How to Manage Worm Resistance in your flock.

INTRODUCTION

Drench resistant worms are now widespread in Australian sheep flocks. For at least the last decade the poor efficacy of B-Z and Levamisole drenches has been widely recognised, despite only a minority of producers testing for resistance on a regular basis.
The introduction in the 1980's of ML drenches brought relief just when some flocks were facing substantial problems maintaining productivity in the face of resistant strains of worms. By the mid 1990's there were more ML products available and gradually the price has been reduced and the MLs have enjoyed widespread use.
However, the last few years have seen the start of a new phase in the battle against worms –that of the emergence of ML resistance. At this stage it is rare for a product from the ML group not to be sufficiently effective to provide reasonable levels of worm control. Based on drench resistance testing a different picture is emerging with over 300/0 of flocks in some areas with detectable resistance to Ivomec. It appears that we are now on the cusp of a new era which will see the decline in efficacy of the ML group.

Chemical abuse

Recommendations to drench sheep monthly were common not that long ago. As well, for many years companies stated that the adult sheep dose peaked at a live weight of 40 kg. Historically, in regions where Barber's Pole worm is endemic, it was recommended that Levamisole could be used at half the normal dose rate for specific Barber's Pole control and in the meantime it was effectively selecting for resistance in the other worms present in the sheep.
Most of the broad and narrow spectrum products for worm control in flocks were highly effective at the time of their release. Their launch price rapidly eroded and they became relatively inexpensive. Therefore they were quickly regarded by most sheep producers as commodities. This process did little to preserve the integrity of the compounds and ensure that they were treated with respect over time to prolong their active life.
The track record of the Australian sheep industry in destroying useful active compounds is shameful to say the least. Producers cannot be asked to bear all the blame for this. The companies manufacturing those active compounds have, in the past, heavily contributed to the demise of their products.

Herbicides and weeds versus worms

Resistance problems are not unique to sheep worms. Significant problems have emerged with blowfly and lice control chemicals and weeds have developed resistance to commonly used herbicides. Development of resistance is natures way of a species changing in order to survive in an altered environment.
Herbicide resistance in weeds provides an interesting comparison to drench resistance in worms. Most producers who grow crops are well aware of herbicide resistance and test for it. Failure of a herbicide is quite obvious because the surviving weeds can be seen in the crop.
Resistance to drenches is not obvious, unless very severe, because even if 500/© of a worm population is removed the sheep will respond to treatment. Just because surviving worms are not as easy to see as surviving weeds, it doesn't mean the resistant worms should be overlooked.

The present and the future

Despite a track record of which we can be less than proud, and despite real and perceived ongoing problems with worm control, the situation is not as serious as it seems. The future even looks positive. Whilst it is unlikely that we will be saved by a constant stream of new compounds becoming available, it is possible to work with existing compounds, combined with knowledge of worm control to minimise the potential economic loss. In the last few years considerable effort has been put into evaluating the best option to prevent and slow the onset of ML resistance. From this work there have been many developments which can be adopted at farm level to slow the onset of resistance. This information package is designed to bring that knowledge together into one publication to help you better manage the issue on your farm.

THE CURRENT RESISTANCE PICTURE

Resistant worms are becoming increasingly widespread, not only in Australia, but in other countries
where sheep are farmed. Some countries have seen resistance emerge more rapidly than others, due to a combination of factors, particularly over-reliance on drenches with insufficient alternative management strategies, and in some cases use of products which have poor quality control.
Internationally the picture for ML resistance is shown below:

% FARMS WITH ML RESISTANCE

• Argentina 6%
• Brazil 13%
• Paraguay 70%
• Uruguay 1.2%
• South Africa 73%*

* for Barbers Pole

These results are concerning, particularly in the case of Paraguay and South Africa, given the relatively short period that ML products have been available.
In Australia the picture with ML's is not yet as severe as the worst affected countries, although the indications are that we are just on the edge of a rapid increase in the prevalence of ML resistance. Given that we have pretty effectively demolished the B-Z and levamisole groups of drenches, we don't have many options left. The last survey of resistance in Australia showed:

% OF FARMS

• B-Z resistance 85%
• Levamisole Resistance 65%
• Levamisole and B-Z resistance 60%

There have been no national surveys of ML resistance, however most states have now detected ML resistance in one or more sheep flocks. Western Australia seems to be at the forefront of the problem, possibly because they have carried out survey work, but maybe because of the extremely Mediterranean type climate in much of the west sheep raising areas. In the Kojonup district 170/0 of flocks have been found to have ivermectin resistance. At half-dose rates, which is a more sensitive test, 380/0 of properties had worms surviving. The half-dose test is a useful screening test for resistance because normally only a 25% dose should be sufficient to kill 100% of Ostertagia in sheep, so resistance has to be quite severe before it shows up at a full dose. This indicates there are a lot of flocks with early stage, low level resistance which will become more severe over time, particularly if appropriate management strategies are not implemented.

RESISTANCE DEVELOPMENT AND REVERSION

What is resistance?

