Researchers, consultants and others working in Australian states have developed worm management programs and information documents that will assist you to effectively manage worms on your property.

This information is additional to the information in the 'Ask the Boss' section of WormBoss which mainly focuses on the 'drenching decision' part of worm management.

Environment

Western Australia (WA) experiences a strongly seasonal pattern of hot dry summers and cool wet winters.

Most sheep are grazed in areas that receive between 350 and 600 mm of annual rainfall. This falls between the autumn 'season's break' (usually April or May) and 'pasture hay-off' in late spring or early summer.

Sheep pastures in WA are predominantly sown with annual species such as subterranean clover, medics and grasses. These pastures are typically dry from November or December until some weeks after the autumn 'break'. Crop stubbles (typically cereal or lupin crops) are also extensively used for sheep production after harvest in late spring or summer however perennial pastures are still relatively uncommon.

There are broadly two regions in WA.

In most sheep growing areas daily maximum summer temperatures frequently exceed 35oC and summer rainfall is rare.

In the more temperate areas, usually within about 50km of the coast, rainfall is higher and occasionally this falls in summer. Maximum temperatures are cooler.

Also see:

• Worms of sheep in Western Australia
• Drench resistance in Western Australia
• Why is drench resistance such a problem in Western Australia?
• Principles of worm management in Western Australia
  • Weaner and hogget sheep
  • Mature ewes and wethers
• Non-chemical strategies for a worm control program

 WORMS OF SHEEP OF SHEEP IN WA

Seasonal conditions have a major impact on worm lifecycles.

In most inland areas and other locations north of Perth virtually no infective worm larvae survive on paddocks during the hot, dry summer.

In coastal regions a small number of infective larvae will survive and after autumn rains are available to infect sheep.

Black scour worm (Trichostrongylus spp.) and brown stomach worm (Ostertagia spp.), and to a lesser extent, large mouthed bowel worm (Chabetia ovina) and large bowel worm (Oesophagostomum spp.) occur throughout the agricultural areas of WA, especially during winter and spring.

Signs of infection include ill-thrift, diarrhoea and in severe cases death. However significant production losses (decreased liveweight gains and wool growth) usually occur before clinical signs become obvious.

Barber's pole worm (Haemonchus contortus) occurs mainly in areas that have significant rain during warm weather and pastures that have areas that remain green over summer.

Larvae are most abundant on pasture in autumn and spring and after significant summer rainfall. Barber's pole worm sucks blood from the sheep and can cause anaemia (visible as pale mucous membranes of the gums and around the eyes), subcutaneous oedema (bottle jaw) and sheep deaths with little warning if environmental conditions are favourable.

The thin-necked intestinal worm (Nematodirus spp.) is common but typically causes only occasional problems in young sheep.

Tapeworms (Moniezia) are also common in young sheep but there is little scientific evidence that tapeworm infection significantly affects sheep production.

There are high levels of drench resistance in most species of sheep worms to many of the current available drenches on most farms throughout WA.

Virtually all farms have worms that are resistant to white (BZ or benzimidazole) and clear (LEV or levamisole) drenches.

Worms on about 80% of farms show resistance to BZ/LEV combination drenches.

Resistance testing during 1999 showed that up to about 40% of tested properties had indications of resistance to the ML group of drenches (the macrocyclic lactones - ivermectin, abamectin and moxidectin) in brown stomach worm. Drench resistance test results during 2002 showed this figure likely to be increasing above 50%.

Because drench resistance in widespread in WA, you should test all drenches you use to ensure that they are fully effective on your property.

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 DRENCH RESISTANCE

In WA, resistance to BZ (benzimidazoles - white drenches) and to LEV (levamisole - clear drenches) products is thought to be present on more than 95% of properties and on most of these the drenches are of little value even for tactical treatments.

Tactical or 'winter' treatments merely aim to control the worm burden in animals at a time when pasture contamination is high (such as winter or spring) and re-infection with worms will occur. For these treatments drench efficacy is should be 75% or greater.

Resistance to combination drenches (BZ/LV) is present on about 75% of properties but this product remains an option for strategic and tactical drenching on some properties. It should only be used however after a drench resistance test has confirmed satisfactory efficacy.

Resistance to naphthalophos (eg. "Rametin" or "Combat Se") has not yet been reported in WA. This product, particularly when mixed with other drench groups, is often useful, but can be variable in effect, and should be used for treatments only after testing its efficacy.

Resistance to the macrocyclic lactone group (the ML's - ivermectin, abamectin and moxidectin) by brown stomach worm is increasing dramatically and is suspected now to affect up to at least 50% of properties in the main sheep regions of WA. Resistance is best detected using a faecal egg count reduction test (FECRT). Seek advice on this from a local veterinarian or sheep consultant or the Department of Agriculture Western Australia. The Drenchrite? resistance testing kit is not satisfactory for detecting ML resistance.

Because the ML group is the last remaining effective treatment option on many properties resistance is a major concern. ML resistance in brown stomach worm is common in WA but there are no reports of ML resistance in black scour worm.

Resistance in barber's pole worm to the BZ and LV drenches has been detected in WA but is still relatively rare. Resistance of this worm to any other drench classes has not been confirmed in WA.

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 WHY IS DRENCH RESISTANCE SUCH A PROBLEM IN WA?

The problem in WA appears to be the result of a combination of climatic and management factors.
Common predisposing factors for drench resistance such as the use of an excessive number of treatments and/or under-dosing of sheep are thought to have played a less significant role in the development of drench resistance in WA.

