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Drenching to avoid drench resistance in your stock

microscopeHow do internal parasites work?

Most adult round worms live in the gut of the animal, (usually the small intestine) where they suck the animal’s blood, reproduce and shed eggs that pass out in the faeces on to the pasture. When conditions are favourable (wet and warm) the eggs hatch and the larvae climb up the pasture plants and are eaten again by the animal.

Some worms live in the lungs (lungworm), and liver flukes live in the tubes of the liver. The head of a tape worm is attached to the gut wall and it sheds body sections on to the pasture. Each section is full of eggs.

What is drench resistance?

This is when internal parasites of animals become resistant to the chemicals in the drench – so the drench fails to work and the parasites continue their damage.

Is the problem getting worse and why?

Yes it is getting worse, and the reason seems to be that people have drenched rather indiscriminately in the past. If stock were scouring or were not thriving, it was always assumed that a drench would stop the problem. What has surprised many people is how quickly parasites have developed resistance to some of the drench chemicals.

What are the drench chemical families?

There are three "action families" in drenches. These are the:

  • Benzimidazole group
  • Levamisole/Morantel group
  • Milbemycin/Avermectin (Moxidectin) group.

All "broad spectrum" drenches produced have a wide range of action and belong to one of these chemical families. So you need to read the label to see what chemical family is in the drench. The commercial name on the drench container – whether it’s an oral drench or a pouron may not make this immediately obvious. Clearly the commercial names have to be a lot easier to read than the chemical names. The chemical names will appear in the small print on the container.

Will new drench families be on the market soon?

No! Currently there are no new chemicals coming on stream, and this is the real worry. Finding new chemical families is going to take time and large amounts of money. So it’s essential not to let drench resistance develop any further in your stock.

How do you know if your stock have drench resistance?

You may not, unless you test for it. All you’ll notice is that stock will continue to show ill-thrift and scour after drenching.

How do you test for drench resistance?

You have to do a faecal egg count (FEC) before you drench, then you drench, and then you do another FEC (7-10 days after drenching). This will show if the parasite eggs have been reduced. If they have, then the drench has done its job. You can get a FEC done at your vets for about $10/sample (from one animal or a composite from a few), or you can purchase a FEC kit to do your own, or share with your neighbours.

The other alternative is to slaughter the animal and check the worm count.

Can you bring drench resistance on to your farm?

Yes you can, by buying in new stock or bringing stock back from grazing. They may carry resistant worms, and they’ll pass eggs which will re-infect your stock and reproduce with your drench-resistant parasites in the gut of the animal.

How can you reduce drench-resistant worms entering your farm?
  • Drench all introduced animals on arrival with a second generation milbemycin/avermectic (moxidectin) group drench.
  • This includes your own stock that have been away grazing.
How can you protect your stock from drench resistance?
  • Discuss your drenching programme with a veterinarian or animal health adviser.
  • Don’t be tempted to buy drench because it’s on a "special" at the vets.
  • Drench only if needed. Determine this by a feacal egg count (FEC).
  • Change to a different drench family each year – read the small print on the drench.
  • Don’t change drench families without checking that the new drench will be effective.
  • Don’t drench by the calendar – parasite burdens are dictated by the season and vary from year to year.
  • If you have used a drench and it clearly has not worked, seek veterinary advice and consider a drench resistance test.
  • Young stock are most vulnerable to internal parasites – try to run them on clean pasture that has not been grazed.
  • Older mature stock are more resistant to worms, so should not need drenching. Take veterinary advice.
  • The drench dose should be based on the weight of the animal. Drench to the heaviest in the mob to be sure not to under-drench the small ones.
  • Make sure the drench gun is working properly and delivering the correct amount.
  • Do not force the animals to accept the drench gun, as they may not swallow correctly and the drug may be diverted past the rumen where it is absorbed. This is especially important for the benzimidazole and milbemycin/avermectin groups.
  • Avoid drenching stock with a full gut. Let them empty out for about half a day before drenching.
  • Take special care to drench all stock coming on to your farm with an appropriate drench.


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