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Farm Vet Carlisle

Zolvix, SCOPs and Anthelmintic Resistance

Until March 2010 there were three groups of anthelmintics (wormers):

  • Group 1 (1-BZ) – the benzimidazoles, or white drenchs
  • Group 2 (2-LV) – levamisole, the yellow drench
  • Group 3 (3-ML) – the macrocyclic lactones (the avermectins and milbemycins), or clear drenches

Since March 2010 a fourth group became available:

  • Group 4 (4-AD) – monepental (trade name Zolvix®). Zolvix® has become known as the orange drench.

Sheep farmers have come to rely on anthelmintics to control roundworms and maintain productive farms. Unfortunately, in the last decade or so, worms have developed widespread resistance to the established anthelmintics. One study estimated that 80% of lowland sheep farms had worms that were resistant to white drenches and 50% had worms that were resistant to levamisole. For hill or upland sheep farms the estimates were 55% and 30% respectively. Resistance to the clear drenches is currently at a much lower level but does occur and is certain to be more of a problem in the future.

The first signs of resistance usually noticed by shepherds are poor weight gain, ill-thrift and scour, despite lambs having received worming treatments. Resistance can be detected long before it becomes a clinical problem so resistant worms have been established in the flock for some time before these signs are noticed.

For many years the strategy advocated by the livestock industry to control production losses resulting from roundworm infections was to limit the exposure of livestock to the infective roundworm larvae on the pasture; this was mainly achieved by the use of broad spectrum anthelmintics. The widespread emergence of resistance to the first three groups of anthelmintics has forced many producers to change their practices and the wider livestock industry has had to re-evaluate its approach to the management of roundworms.

The rate of selection for resistant worms has been highest on farms that achieved the best control of roundworms in the short term; these tended to be the farms that used the most anthelmintics. Sadly, the development of resistance has been an inevitable consequence of good short term worm control and not necessarily the result of bad farming practices.

Unfortunately, most of the recommendations given to help slow down the development of resistance involve compromising the control of worms in the short time.

SCOPS (The Sustainable Control of Ovine Parasites) was established in 2003 by the sheep industry and DEFRA to provide expert advice on the sustainable control of roundworms and other parasites in response to the emerging problem of anthelmintic resistance.

There a currently eight recommendations, which are discussed later:

  • Use anthelmintics only when necessary.
  • Administer anthelmintics effectively.
  • Select the appropriate anthelmintic for the task.
  • Test for anthelmintic resistance on your farm.
  • Reduce dependence on anthelmintics.
  • Adopt strategies to preserve susceptible worms on the farm.
  • Use effective quarantine strategies to prevent importing resistant worms.
  • Develop a control strategy with your vet.

Treatment of ewes

Whole-flock pre-tupping anthelmintic treatments are generally not considered necessary. While an increased lambing percentage is likely to result from treating “wormy” ewes (as long as they receive adequate nutrition), unnecessary treatments incur extra expense and effort and encourage the development of resistance. It may be advisable to only treat immature or lean ewes and leave older, fitter ones untreated. Treating ewes between October and April strongly selects for resistant Heamonchus worms in particular.

Treatment of lambs

It may not be absolutely necessary to treat lambs late in the year as by then they a likely to be developing a degree of immunity. The need for worming will be less if lambs can be moved on to relatively clean pasture. After warm, dry conditions there are fewer larvae on the pasture. This means a reduced challenge for the lambs and less need for worming.

It can be difficult to decide when lambs are due an anthelmintic treatment. There will inevitably be some production losses if anthelmintics are administered only when a few lambs have started to scour or “fall back”. Exploring strategies to extend the period between treatments to four or five weeks or more will reduce overall anthelmintic usage and reduce the selection pressure.

It is possible to monitor worm egg counts in faeces and administer anthelmintics in response to a rise, but this is difficult in practice as egg counts can increase very rapidly and the count may not correlate well with the harm the worms are doing or the need for worming. Faecal worm egg counts (FWEC) can be used to establish the pattern of pasture contamination on individual farms with a view to setting “intervention targets” for use in subsequent years.

Administer anthelmintics effectively

When worming lambs or ewes it is always advisable to dose to the heaviest animal in the group. There are wide safety margins on the benzimidazoles and the “mectins” (group 3) so be sure not to underestimate the weight of the sheep.

Anthelmintics should be stored properly and not used after their expiry dates.

Lambs should ideally be yarded for 12 to 24 hours before being wormed with oral drenches, especially if they have been on lush pasture; this slows the passage of gut contents (and the wormer) and helps the wormer to work better. Heavily pregnant ewes should not be fasted before anthelmintic treatments as this could precipitate twin lamb disease.

Anthelmintics work much less effectively if they bypass the rumen and enter the abomasum directly; this is most likely to occur if lambs receive high volumes of anthelmintics or if they are wormed straight from the pasture.

