- Drench (anthelmintic) resistance is a huge and growing problem on livestock farms, particularly with sheep and goats.
- If you have drench resistance on your farm it means that some of the worms on your pasture and in your sheep or goats have developed resistance to a particular type of anthelmintic, so drenching with anthelmintics in that drench family will not be effective in getting rid of these worms.
- Once you have drench resistance in the worms on your farm, it is very difficult or impossible to get rid of it.
Testing for drench resistance/drench effectiveness
- To find out if your livestock are carrying worms that are resistant to the drench you have used, you will need to test the effectiveness of the drench.
- You will need the help of your veterinarian or a farm consultant.
- Testing for drench effectiveness involves laboratory tests on faecal (dung) samples collected from ten animals before drenching (so that you know what worms you have) and again about a week after drenching (when there should be no worm eggs in the samples).
- If worm eggs are present, your veterinarian will probably advise using a drench from a different drench family.
If drenching is not effective
- If drenching is not effective in reducing faecal egg counts to zero, then either you are not drenching properly or you have drench resistance.
- Check your drench technique, ie make sure you are drenching properly (see below).
- If you have drench resistance you will need to drench again using an effective drench from another drench family (see below).
- It is important when drenching to give each animal a full dose for its body weight, because underdosing encourages survival of resistant worms.
- If the animals in the group you are about to drench are very similar in size, you can give them all the same dose, but it must be the correct dose for the heaviest of them. You will need to weigh a few of the biggest to find which is the heaviest, work out the correct dose for this animal and give this dose to every animal in the group.
- If the animals in the group are very varied in size, weigh each animal before dosing to make sure it gets its full dose.
- If the animals are small enough for you to lift easily, you can weigh them using bathroom scales. Carry the animal in your arms and get someone to weigh you with the animal, them subtract your weight.
- You must make sure your drench gun is delivering an accurate dose by testing it before use. Draw up a calibrated dose, ‘drench’ an empty syringe barrel and check that the dose it delivers is the calibrated dose.
Don’t bring resistant worms onto your farm
- Drench resistance can be brought onto farms in introduced stock.
- Brought on sheep and goats should be drenched with an effective drench immediately on arrival.
- They should be held in a quarantine area for a few days and released onto the farm only when their faecal egg count is zero.
What are the three ‘drench families’?
1. White drenches (eg bendimidazoles)
These act quickly to kill worms in the animal’s stomach and intestine, but do not kill any worms in the animal from a day or so after drenching. This means that young animals like lambs and calves will need to be drenched at 4-week intervals.
2. Clear drenches (eg levamisole)
These drenches kill worms quickly with no lasting activity. Their effectiveness doesn't appear to be as great as either the white drenches or third generation drenches, and they should be used only at weaning and in early summer. In autumn when the worm burden on pasture tends to be greatest, a white drench or third generation drench should be used as the clear drenches do not kill larvae that have lodged in the stomach wall (ostertagiasis).
3. Third generation drenches or endectocides (eg ivermectin, abamectin, doramectin, moxidectin and eprinomectin)
These are more expensive than the white or clear drenches but they continue to kill worm larvae in the animal for about 2 to 3 weeks after drenching. Because of this, the interval between doses can be extended to 6 to 8 weeks, which makes the cost per day comparable with other drench families.