Drench resistance can occur in very dry areas and this is mainly due to:
Selection for drench resistance happens when worms in a sheep are exposed to a drench. Some worms can survive a drench active because they have genes conferring resistance to that active. This may initially be just one worm in 100,000 or even 1,000,000 worms. Some worms may be partly drench-resistant: they can survive lower (sub-lethal) but not full doses of the treatment.
Worms that survive treatment produce eggs that give rise to infective larvae on a pasture. These are eaten by sheep and so the worm life cycle continues. In this way each treatment causes an increase in the proportion of the worm population that is either partly or fully drench-resistant.
If resistance to a drench active is already present, it will likely remain, even if the drench group is not used for years. Drench resistance probably cannot be prevented, but the rate at which it occurs can be greatly reduced.
The first step is to know what drenches are effective on your property.
Each property has its own drench-resistance profile based on its own drenching history and that of properties from which sheep were sourced. The profile of neighbouring properties can be quite different.
The extent of resistance is only known by testing. Obvious worm control failures may only occur when resistance is quite advanced. In this region, a DrenchCheck-Day10 is the preferred method to check individual drenches at any time. DrenchCheck-Day10s should be considered when any drench is given and it is the most practical and cost-effective method of testing drenches in this region.
While a DrenchTest or Worm Egg Count Reduction Test (WECRT) is the most accurate test for drench resistance, this test is rarely feasible in this region as infections are often not high enough and when they are, they may be unexpectedly high and need swift treatment or are in lambs at weaning, which should not be put at risk in a DrenchTest.
This simple and inexpensive test gives an indication of drench effectiveness.
The DrenchCheck-Day10 involves two WormTests: the first up to 10 days before drenching (usually at a routine WormTest time) and the second between 10 and 14 days after the drench.
The results from the two WormTests are compared to gauge the extent that worm egg counts have been reduced by the drench. Discuss the results with a worm control adviser.
For more detail see the fact sheet ‘Checking for drench resistance with a DrenchCheck-Day10’.
Keeping drench-resistant worms out of your property is part of sustainable worm control.
Assume that purchased sheep are carrying worms with some degree of drench resistance to one or more drench groups. See Drench groups and actives.
If sheep have come from high rainfall (>600 mm) or irrigation areas in eastern states, consider a liver fluke treatment using triclabendazole.
Use all 3 principles where possible.
They are equally important and greatly slow the development of drench resistance.
A small benefit can be gained by rotating drench groups providing you also rotationally graze stock across the property so that paddocks are exposed to sheep that have received different drenches. However, if you set-stock, drench rotation will not slow the development of drench resistance.
While not affecting resistance, it is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the animals may go to slaughter, or their milk may be used for human consumption.
Search for drenches based on the worms or other parasites targeted, drench group or active and product name.
Follow all 5 principles where possible:
1. Avoid unnecessary drenching, especially:
2. Calibrate drench guns to ensure the correct dose is delivered.
3. Calculate the dose based on the heaviest animals in the mob. Split mobs for drenching if there is a large weight range, so that heavy animals are not underdosed, and light animals are not overdosed.
4. Follow the label instructions to ensure correct dose and use of treatments.
5. After animals have been drenched, graze them initially on paddocks already contaminated with worms, not on paddocks that are being specifically prepared as low worm-risk. Eggs deposited on pasture from surviving drench-resistant worms in the animals will be diluted by eggs and larvae already on the paddock (these should be susceptible, or at least, less drench resistant).
i. When the sheep eventually leave these low worm-risk paddocks, treat them with an effective drench that is from a different group to the drench used when the sheep first went onto the paddock. The aim is to remove any drench-resistant worms surviving in the sheep after the first drench.
ii. Ensure that the next time the paddock is grazed it is with a different mob of sheep. This second mob should have a moderate to high worm burden and their last treatment must be different from the treatment used on the first mob that grazed the low worm-risk paddock. This will dilute drench-resistant worms already on the paddock with more susceptible worms that the second mob is carrying. Note that grazing with cattle will not dilute the proportion of drench-resistant worms, but they will decrease the total number of worm larvae on this paddock.
In this region the only time a long-acting treatment should be considered is in the summer rainfall areas where there has been a history of barber’s pole worm outbreaks and extensive flooding threatens to isolate and crowd sheep for a number of weeks.
Fortunately, producers often have some days notice of large floods, so in a situation where sheep are likely to be inaccessible for a month or more, the sheep can be treated with a long-acting product before being moved to a safer paddock. A fly preventative treatment is also warranted at the same time.
Do not use a long-acting drench more than once a year.