- Weaner and hogget sheep (current and last year’s lambs): give a single fully effective summer drench once the pasture has dried off or as sheep move onto a crop stubble.
- Adult sheep (2½ years or older): either give a drench in early April (no summer drench), or if drenched in summer, leave at least 10% of the mob undrenched. (Note: worm egg counts may show this drench is not needed in some mobs.)
- Graze summer-drenched sheep in autumn and winter on paddocks occupied by sheep not given summer drenches.
Resistance by sheep worms to drenches continues to increase and is a major cause of worm problems due to ineffective control programs. The white (benzimidazole) and clear (levamisole) drenches and the older macrocyclic lactone (ML) drench ivermectin are not recommended for use by themselves, as resistance occurs on virtually all farms in Western Australia (WA). Resistance now affects abamectin on up to 50% of farms and moxidectin on more than 30% of farms. Cases of resistance to the ‘triple combination’ (abamectin, a white and a clear drench) are also found. On any one farm, it can no longer be assumed that any of the currently available products will be fully effective, apart from the newly-introduced monepantel, or (in most cases) the new abamectin-derquantel combination drench.
For effective drenches to continue to work the rate of increase of drench resistance needs to be reduced. The causes of resistance in particular situations should be identified so alternative management plans can be developed, and the maximum use made of non-chemical options. The recognition that the widely-used ‘summer drenching’ program is itself a major factor in the development of resistance indicates the need to change to more sustainable strategies.
The ‘refugia’ concept
This concept forms the basis of managing drench resistance: ensuring that resistant worms (that survive drenches) are always diluted by non-resistant worms 'in refugia' from drenches. Hence, resistant worms are always kept in the minority in the total worm population on the farm.
Worms in refuge from drenches may exist as larval worms on the pasture when the climate or season permits them to survive, or as worms in sheep not recently drenched. In WA there are typically many worms in refugia on pasture from late winter until late spring. This is because mild temperatures and moisture at ground level favours the development of worm egg to larvae and the prolonged survival of larvae. Drenching sheep at that time has little risk of increasing resistance as any resistant worms remaining in the sheep after treatment are quickly replaced by worms taken in as larvae from the pasture (less resistant as they have not been recently exposed to a drench). This heavily dilutes the resistant worms so they do not become a significant proportion of the total population.
However, in summer there is no refugia for worms on the pasture in most situations in WA as the hot, dry conditions kill the eggs and larvae. Any worms surviving in sheep drenched then are not replaced by worm larvae ingested from the pasture and can remain in the sheep for months. Although they are usually in small numbers (depending on the effectiveness of the drench), the resistant worms are the major source of next year’s worm population. If all sheep on the property are given summer drenches, the drench resistance level increases permanently.
Reduced refugia for non-resistant worms also occurs where pastures are deliberately managed to be worm-free, such as by grazing for some months with cattle, or as forage crops.
In many situations outside WA there is the potential for refugia in worm larvae all year round because the mild environmental conditions allow continual survival of worm larvae. Under circumstances highly favourable for worm survival, drench resistance is mostly a result of a high drenching frequency or worm-free pasture policies.
In more seasonal climates, including in WA, the management of drench resistance depends on deliberately ensuring that sufficient refugia is available, by changes to worm control programs or keeping some sheep untreated when a mob is drenched.
Summer drenching: a double-edged sword
In the past the major causes of resistance throughout Australia have been under-dosing with drench and treating too frequently. However, these are no longer significant in WA as the need for correct dose rates is widely accepted and frequent drenching is now uncommon.
The major cause of resistance in this state has been the use of the traditional ‘summer drenching’ program, which has been the foundation of good worm control in the state for many years. The program was developed to reduce the number of drenches needed by removing worms at a time when sheep will not pick up a replacement worm burden from pasture. Sheep treated when grazing crop stubbles or on dry summer pastures stay worm-free for an indefinite period.
However, the recognition that the lack of refugia for non-resistant worms in summer leads to drench resistance has required a re-evaluation of the program. Some worms with genetic mutations that allow them to tolerate a particular chemical family (drench group) are present in all worm populations and these worms survive when drench-susceptible worms are killed and not replaced. The extreme selection pressure for drench resistance associated with summer drenches is believed to explain why WA has some of the highest levels of resistance in the world to the macrocyclic lactone (ML) drenches.
However, the answer is not simply to abandon summer drenching. Research conducted by the Department of Agriculture and Food, Western Australia (DAFWA) that confirmed the development of resistance due to drenching in summer also showed that if sheep were simply not drenched then, there was an immediate risk of worm disease in lambs due to typically high worm burdens in early summer. Also, worm contamination of pastures during autumn from sheep not given summer drenches resulted in significant worm problems in the following winter and spring. The challenge is to get the best of both worlds: good worm control without an increase in drench resistance.