The practical definition of resistance is when an anthelmintic treatment fails to reduce worm faecal egg counts by at least 95%. This is really an on farm definition which indicates how efficient a drench will be at reducing the worm population in a sheep. In reality resistance can be present while egg counts are reduced by greater than 950/o. A stricter definition is the decline in efficacy of an anthelmintic which is genetically determined in a worm population that is normally susceptible to the anthelmintic.
The major difference between these definitions is that, in the early stages of the development of resistance, anthelmintics will have greater than 95% efficacy but a proportion of the worm population will be carrying one or more genes which confer resistance. In these situations, which are difficult to detect on farm, the continued use of the anthelmintic will rapidly result in the further selection for resistance and ultimately clinical failure of the anthelmintic.

The development of resistance

Unfortunately there is a fundamental conflict between achieving good worm control and maximising the life of anthelmintics. Good worm control programs will inevitably lead to the development of resistance, it is just a
question of how long it will take.
In its simplest sense this occurs because the anthelmintic selects out those worms which are capable of surviving in the presence of the anthelmintic. These resistant worms then go on to produce eggs and they contribute to an increased proportion of resistant worms in subsequent generations.
It is now accepted that climatic conditions in much of Australia, which are not very favorable for larval survival on the pasture, have helped provide a greater influence of the survivors of the anthelmintic on the next generation of worms. This appears to have been most extreme in WA with the summer drenching program.
Various management strategies can be used to slow down resistance development and some of these can also provide good levels of worm control. However, other management practices which may provide very good worm control can select rapidly for resistance. Monthly drenching's, shown to provide very good worm control and high levels of animal performance is an old example of a very effective program, but a flawed one in terms of resistance management in the longer term. A more recent example is capsules which have provided very good worm control but have been demonstrated, both theoretically and practically to select very rapidly for resistance.
There is no avoiding this fundamental conflict which in essence means finding some compromise in current worm control, and most likely profitability, in order to help ensure profitability in future years. The challenge is to find the happy medium where both long and short term worm control and profitability are acceptable. Unfortunately, sheep producers in Australia have a history of focusing on the short term issue and worrying about potential future problems when they get a bit closer. This is not to say the fault lays completely with the farmers – there has been and continues to be plenty of information from the companies selling products which has put short term company profits ahead of the longer term interests of the industry.

Do drenches regain their efficacy?

Once a worm population has developed resistance, if the use of the anthelmintic is discontinued, is it likely that the worm population will once again become susceptible to the anthelmintic?
The short answer is no, although there is some evidence that drench efficacy will improve in the short term, only for resistance to rapidly return to previous levels.

Reversion may occur due to two possible reasons:

1. Reduced 'fitness' of resistant strain. If the resistant strain which has developed is less 'fit', it is not as capable of surviving in the sheep or in the environment as the original susceptible strain of worms. Because it is not as well adapted to the environment, the resistant strain loses its competitive advantage in the absence of the drench and declines as a proportion of the worm population over time.
Considerable research work has been done in this area and the overall conclusions which can be drawn are that there is little or no evidence for decreased fitness of resistant strains, so ceasing the use of an anthelmintic will not make any substantial differences to the resistance status of a strain of worms.
2. The use of one anthelmintic selects actively against the strain resistant to another anthelmintic. This would occur, for example if the B-Z resistant worms in a population were more susceptible to Levamisole than the rest of the worms in the population. Therefore, the B-Z resistance is a disadvantage in the presence of levamisole and is actively selected against.
This would provide an ideal scenario for reversion to susceptibility. The research evidence to support this is not strong and is backed up by field observations. Also, it is likely that the more severe the resistance, the less opportunity for any counter-selection.
Reversion needs to be distinguished from 'apparent reversion' which is an apparent improvement in drench efficacy from one drench test to another. Often this is due to a change in the mix of species present and should not be confused with real reversion to susceptibility.
From all the available information it can be concluded that reversion to susceptibility is e slow process and cannot be relied upon to resurrect drenches after a five or ten year period of no use. In cases where resistance is present and drenches have not been used for long periods, re-introduction of the drench has resulted in resistance levels quickly rebounding to previous levels. This may provide ar opportunity for the short term use of a product which may previously have been of little or nc use, but the selection of the population for the resistant worms which remain, and their subsequent competitive advantage, means that very quickly resistance levels are back to where they originally were.

PREVENTION OF RESISTANCE

As discussed in section 2, resistance to B-Z and Levamisole drenches is already widespread in Australia. In most flocks it is too late to do anything about preventing resistance to these drench groups. It is a case of trying not to make the problem any worse than it already is so that those products can be used at times instead of relying completely on the ML group of drenches.
Preserving the efficacy of the ML group of drenches is a priority because resistance is still not widespread, although possibly emerging rapidly. While it remains the best option for many flocks it is important its efficacy is preserved for as long as possible. There is a range of management strategies that can be used to delay the emergence of resistance. Note that we are talking about delaying rather than preventing resistance. The only sure way to prevent the development of resistance is not to use a product at all. That way there is no selection pressure for resistance but it won't do much for the worm control program on the farm. The next best option is to use the anthelmintic in such a manner that it does little for flock worm control but its poor efficacy means that it will also be inefficient at selecting for resistant strains within the population.
Therefore the aim should be to devise a program which, for as long as possible, delays the development of resistance whilst maintaining flock productivity, not necessarily at the maximum, but close to it.

