

Recent studies of the diseases caused by worms, their treatment and reported instances of resistance has led to an understanding of the rate at which resistance builds up in different species (Cabaret J, 2000).

With evidence that resistance develops at a faster rate in sheep than horses, experience in the sheep industry gives a good indication of how the situation will develop in horses.

In sheep farming the strategy of rotation has not been widely shown to make a significant difference in delaying the onset of resistance. In addition computer modelling has suggested that either alternatives of “no drug rotation” and “frequent drug rotation” make no difference to the onset of resistance (Barnes E et al, 1995).
Rotating! Then what to rotate on to:

If planning to adopt the rotation strategy during the grazing season then it is important to ensure you actually change the active ingredient and don’t just switch brand names, as many brands contain the same active ingredient.

A rotation strategy would be to rotate your routine worm control treatments on an annual basis between a macrocyclic lactone, pyrantel and benzimidazole-based wormer. However, you shouldn’t rotate onto a wormer with known resistance. As such, with small redworm resistance to benzimidazole (Fisher M et al, 1992), small and large redworm resistance to pyrantel
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