We've already discussed the real star performers among equine drenches - the macrocyclic lactones, which comprise moxidectin, ivermectin and abamectin.
It is a highly effective family of drenches and resistance is, to date, limited.
It is tempting to dose only with a member of this family, but we have already discussed the importance of occasional drench rotation to slow the advance of worm resistance.
But even within a family of drenches, the choice can be important. Moxidectin is more powerful and longer-lasting than ivermectin, but that is no reason to head straight for moxidectin.
By doing follow-up faecal egg counts we can determine the effectiveness of each of these drenches.
If your testing shows ivermectin to be highly effective, why use a bigger hitter?
As a general rule, don't use a bigger gun than you need to kill your parasitic prey. If ivermectin is doing the job effectively, keep using it, and keep moxidectin up your sleeve.
It's important to realise that no new drench families are on the immediate horizon. We have to make do with what we have and it's our responsibility as horse owners to use them as effectively as possible, and to minimise the growth of worm resistance.
The four key strategies for minimising the growth of drench resistance in worms are:
- Never underdose a horse, except on veterinary advice (such as trying to gradually reduce a heavy ascarid burden in foals).
- Avoid unnecessary treatments. Drench only when circumstances (such as a new arrival) or faecal eggs counts indicate a need for drenching.
- Never use the same drench family year after year without changing.
- Monitor eggs counts after drenching to ensure that the drench family you're using is proving effective, and you haven't already got a resistance problem on your property.
» Next: The perils of youth
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