The essential steps to delaying the development of resistance are:

1. Know what works and what doesn't in your flock. This involves setting up a resistance test on your flock if you haven't already carried one out.
2. Design a worm control program to minimise selection pressure for resistance. This is a complex area which needs to be addressed in detail at the individual farm level. Factors which need to be taken into account at the farm level include:

• Seasonality of rainfall as well as total annual rainfall.
• Enterprise mix -cropping and sheep or cattle and sheep versus sheep only, merino sheep versus only prime lamb production.
• The timing of management activities including shearing, lambing, weaning and sheep sales.
• Labour availability and flock size. The objective when it comes to developing a control program which delays as long as possible the development of resistance is not to focus simply on minimising drench frequency, but to aim to focus on minimising the selection pressure for resistance. This requires not only minimising drench frequency but also consideration of a number of additional factors.
• Timing of drenches. Drenches given at times of the year when few larvae survive on the pastures will enhance selection for resistance. Once again however, this presents a trade-off. The two-summer drench program recommended through much of southern temperate Australia relies on low worm survival over the summer period. The principle of killing the worms in the sheep while the worms die on the pasture has been the reason for the success of summer drenching programs. Of the two drenches which have been recommended for summer, the second selects most rapidly for resistance. To minimise this selection pressure, avoid drenching unless it is necessary and if it is use a product other than an ML, even if it's efficacy is reduced.
• Provision of low risk pastures, where they can be provided enable fewer drenches to be used and tend to reduce selection pressure for resistant strains. Once again this is not simple. Putting sheep onto clean pastures after drenching means that the future population is derived almost exclusively from those worms which survive the drench. In fact some work has shown that a drench and move strategy based on two drenches selects just as rapidly for resistance as does five drenches under a rotational grazing strategy where pastures are much more likely to be contaminated.

3. Choose the most appropriate product. The standard recommendations include annual rotation between effective drench groups. This has been promoted as a means of delaying the development of resistance, however, the basis for this recommendation has increasingly come under question. The current scientific consensus is that rotation provides no significant benefit in delaying the onset of resistance. In other words, if you start with two effective drench groups, say white and clear, and rotate annually you will have significant resistance to both in 20 years. Alternatively you could use say white drench continuously for ten years by which time resistance would have then emerged. For the next ten years you change to clear drench, to which resistance develops over that time. At the end of the twenty years the worm resistance situation in the flock is similar, despite rotations in both of the options. Rotation will not prevent resistance developing and is not necessary for delaying resistance development. A more appropriate strategy is to choose the most suitable product. This may involve the use of a number of drench groups through the year, that is, rotation within a year rather than between years.
4. Use the drench correctly with all the standard recommendations including dosing delivery to the average of the heaviest 10% of the mob, and checking the accuracy of drench guns.
5. Use of specialist advisors to tailor a program to your farm especially if there is widespread resistance to a number of drench groups.

Options for anthelmintic treatment.

Producers are strongly encouraged to seek individual consultation with an adviser to plan worm control programs because of the variation between properties in terms of environment, nutrition, management and worm population size, composition and resistance. Note: For the following guidelines, only two levels of ML resistance are considered, 'resistance confirmed' or 'resistance unlikely to be present. In practice, once ML resistance is confirmed it is most likely to be at a moderate to high degree (resistance present in 100/0 of the population). If ML resistance is not suspected or detected ('resistance unlikely to be present'), it may be present but at a low frequency (less than 50/o of the population). Four generic situations are considered:

1. Quarantine treatment,
2. Treatment on/to clean pasture,
3. Treatment on/to safe pasture,
4. Treatment on/to moderate/heavily contaminated pasture.

1. Quarantine treatment

Principle - remove all worms with a highly effective combination of drugs (efficacy > 99.9%), and introduced sheep should be placed on a contaminated pasture.
A quarantine strategy is considered essential, so that resistant strains are not imported to a farm. This treatment aims to eliminate all worms in introduced sheep, and after treatment introduced sheep should be placed on a contaminated pasture and not a paddock with minimal contamination. It is recommended that anthelmintics from at least three broad-spectrum groups be given to sheep as sequential treatments unless they are commercially available in a combination formulation. From the ML class of anthelmintics, only MOX has consistently demonstrated high efficacy against ML resistant worm strains, and MOX in combination with other drugs would thus be the treatment most likely to prevent importation of ML resistant worms. Increased costs of using several anthelmintics is justified by the importance of maintaining effective anthelmintics on a property and avoiding the introduction of drug resistant or new worm species.