Select the appropriate anthelmintic for the task

It is not advisable to use a combination treatment (fluke and worms) if you only actually need to treat ewes or lambs for roundworms and not for fluke.

If anthelmintics are employed to control Nematodirus infection (typically in May), BZ wormers should be considered as BZ resistant Nematodirus worms are not currently widespread. This avoids the exposure of all worm types to clear and yellow drenches.

Test for anthelmintic resistance on your farm.

Worm egg counts can be performed at our Rosehill surgery, and we should have the results back to you within 24 hours. We charge £5.00 per pooled sample.

We would like samples from 10 individual sheep.

To collect the samples hold at least 10 sheep or lambs in the corner of a pen or field for 10 minutes and collect fresh samples.

We usually examine two pooled samples, each containing faeces from five sheep. Ideally the samples should be pooled at the surgery after weighing; if you pool them on-farm try to make sure each sheep contributes a fifth to the pooled sample.

The samples should be collected 7 days after treatment with levamisole or 14 days after treatment with either the white or clear drenches.

Samples should be delivered to the surgery within 48 hours of collection. The samples should be refrigerated and kept in an airtight container if they are not delivered immediately.

Reduce dependence on anthelmintics

Try to use safe pasture if it’s available. Few commercial farms can provide sufficient safe pasture and rely on the use of anthelmintic drugs to minimise pasture contamination.

Use a ram that has been bred for worm resistance when breeding replacement ewes.

Incorporating chicory into normal grass leys can reduce worm egg counts in lambs grazing with their mothers. Other plants have also been shown to be beneficial and research is on-going.

Adopt strategies to preserve susceptible worms on the farm.


Don’t move sheep on to clean pasture after worming.

Avoid worming sheep from late autumn to spring.

Leave a proportion of the flock untreated.

“It is likely the alleles (genes) conferring anthelmintic resistance are already present in most, if not all, UK flocks, although often at low or clinically insignificant levels”. Neil Sargison, Professor of Farm Animal Practice at the Royal (Dick) Veterinary School in Edinburgh.

At one time the underlying principle of nematode control was to limit the exposure of susceptible lambs to infective larvae on the pasture using a ‘dose and move’ strategy. It is now advised that sheep are moved to contaminated pasture after anthelmintic treatments. Why is this?

Roundworms are either resistant to a particular anthelmintic or susceptible; this applies to eggs as well as the adults.
In order to prevent or slow down the emergence of resistant worm populations it is important that worm control strategies minimise any advantages resistant worms have over susceptible worms.

Roundworms exist as adults within the sheep and as eggs or larvae on the pasture. Parasitologists refer to the population of roundworms on pasture as the “in refugia” population.

Employing worming strategies that preserve a population of susceptible in refugia worms is an important part of reducing the rate at which worms develop resistance to anthelmintics.

Moving stock to clean pasture after worming gives resistant worms a huge advantage over susceptible worms as eggs from the resistant worms are the only ones passed on the clean pasture; they have no competition from susceptible worms when they develop into larvae and are eaten by sheep. By maintaining a population of susceptible worms the efficacy of anthelmintics is preserved.

For the same reason the best time to treat sheep with anthelmintics in order to prevent the development of resistance is when the largest proportion of the worm population is on the pasture (in the forms of eggs) rather than in the sheep. This reduces the proportion of the worm population that is exposed to the anthelmintic. In practice this means avoiding worming during winter.

Another way to reduce any survival advantage resistant worms might have is to leave a proportion of the flock untreated. Some of the worms they are carrying will be susceptible and seed the pasture with eggs that will develop into susceptible worms. One would expect older, immune ewes and the fittest lambs to be producing the fewest eggs but the eggs they do produce will still add to the in refugia population.

The high levels of sheep scab currently being seen in the UK are treated on many farms by repeated treatments with moxidectin injections often in the winter. This practice strongly selects for the development of resistance and alternative control measures for sheep scab should be used if possible. Fortunately resistance to moxidectin in currently at quite low levels.

Use effective quarantine strategies to prevent importing resistant worms

Avoid the introduction of resistant worms on to you farm.

When buying in sheep treat with Zolvix® (no known resistance) then yard them for 48 hours before turning them onto contaminated pastures.

Consider also using moxidectin to prevent the introduction of sheep scab. (Using only moxidectin increases the likelihood of introducing worms resistant to group 3 wormers.)


Zolvix® has an important role to play in managing internal parasites.

It has three main uses:

  • Firstly, it is used as a mid-season drench for lambs at (or shortly before) weaning. There is currently no known resistance and it should eliminate worms resistant to the other anthelmintic groups.
  • Secondly, it should be used as a “quarantine” drench as outlined above.
  • Finally, it can be used as part of the worm control program on farms with double or triple resistance to the other three groups of anthelmintics.

To discuss the management of metabolic diseases or another other sheep problem please contact a vet at the surgery.