Some sheep producers, particularly those in drought-affected areas, have already been caught out with drench-resistant worms when bringing sheep back onto their property.
In some areas, many properties partially or fully destocked their sheep during drought, but since rain has occurred in these areas, large numbers of sheep are moving back and these have the potential to bring drench-resistant worms with them.
During the long destocked period, almost all worm larvae remaining on home-property paddocks will have died; meaning that any worms brought in by new sheep will be dominant. If these worms are resistant to drenches, the drench resistance status of a farm can very quickly go from a good to a bad situation.
If you want to keep drenches working as effectively as possible on your farm, the best approach is to give a quarantine drench and stop drench resistance from coming home with the sheep.
On properties that do need to drench weaners each year, and occasionally (or more often) other mobs, the drench resistance will make it more expensive and difficult to kill all of the worms.
You won’t know the drench resistance status of the worms in the introduced sheep, so the strategy is to use a number of different drench groups so that no worms survive to establish on your property. Since viable worm eggs will still be passing from the sheep for a few days after a drench, these also need to be prevented from contaminating your paddocks.
The first thing is to avoid contaminating your paddocks before the drenching occurs.
Drench the introduced sheep with at least 4 different drench groups. Importantly, at least one of these should be either monepantel (Zolvix®) or derquantel (with abamectin—Startect®), because they are relatively new products with very few reports of resistance.
This sounds excessive but drench resistance is widespread and especially severe in higher rainfall regions, where over half of all properties report resistance to many of the older drench groups and combinations.
To drench with 4 actives (including one of the new products) will require the sheep to be drenched twice—up the race with one product, then up the race again with the next. The added benefit from this is that missed sheep are much less likely to occur.
See Detail about the quarantine drench at the end of this article.
Leave the drenched sheep in a holding paddock for 4 days after drenching, but ensure they have access to feed and water. Although the drench will have killed the worms, it won’t have killed the worm eggs in the sheep and the 4 days is required for worm eggs to pass out of the sheep.
The holding paddocks won’t be able to be used again for 2–5 months (shortest when daily maximum temperatures are 30°C or more and longest when 15°C or less) to let any resistant infective larvae time to develop and die.
If you bring more sheep in and quarantine drench them, you won’t be able to use those holding paddocks if it is within 2–5 months of their first use, as the sheep will become reinfected with resistant worms from the first group of sheep.
If bringing multiple mobs in over time, the best option is to feed and water them in a yard for the few days. The bare dirt is not favourable for worm larvae to develop and reinfect other sheep and it avoids the problems of managing the holding paddocks.
If you have already brought sheep in without drenching them or only used a single or double combination product, the paddock/s with these sheep could already be contaminated with resistant worm eggs and infective larvae from the worms that survived this treatment.
Firstly, do a WormTest on the mob now using a bulk method where dung from at least 20 sheep is included for testing (talk to your testing lab for details).
If the WEC result is positive, use the quarantine protocol outlined in this article and then move the sheep to a clean paddock. Do not use the paddock/s the sheep have grazed since they arrived for a further 2–5 months.
If the WEC result is negative, you were lucky this time and no action is required.
WormBoss recommends that if Startect® is one of the quarantine drenches, that you also use a double combination drench containing a benzimidazole (BZ or white) and a levamisole group, rather than a triple or quadruple active, as the latter contain a macrocyclic lactone drench active, which is already included in Startect.
If Zolvix® is one of the quarantine drenches, WormBoss recommends that the other drench be either one of the triple actives (containing a BZ, levamisole and either one of the MLs: abamectin or moxidectin) or Q-Drench®, which is the only quadruple active product and it contains BZ, levamisole, ML and closantel (closantel only targets barber’s pole worm).