Wormwise Advice: Managing Resistant Worms After Drench Testing

One of the key risk factors for accelerating drench resistance on a farm is the continued use of ineffective products.

Here at Wormwise, we’ve been hammering the messages ‘you can’t tell by looking’ (whether your drench is effective) and to ‘check your drench performance’. But then what?

What’s next if the drench testing news is a story you didn’t want to hear?

If faecal egg counts (FECs) after your routine drench show that there are still eggs in the poo samples, worms are surviving your drench.

Nowadays, the most common reason for that result is resistant worms. However, it’s still important to rule out hiccups with dosage or administration. Occasionally problems with dose calculation, gear malfunctions or administration technique can lead to animals being under-dosed, or missed.

Things to double-check are:

• Was the weight of the animals known and were they dosed to the heaviest in the group, or their individual liveweights? Or was liveweight guessed?
• Was the drench gun delivering the volume it was supposed to? Calibrating guns is an important job and should be done before every drenching episode. A medical/veterinary syringe or laboratory grade cylinder should be used to check the volume, not a jug from the hardware store.
• Were the people doing the drenching taking time and care to do the job properly? Speed does not win the race in the war against worms. Bob Marley is better background music than Iron Maiden!

The pattern of FEC results can be a clue as to whether the eggs left behind are from resistant worms or from animals that have missed being drenched.

In the ‘resistance’ situation, few to many low-to-moderate FECs (depending on how bad the resistance is) are common.

In the ‘missed dose’ situation, mostly zero FECs with one or two moderate-to-high FECs can be more common. If you are using composite FECs, it is not always possible to know which of these scenarios might be
at play.

Another check that can help indicate whether eggs in the poo post-drenching are from resistant worms or a dosing mishap is worm species identification.

This is done at the laboratory (via larval culture or the new GIN PCR test). In sheep at least, the presence of a large range of worm types in the samples is more likely to indicate a drenching slip-up (e.g. animals being missed).

The presence of only one or two species is more likely to indicate resistance. Calves tend to have less of a species mix on board to begin with, but it’s still worth doing this check on them too.

Finally, repeating your drench performance check to a higher level of detail is a good idea. This involves dosing another group of animals and recording FECs and worm species identification both on the day of drenching (what was there at the start?) and then again 7–14 days later (what is being left behind?). Your vet can help you set this up correctly.

OK, so you’ve checked all these technical aspects, and it looks like you have resistant worms. Now what?

There are two main questions:

Is the situation bad enough that I need to get these animals back in now and dose them with something more effective?

What management strategies do I need now?

The answer to the immediate treatment question is ‘it depends’. It depends on how many eggs are left behind, and maybe what species the worms are. Lambs or calves on good feed, growing well and otherwise bright and healthy-looking, with just a smattering of eggs post-drench, may be left untreated until their next drench is due, for which a conversation is needed with your animal health advisor about a more appropriate choice. Remember, the longer you go on using an ineffective product, the more quickly you are building a future problem.

On the other hand, lambs or calves with moderate to high post-drench FECs, or on poor feed (why??) are likely to be better off treated immediately with a more effective product. This is both for their own health, and to cut in on the pasture contamination that they are creating with those resistant worm eggs.