Key recommendations
- 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.
Background
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.
Results from sustainable control research
Investigations funded by DAFWA, Australian Wool Innovation and the Sheep Cooperative Research Centre have produced several key findings that form the basis of sustainable programs.
The vast majority of adult sheep mobs (about 95%) had worm egg counts in early summer of less than 200 eggs per gram, indicating that there is little reason to drench them to prevent production loss due to worms. First monitor the level of infection of the mob before you decide to drench. You will save yourself money and a lot of hard work if the mob is not under risk of a worm challenge. But most importantly, buy rams from breeders that use Australian Sheep Breeding Values (ASBV) for faecal worm egg count (FEC) in their breeding program to breed for increased worm resistance. This will pay off handsomely in the long term as any genetic gains made are permanent and accumulate over time.
However, worm egg counts in adult sheep usually rose from December to April, even though no new worm larval pick up would be expected during the summer period. It is believed this is due to the maturation of immature worms inside the sheep that are dormant during adverse seasons.
As a result of the late-summer worm burden increase, at least 80% of mobs tested had counts of over 200 eggs per gram in autumn. This count does not necessarily indicate a significant parasitic effect in sheep of this age but unless the worms are removed, pasture contamination with worm eggs will continue. This may lead to worm problems later in the year, so pre-emptive treatments are recommended (except in mobs shown to have low worm egg counts, or in lower-rainfall areas where worms are rarely significant). The development of worm eggs to the infective larval stage in sheep dung commences in early March, and some of the larvae can remain in dung pellets on dry ground for many weeks, until green pasture growth occurs and the larvae can migrate onto the herbage. Pre-emptive programs therefore need to ensure that adult sheep mobs have low average counts before larval development commences, well before the usual time of the season’s break.
Research showed no production penalty associated with drenching ewes in mid-autumn, rather than in December. This accords with their low worm burdens in summer and there is little effect due to immature worms. More recent research showed that an alternative is to leave a percentage of adult ewes undrenched in summer ('targeted treatment'), so some worm eggs from worms not exposed to a drench continue to be shed onto the pasture. Providing that the sheep not drenched were in high body condition score (3.5 or over), no loss of production (wool growth, condition score or lambing performance) occurred at a mob level.
Sustainable programs
Weaner and hogget-age sheep
- Give current and last year’s lambs a single fully effective summer drench after the pasture has dried off, or when a crop stubble is available, towards the end of spring or early summer. Growing sheep are susceptible to worms and usually require a drench at this time. A second summer drench is very rarely needed.
- Graze these mobs in winter and spring on paddocks occupied by adult sheep. The worm larvae on these paddocks originate from worms of the lowest resistance level on the farm and will dilute worms surviving in summer drenched sheep, hence reducing the overall resistance level.
- Monitor worm egg counts at 4-6 weekly intervals after the season’s break, to check the need for further treatments.
Adult sheep
There are two major options for sustainable worm control in adult sheep in WA; 'autumn drenching', or 'targeted treatment'. These have a similar effect in reducing the development of drench resistance, without an impact on sheep production. Which program is most suitable for a particular property will depend on management and logistical considerations, as neither requires additional time or effort to apply compared to traditional programs.
Autumn drenching
- Drench all adult sheep between the end of March and the second week of April with a fully-effective product. If some mobs are in especially good body condition, it may be feasible to leave them undrenched, provided that a test shows they have low worm egg counts.
- Pre-lambing drenches to ewes: if lambing in April or early May, it is unlikely that pre-lambing treatments will be needed. A worm egg count will confirm whether drenching is justified. Ewes lambing later are more likely to require a pre-lamb drench.
'Targeted treatment' in summer
- Give a single fully effective drench to adult sheep after the pasture has dried off (the end of spring or early summer), or when a crop stubble is available, but leave 10-20% of the mob undrenched.
- Sheep to be left undrenched should be at least of condition score 3.5. If the mob average condition score is less than 2.5, do not leave any undrenched, but treat all in the mob in autumn.
- In areas where barber's pole worm has been a significant problem, check worm egg counts before drenching, and do not leave sheep undrenched if counts are high.
- Pre-lambing drenches to ewes: check worm egg counts 2-3 weeks before lambing, to indicate whether drenching is justified. Ewes lambing in winter are most likely to require a pre-lamb drench.
Other low-refugia situations
Summer drenching has been identified as the major risk period for drench resistance development in WA but all low refugia situations potentially increase the resistance level. Pasture management for worm control involves keeping sheep out of a paddock until most worm larvae have died off and drenching sheep as they move in. ‘Worm-safe’ paddocks can be created by grazing with cattle for some months, using forage crops, or temporarily grazing sheep on cereal crops during the growing season.
The benefit of drench-and-move tactics should be considered against the drench resistance risk on a mob-by-mob basis. It is obviously a major risk if all sheep on the farm were drenched and moved onto worm-safe paddocks at the same time. However, where applied in single mobs there is likely to be dilution of any resulting resistant worms as mobs cycle through different paddocks. If worm-safe pasture tactics are considered a risk, drenches can be delayed for a week or so after the sheep move to the new paddock to allow some pasture contamination with worm eggs from worms not recently exposed to a drench.
High worm-risk situations
- Where barber's pole worm has been a significant problem in the past, the basic programs outlined here may not be adequate. Seek local advice regarding additional worm egg count monitoring or specific treatments.
- Unusual weather conditions can reduce the effectiveness of programs planned for 'normal' seasons. If summer rain or early season’s breaks lead to a sustained pasture germination, worm larvae may develop and survive for some time. Worm egg counts will indicate whether additional treatments are required, or summer/autumn drenches should be given earlier than usual.
- On perennial or irrigated pastures, worm development may continue for most of the year, although it will be reduced in very hot weather. Frequent checks of worm egg counts are needed to manage the risk of unforeseen worm problems.
- Use worm egg counts to confirm that this program is applicable under unusual weather conditions (for example summer rain, or early season’s breaks), situations especially favourable for worm development (perennial or irrigated pastures), or for sheep in poor nutritional condition.
- Buy rams that have negative values for Australian Sheep Breeding Values (ASBV) for faecal worm egg count. That will ensure that their progeny will be more resistant to worms.