The cornerstone of effective broad-spectrum worm control in WA has been the summer drenching program (used in worm control strategies such as the CRACK program in the past). This "strategic" program involves a treatment when there are likely to be few infective worm larvae on the pasture to re-infect the sheep.

Whilst summer drenching provides excellent worm control, the downside has been the accelerated development of drench resistance. The only worms left in the sheep after a treatment are those that are resistant to the given drench and during a hot, dry summer there are essentially no larvae left on the hot dry pastures. Therefore the eggs put out by the remaining resistant worms form the bulk of the worm population in the next season as conditions for worm survival improve.

In contrast, treatment during the winter and spring selects more slowly for resistance. At this time of year the weather conditions are more favourable for egg and larval survival on the pasture and therefore more parasites that have not been exposed to the previous treatment remain to dilute the egg output from resistant survivors in the sheep.

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 PRINCIPLES OF WORM MANAGEMENT IN WA

There are three parts to effective worm management in WA - summer drenching, winter drenching and non-chemical strategies.

Summer drenching

Because summer drenching has led to drench resistance in WA, the Department of Agriculture Western Australia is currently conducting research, including investigations on up to 20 'demonstration farms' throughout the State, to find alternatives to the traditional summer drenching program. This is supported by the national, AWI-funded Integrated Parasite Management of Sheep project.

Results to date suggest you should adopt the following approach:

Weaner and hogget sheep - give a single, fully effective summer drench after the pasture has dried off towards the end of spring or early summer.

Mature ewes and wethers - no summer drench is usually necessary, provided they are in good condition and appear healthy. If barber's pole worm is present on the farm then local advice should be sought to include treatments for this worm and maintain barber's pole worm control.

Mobs that are not summer drenched should be monitored visually and faecal worm egg counts checked no later than the first week of April. If results are greater than 200 eggs per gram then a treatment should be given. If it is not possible to do a worm egg count at this time then a routine autumn drench should be given.

Research into the modifications of traditional summer drenching are ongoing and recommendations might change. Further information can be obtained from the WA Department of Agriculture Factsheet on summer-autumn worm control. If you have any doubts, then it is advisable to measure worm egg counts and seek advice from your local veterinarian or sheep consultant.

'Winter' or tactical drenching

Strategic drenching, such as summer drenching, aims to control worms by lowering pasture contamination. Tactical drenching is used to keep worm burdens at a level where there is little impact on animal production. For tactical drenching to be worthwhile, you should use products with an efficacy of about 75% or greater.  Below this level little parasite control is achieved.

 WEANER AND HOGGET SHEEP

Given the higher susceptibility of these sheep to worms and the very frequent need to treat them on the 'demonstration farms' during summer it remains strongly recommended that all sheep of this class receive a single, effective summer drench.

No mob of sheep on any of the 'demonstration farms' has yet needed a second summer drench. If a second summer drench is considered necessary on a property then faecal worm egg counts should be checked first to make sure that the drench is worthwhile.

Results also show that it remains important to maximise the efficiency of any summer drenches given:

• Treat sheep with a fully effective drench (as close to 100% efficacy as possible) at the correct dose rate based on the heaviest sheep in the mob.
• Sheep should be held off feed before treatment with BZ and ML classes of drench to maximise efficacy.

 MATURE EWES AND WETHERS

Up to two-thirds of the mobs of mature ewes (not weaners or hoggets) that were monitored using worm egg counts on the 'demonstration farms' did not need a treatment during summer or early autumn.

Therefore, if ewes appear healthy (good condition and no scouring) at the normal expected time of summer drenching then a routine summer drench should not be given. Instead carry out a worm egg count on a couple of the most 'at risk' mobs of mature sheep (eg. two or three-year old ewes or mobs that are poorer than the rest) and use these results as a guide. If average levels are above 200 eggs per gram then consider a single, fully effective summer drench for these sheep.

It would also be worthwhile to carry out some worm egg count monitoring of other mobs of mature sheep on the farm.

Mobs that are not summer drenched must be monitored visually and worm egg counts checked no later than the first week of April. It is a good idea to monitor at least a couple of mobs on the farm at this time of year regardless of previous treatment history. If these results are greater than about 200 eggs per gram, a treatment is recommended. If it is not possible to do a worm egg count at this time then a routine autumn drench should be given.

Extra worm egg counts are also advisable if sheep or seasonal conditions are poor and following 'atypical' events such as summer rain or false breaks.

Seek the latest advice on modifying summer drenching from your local veterinarian or sheep consultant.

If barber's pole worm (Haemonchus contortus) is present on your farm, then you should also seek professional advice to include barber's pole worm treatments in the program.

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 NON-CHEMICAL STRATEGIES FOR A WORM CONTROL PROGRAM

Effective worm control is much more than drenching of sheep.

A sustainable worm control program aims to reduce the number of chemical treatments and incorporate as many non-chemical control measures as is possible.

For example:

• Modified summer drenching - summer drenches should be avoided if possible in mature sheep by using regular worm egg counts to monitor worm populations. Remember that any departure from traditional summer drenching should be planned carefully with your local veterinarian or sheep worm consultant.
• Modifications to traditional summer drenching allow more worms to survive and these have the potential to cause production loss and disease.
• Monitor faecal worm egg counts and treat only when necessary
• Perform drench resistance tests to be able to use treatments that have been measured to be effective
• Pasture management - plan paddock grazing rotations to reduce larval contamination of pastures
• Ensure good nutrition to help sheep combat worms
• Breed and/or buy worm-resistant sheep