ML resistance considered unlikely to be present:

Use any highly effective combination of anthelmintics including MOX (ie: BZ+LEV+MOX) and put sheep on contaminated pastures.

ML resistance has been confirmed:

If ML resistance is suspected on the property from which sheep were purchased, then it is prudent to include NAP in the combination (BZ+LEV+MOX+NAP) and put sheep on contaminated pastures.
In either situation, where possible, sheep should be held for at least 24 hours after treatment (with access to water), before release onto contaminated pasture. If NAP is used in the combination, then sheep will require access to food as well as water in the 24 hour quarantine period. If purchased sheep are being placed on farm that has been destocked for a period sufficient to reduce L3 to very low numbers, for example because of OJD eradication, then after treatment the sheep should be withheld from pasture for 48 to 72 hours. This is to ensure eggs laid before treatment have passed through the gastrointestinal tract. Because it is unlikely that contaminated pasture will be available, it is recommended that animals introduced to a destocked farm be held in a small paddock until success of the quarantine strategy can be confirmed by faecal worm egg count 10 days after treatment. If sheep have zero worm egg counts they can be released onto the destocked property. However, if worm egg counts are still positive then further treatment is necessary to prevent introduction of worms with multiple resistance onto pastures likely to have extremely low numbers of L3.

2. 'Clean' pastures

A clean paddock is defined as one on which no larvae are expected to be present, for example, crop stubbles, paddocks only grazed by cattle and very dry pasture paddocks, particularly in South Australia and Western Australia where second summer treatments are commonly given.
Principle – remove all worms with a highly effective combination of drugs (efficacy >99.9%), to reduce the contribution of resistant worms to future worm populations.
Note that it takes at least three months in summer and six months in winter for contamination of a paddock to be substantially reduced.

ML resistance has been confirmed:

MOX is the preferred ML for use in a combination because of its higher efficacy against ML resistant worms than IVM and ABA, and because any selective potential from its persistent effect is not relevant on clean pasture.

3. 'Safe' (lightly–contaminated) pastures

'Safe' paddocks are defined as those in which Worm infection is likely to be low, for example, after treatments in dry-summer climates when larval numbers are expected to be low; lambs weaned onto paddocks previously grazed by healthy (immune) adult wethers or dry ewes. Principle – use an effective combination of anthelmintics (efficacy > 98%).

ML resistance considered unlikely to be present: If the population of larvae on pasture is declining such that little re-infection is expected after treatment, for example after summer treatments in a winter rainfall environment, use an effective combination without an ML (NAP+BZ or NAP+LEV). In alternate years a combination including MOX would be advised, because selection during`its persistent phase is unlikely. If re-infection after treatment is likely, for example after weaning in a summer rainfall environment, an effective combination should be used. If other combinations are effective (such as NAP+LEV), they should be used in alternate years.

ML resistance has been confirmed:

An effective combination, which does not include an ML should be used if available. In alternate years an effective combination including ABA, if shown to be effective against ML resistant worms, or MOX is advised.
To control Haemonchus contortus, a narrow spectrum anthelmintic should be used wherever possible, because using a broad-spectrum anthelmintic may accelerate the development of resistance in other worm species. Narrow spectrum anthelmintics that are active against Haemonchus are closantel (Seponver®, Razar®, Sustain®, Closantel® and Closicare®) and naphthalophos (Rametin®). In some areas LEV can be regarded as a narrow spectrum anthelmintic because commonly LEV remains highly effective against Haemonchus though LEV-resistance in Trichostrongylus and Ostertagia is present.

4. Moderate/heavily contaminated pasture

These are moderate to high-risk paddocks in which substantial larval contamination is expected, for example, potentially during winter and spring in winter rainfall regions and Haemonchus-endemic areas of Australia.
Principle - reliance on a persistent drug, capsule or multiple treatments may be necessary to achieve satisfactory worm control, although this is not advocated as an on-going substitute for a broader based worm control strategy.
If substantial populations of infective larvae are expected (such as summer rainfall regions where closantel resistance in Haemonchus is a problem), then it may be necessary to rely on a drug or device with persistent activity to protect sheep while natural immunity develops or safe pastures become available.

ML resistance considered unlikely to be present:

If BZs are effective or BZ resistance is low (BZ efficacy >800/o), then a combination treatment followed by a BZ-capsule is an alternative to relying on persistent MLs, but is not advocated as a strategy to be repeated each year.
The persistent ML options are: MOX Drench, MOX injection and IVM-capsule. The order above reflects increasing persistence of drug availability and protection and also the theoretical selection pressure. These are tactical treatments for susceptible stock grazing contaminated pasture and use of any persistent/capsule-ML should be excluded in the following year and if possible, the paddock should be used for crops or cattle to avoid infection with resistant larvae that may arise. In subsequent years, when possible, an Integrated pest management (IPM) program should be followed (for example, preparation of a safe pasture) to avoid build up of high L3 populations. If IVM-capsules are used it
strongly recommended that a worm egg count is carried out 50 and 80 days after capsule administration to determine if ML resistance is developing. Strategies similar to those aimed at controlling closantel resistance could be considered as alternatives to relying on persistent MLs in the face of heavy contamination.

