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Drench Resistance

Worm resistance to triple combination drenches is here! This year we have seen a number of farms
with resistance to triples. That means their animals are still being affected by worms, and have high
egg counts, 7-12 days after drenching with a triple combination drench. Many of these are high-
performing farms with good animal health protocols. Most of them did not think they had a problem
and had not seen lots of dead lambs or massive decrease in growth rate. This problem is not
confined to the Wairarapa but seems to be increasingly common across the country.

So, you have been diagnosed with resistance to triple combination anthelmintics – what do you do

There is no blueprint for what to do once the drenches stop working. However, we do have a lot of
knowledge and a lot of tools that we know will prevent this resistance from occurring. Using these
tools will reduce the effect of resistance once it has happened. We do not know whether we can go
back to a time when your drench is 100% effective again. But these tools will help maintain
production in the face of resistance. Using these tools means you will be able to keep growing lambs
in New Zealand.

These tools include but are not limited to refugia, pasture management, reducing larval challenge in
lambs, avoiding the use of long-acting treatments in adult ewes, improving nutrition, exit and/or
entry drenching, getting store lambs off farm early, changing sheep:cattle ratio, genetics, refugia, oh
and did I mention refugia?

A faecal egg count reduction test (FECRT) tells you which worms are resistant to which drenches on
your farm. A FECRT is the best way to find out if you have resistance. If you didn’t manage to get a
FECRT going this year, drench checks (collecting faecal samples for a worm egg count (WEC or FEC)
7-12 days after drenching) will tell you if any worms are surviving the drench.

Some people are afraid to do a FECRT because they don’t want to know that they have resistance.
With worms, as many biological systems, you don’t get a gradual and incremental reduction in
efficacy. In other words, you don’t see lambs getting slowly a little bit wormier after each drench.
The proportion of resistant worms is small at first, gradually increases, then if you keep using an
ineffective drench it suddenly increases exponentially. Then you get dead lambs. Catching resistance
early means it’s a lot easier to control.

I have heard a number of farmers tell me they don’t need to do a FECRT because they are already
doing all they can to prevent resistance. Most of these farmers are doing very little to prevent
resistance, and in some cases nothing at all! In some cases their worm management practices put
them at very high risk for resistance. In all cases there are worm management tools they could be
using that they are not.

If you would like to know more about worm management on your farm, you can talk to us any time.
I also encourage you to attend a Wormwise seminar, or look at the wormwise website There is lots of good information out there and lots of tools available that
you can implement to control worms, maintain production and limit resistance.

Published Saturday 28th of July 2018

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