A single treatment with a combination including a non-persistent ML (IVM or ABA) may be adequate, but it is then essential to monitor worm egg counts. Selection pressure may be lower in this case, but if multiple treatments are necessary the selection for ML resistance is likely to be increased.

ML resistance has been confirmed:

Avoid the use of IVM oral or capsules if ML resistance is present because these are unlikely to be fully effective and may increase selection for resistance. The use of MOX, even though effective against resident worms, is unlikely to provide extended protection against ML resistant larvae, and may increase selection for ML resistance.
Testing to indicate an effective combination of drugs, which may include MOX or ABA, is advised in this situation.
* This section 'Options for anthelmintic treatment' was drawn from Australian Veterinary Journal. Volume 79, No 11. November 2001.

Monitoring your program

Worm management programs are not something you can 'set and forget' Each season is slightly different, some are so favorable for worms that they provide a constant challenge to keep sheep healthy and productive, while other years not every drench in the standard program will be required. To ensure you get this fine tuning of your program right you need to monitor. Drenching simply because a mob is 'not doing' or because the neighbor just found high egg counts is not a satisfactory approach. Monitoring is the key to minimising drench frequency as well as ensuring the program you have in place is actually working.
In most cases monitoring is done by counts of the number of worm eggs in the dung. In some cases monitoring of liveweight's and/or condition scores will be useful, as will monitoring of pasture quantity and quality.

Principles of monitoring

There are a number of principles you should understand as a basis for making decisions when you use monitoring.
These are:

Egg counts lag worm burdens

Worm egg counts lag the total worm burden by about three weeks. This is because the time required from larval pickup off the pasture through to an adult worm capable of laying eggs is approximately 21 days for most species of worm. Therefore a worm egg count of 500 epg in weaners tells you that the weaners should have been drenched some time ago the burden may now be equivalent to 700-800 epg or more, depending on the numbers of worm larvae on the pasture. An average count of 300 epg in weaners at a range of times has differing interpretations

(Table 1) Possible interpretation of a 300 epg burden in weaners.

Egg count reliability

As weaner sheep mature, they develop a greater immunity to worms. The effects of this immunity are to:

• Reduce the number of worms establishing.
• Reduce the life-span of the worms.
• Decrease the egg output of the worms.

(Table 2) Egg count results for two mobs

This means that egg counts in adult sheep can underestimate the number of worms present.
For example, a mob of mature wethers with an average egg count of 150 eggs per gram may have a much higher worm burden than the count indicates. However, a high egg count still means a high worm burden.

Always look at the average

Egg counts vary widely between individual sheep in a mob. Table 2 shows the results of monitoring two mobs.
When interpreting these results the important figure is the average and that should be used as a basis for deciding whether or not to drench. Do not use the highest counts because there will always be a few sheep with much higher counts than average.

Use of paddock

The subsequent use of the paddock can have a major effect on whether a drench is necessary. For example if you are using wethers to prepare a paddock for weaning later in the year a count of 100 epg needs to be managed quite differently compared to a paddock which is just going to be used for wethers for the rest of the year. If the paddock is grazed at 10 wethers/ hectare for 8 months with an average epg output of 100 epg, a total of 1242 million worm eggs are deposited per hectare. Obviously not all survive but even if 10/o do, the pasture could not be considered low risk for weaners.
Therefore when interpreting epg counts, keep in mind what the plan is for the paddock over the next 3-6 months.

Worm species

Under the microscope all worm eggs look the same, except Nematodirus, therefore a worm egg count will not indicate species present. A larval culture and identification is necessary to determine the species. This takes 10-14 days. The only time species really matter is if Barber's Pole is suspected OR as an aid in interpreting the results of a resistance test.
The main points about each of the species are:

• Ostertagia (Small Brown Stomach worm) and Trichostrongylus (Black Scour worm) are the main problem species in the non-seasonal and winter rainfall areas.
• Haemonchus (Barbers Pole worm) is significant in summer rainfall areas and can be a sporadic problem in other areas, often associated with summer rainfall, eg: Esperance, SE South Australia, central and southern NSW.

Cause high egg counts, often 10-20,000 epg and sheep do not scour.
Sheep anaemic, often in good condition.

• Nematodirus is easily differentiated by faecal egg count because of different shape egg. Most commonly associated with dry seasons (very resistant) or wormy lambs sometimes as early as marking time.

Class of sheep

Table 3 provides a guide to interpreting worm egg counts. Nutrition has a major impact on the ability of ewes to cope with worm burdens.

(Table 3) Guide to interpreting worm egg counts (assumes no Barbers Pole is present)

Nutrition can be assessed by condition scoring ewes and by assessing pasture availability. For example, if ewes are less than condition score 2.5 and/or there is less than 800-1000 kg of dry matter just before lambing they will be very susceptible to a rapid build up of worms.
Figure 1 provides a suggested monitoring program for the winter rainfall and year round rainfall regions. The exact timing of monitoring will vary between flocks, for example as lambing time varies. A blank template is provided on page 29 to use with your own program.

Do it yourself egg counts

Many producers are keen to set themselves up with the necessary equipment to do their own worm egg counts. They see advantages in cost savings as well as quick results. We strongly recommend producers don't go down this path for the following reasons:

• At $20-$25 from most laboratories per mob, egg counts are cheap and probably cheaper than you can do it yourself if you account for your labour and equipment.
• Accuracy is critical. Setting yourself up in the laundry to do a few counts each month often leads to compromises in quality control.
• At times you will want a larval cu re to identify species, in which case the sample will have to be sent to a laboratory anyway.
• Most laboratories offer a turnaround time of 24-48 hours, less if the results are required urgently.

(Figure 1) Suggested monitoring program for winter rainfall / year round rainfall districts

PERSISTENT ACTIVITY - FRIEND OR FOE?

There has been much debate about the merits or otherwise of persistent activity in drench products. Much of the debate in recent years has focused on whether or not persistence increases selection for resistance. This debate has been driven more by commercial interests rather than thorough research and careful thought.
In the last year more work has enabled us to see this debate in a different light.
There are three factors relating to the drench which contribute to the development of resistance by worm populations. In order of importance these are:

• Efficacy against resistant worms
• Persistence
• 'Tail'

We can now take each of these points one at a time to explain their importance:

Efficacy against resistant worms

Resistance emerges in a population because the resistant worms are capable of surviving the drench. If these same worms are killed by an alternative product, the development of resistance must be delayed because the resistant worms have no advantage over the other non-resistant worms. By comparison, if efficacy against resistant worms is reduced the resistant worms which survive the drench are happily producing eggs that go onto the pasture. Therefore, the next generation of worms has an even greater percentage of resistant worms and so it goes on.
Most of the debate about the benefits of persistent activity has centered around the two ML subgroups - the short acting Ivermectin and Abamectin and the persistent activity of subgroup Moxidectin. The efficacy of these two groups as well as the less widely used Abamectin against resistant strains is shown in Table 4.

The average results for Ivermectin across all trials was 45% efficacy while moxidectin achieved an average efficacy of 99%. The Abamectin group, for which there is very limited data, appears to be somewhere in between.
A study by Barnes et al. (2001) showed the critical importance of the differences in efficacy when it comes to the survival of, and hence selection for ML resistant worms.

(Table 4) Efficacy of ML's against some reported cases of resistant worms

(Table 5) Moxidectin and Ivermectin efficacy against resistant worms

It is quite apparent from Table 5 that moxidectin provides much greater efficacy against resistant worms than ivermectin does. This means ivermectin will select more quickly for resistance in a worm population because it effectively screens worm populations for worms carrying a resistant gene (heterozygote). These worms carrying the resistant gene go on to breed and as a consequence, homozygotes, which carry two resistant genes become more common in the population which is when the resistance starts to become a real management issue.

The reason for the differences in efficacy between the products relates to differences in product potency, that is the ability of the product to varying concentrations. The more potent product kills the target worms at lower concentrations than the less potent product. Despite the differences in potency between the products, both ivermectin and moxidectin are supplied at the same concentration in commercially available products in Australia. This concentration is determined by efficacy against what is called the dose limiting species, or species of worm which requires the greatest concentration to provide acceptable efficacy. In the case of the ML group of drenches the dose limiting species are not the ones which are the most important species to control in most sheep flocks. This is shown diagrammatically in Figures 2 and 3.

As resistance begins to develop in the worm population, the dose-response curve moves to the right, that is, closer to the recommended dose rate. Ivermectin is the first of the two products to start failing because it has less room to move, simply because its potency is lower.

This is not to say that resistance to moxidectin will not emerge at all. What it does indicate is that ivermectin will show up with resistance problems before moxidectin but as selection pressure is continued, both curves in Figure 3 keep moving to the right and it is only a matter of time before resistance emerges to moxidectin. It is critical this advantage of moxidectin is not abused because it is not immune to ML resistance developing – in fact it already has been confirmed in one flock in Australia.

Persistence

The contribution of persistent activity to selection for resistance is complex.

If the drench, through its persistent activity, prevents incoming larvae establishing it allows any resistant survivors of the initial dose to breed amongst themselves and contribute resistant eggs to the worm population on the pasture.
However, if the drench has a large initial kill of worms present at drenching, there aren't any resistant worms to breed amongst themselves. Therefore, persistent activity is much less of an issue if most of the resident worm population is removed at the initial drench.

To complicate matters further, the greater the persistent activity of a drench, the less frequently drenches are required. For example capsules have approximately 100 days of persistent activity, thereby reducing the frequency of drench use. It is well accepted that more frequent drench use selects rapidly for resistance. Therefore, there is a trade off, greater persistent activity, may on its own select for resistance but this may be partly compensated for by the fact that fewer drenches are required.

The net effect is that persistence is undesirable if the initial kill is low, but the higher the initial kill, the less persistent activity will select for resistance. If we compare Ivermectin and Moxidectin, Ivermectin has no persistent activity and a lower initial kill compared to Moxidectin. Moxidectin has a mid-range persistent activity, but high initial kill. The combination of Moxidectin features mean that it is less likely to select for resistance than Ivermectin.

Tails

The tail of a product occurs when the concentration declines as the drug is removed from the animals system. It is important to draw a distinction between a 'tail' and 'persistent activity' A tail occurs when the amount of active ingredient in the animal falls below the level required to kill resident or incoming worms. Persistent activity occurs when a product comprises a lethal concentration for worms for an extended period. Products which have persistent activity do not necessarily have a long tail and vice versa. The tail is much less important in selection for resistance than is the initial kill because worms surviving the initial drench have a much greater advantage and produce more eggs than do larvae which are ingested and develop during the tail period. Ideally, drenches should have short 'tails' but a long tail only has a relatively minor impact on selection for resistance.

(Figure 2) Diagrammatic representation of efficacy of Moxidectin and Ivermectin.

(Figure 3) How resistance develops to Ivermectin and Moxidectin. The curve for Moxidectin has further to move to the right and moves at a slower rate.

(Figure 4) Diagrammatic representation of persistent activity versus tail

Conclusion

At the end of the day, no one drench will provide everything needed to prevent the development of resistance. Within the ML group ivermectin provides a short persistent period but poor efficacy against resistant worms. Moxidectin provides very good efficacy against resistant worms, persistent activity which may reduce drench frequency but select for increased resistance. The situation is best summarised by parasitologist, Ian Barger "Clearly, the greater the persistence of a drug, the greater must be its efficacy against resistant worms if it is not to select more strongly for resistance. The available evidence suggests that this balance between persistence and efficacy is probably optimal with a drug such as moxidectin. By comparison, the short persistence of the oral avermectins is not sufficient compensation for their poor efficacy against resistant worms, while the much longer persistence (3 months) of ivermectin capsules exaggerates this deficiency, particularly when the poor efficacy of the capsule against resident adult resistant worms is considered".

THE COST OF RESISTANCE

Work carried out in WA has provided estimates of the cost of drench resistance in a flock.
The trial involved three drench treatments, with a range of efficacy from 1000/0 down to 650/0 (Table 6). The sheep used in the trial were Merino weaners, the most likely age group to be affected by an increased worm burden associated with reduced drench efficacy.
The main points from the table are:

• The small production difference between a product of 1000/0 efficacy and one of 850/0 efficacy. The differences between these two groups were not significant.
• Large production effects were seen when drench efficacy was down to 650/0.

As expected the sheep drenched with the product which was only 650/0 effective had a lower fibre diameter and an associated decrease in fleece weight. One may be tempted to conclude that in times of large premiums for fine wool compared to medium wools, a few worms might be a good thing to keep the fibre diameter down. However, when you consider the cost of higher mortality rates, the added scouring and lower value if you sell them as young sheep, the worms would have decreased the overall profitability.

It is a far better option to control the worms properly with a combination of an effective drench and management program, then use other factors including genetics, stocking rate and supplementary feed as a means of controlling fibre diameter. That will mean you can avoid the scouring and mortality rates that go with the worm burdens but still benefit from the finer wool.

The effect of these production differences on income are shown in Table 7.

These significant financial penalties to the group with low drench efficacy would not necessarily be expected to be repeated through the whole flock because wethers, and to a lesser extent ewes, usually develop and maintain good immunity to worms from 12-18 months of age. However, the effect is large enough to justify testing drench efficacy as part of any worm control program.

Table 6: Production effects of resistant worms

Table 7: Economic effects of resistant worms

It is worth noting that the difference between the financial returns for groups treated with 100% vs 85% effective products were small. The implication is that some reduction in drench efficacy may not have large effects on overall net farm profitability, particularly where the flock contains a high proportion of dry sheep Where grazing management can be integrated with the drenching program, which was not done in this trial, one would expect the penalties to be even smaller.

These results also remind us to use our currently effective drenches wisely to preserve their efficacy for as long as possible. Lets avoid the situation where all the drenches have reduced efficacy resulting in significant production losses. This is particularly important where the Macrocyclic Lactone (ML) products are the only highly effective option for drenching. In this situation, the use of another product with slightly lower efficacy, eg 80-90% would be preferable to continued use of one product over a number of years.

DETECTION OF RESISTANCE

Testing the worm population on your farm should be a routine part of a worm control program. There are two methods of testing for resistance. The first (Drenchrite®) is simple, quick and provides a broad indication of the resistance status, although is of very little use in detecting ML resistance. The second, the faecal egg count reduction test is more complex but may be necessary to provide a more comprehensive result.

Drenchrite®

Drench resistance testing has until now not been an easy or pleasant task.
The Drenchrite® test has been available for several years and makes testing much easier for producers. However, it does have serious limitations for testing ML resistance (see below).

All that is required is to collect a good handful of dung, package it up and send it to the laboratory. Kits can be obtained from rural resellers or from most laboratories.

The sample must be fresh, free of dirt, sticks and grass because the test requires worm eggs to be separated from the dung. The more contaminants there are in the sample, the harder it is to separate the eggs for the test.
Once the eggs are separated from the sample, their development into larvae is determined in a range of drench concentrations.

Resistant worms will develop in the presence of drench whereas susceptible ones will not.

Drench groups included in the test are white (B-Z), clear (Levamisole), combination (B-Z + Levamisole) and ML (moxidectin, ivermectin). Ivermectin is always used in the test because any resistance will show up first with Ivermectin.
Other requirements for the sample include:

• A minimum of approximately 200 eggs per gram. If the sample is likely to contain less than 200 eggs, for example if it is from wethers, a worm egg count should be done before sending samples off.
• Sheep should not have been drenched in the last 6-8 weeks because if resistance is present, the drench would have left some resistant worms in the sheep.

These worms will be contributing significant proportions of the total eggs being passed, and therefore the results may overestimate the severity of the resistance problem in your flock.

Limitations

There are several limitations of the Drenchrite® test compared with the standard egg count reduction trial. These are:

• The test is very little value in detecting ML resistance. If you have used ML drenches for a number of years or have used ML capsules you will need to do a FECRT to check ML efficacy.
• There is no provision for varying the dose rate. For example, double doses of levamisole will sometimes provide sufficient worm kill whereas a single dose will not. The Drenchrite® test only allows for a single dose.
• Rametin cannot be included, which in combination with B-Z's has often provided an effective alternative.

If previous tests show you are in the situation of having a limited number of effective drenches, you will need to set up a small trial testing these additional options at the same time as you do your Drenchrite® test.

However, if you have not previously performed a drench test and it is unlikely you will have ML resistance, or the results indicated that you had a number of drench groups which were effective, the Drenchrite® test is the preferred method of testing.

Cost for the test is approximately $200 but will vary slightly depending on the laboratory.

An example of the report from a Drenchrite® test is shown in Table 6. Note that this particular test result shown is only telling you about the resistance status of the Small Brown Stomach Worm which made up the majority of the worm population at the time of testing. This test does not provide any indication of which drenches will be effective against Black Scour Worm which is likely to be in pasture at other times of the year or in other mobs.

Faecal Egg Count Reduction Tests (FECRT)

This was the initial method for resistance testing and is the most accurate and comprehensive. This test does still have a role where the severity of a resistance problem is such that you need to be using drenches other than those which are available in the Drenchrite® test. For example, you may need to be testing double doses of some products or a combination of B-Z and Rametin. Also it is the only effective method of testing for ML resistance.

Setting up the test

The test is to be done on lambs or weaners, preferably lambs of maiden ewes, at weaning because they will have a fairly typical sample of the worm population on the property.

1. Determine the number of groups to be tested. This will be determined by any previous results and discussion with an adviser. If there is a history of frequent ML use, particularly capsules, a half dose of Ivomec should be included to help detect any cases of ML resistance.
2. 2. 10-12 days before sample collection get a mob of ewes and lambs in.
3. Divide lambs into groups of 15 by drafting. This is NOT just an academic exercise and it's important to randomise the sheep to avoid biased results. It may be easier in some yards to allocate sheep to their respective groups in the race. For example, the first sheep goes into the controls, the second into levamisole, third to double levamisole, fourth to B-Z+Levamisole and sixth to ML, then seventh to new drench group, the eighth to the control and so on. Clearly identify the lambs in each group with different coloured spraymark or Siromark. Marks are better on the shoulder or rump than the head. Siromark is preferred because it is completely scourable.
4. Weigh at least six of the heaviest. DO NOT GUESS.
5. Check that your drench gun delivers the correct dose by squirting 4-6 doses into a measuring container. Do not miss this step. It is easier if you have two drench guns and packs - one with Levamisole and one with B-Z.
6. When treating the sheep, be sure each one gets the correct dose – if you are unsure whether or not you drenched a sheep, or it pits out drench, put it out of the trial.
7. Write down which groups received what treatment, the date and the bodyweight's.
8. Run the sheep back with the mob – they do not need to be kept separate or on clean paddocks.
9. In 10-12 days time collect samples from 10 sheep in each group. Yard sheep no more than 2-3 hours before samples are to be collected.

(Table 8) Example of Drenchrite® report

NUMBERS AND SPECIES PRESENT
Average egg count - 240 epg

There may be some difficulty collecting samples from every sheep, hence the inclusion of 15 to provide 10 samples. A minimum of a heaped teaspoon of dung is required per sample.

Place samples in individual bags and each group of ten in separate labeled bags. Exclude air from bags to prevent eggs hatching.

An example of the results of a test and its interpretation is shown below.

Example of Drenchrite® results