Is your wormer working? Resistance in gut worms in sheep
- What is wormer resistance in sheep?
- The scale of wormer resistance in sheep
- How will I know if there is wormer resistance on my farm?
- Top tips for reducing the risk of wormer resistance
Wormer resistance is an increasingly significant problem in the UK sheep flock. Various UK-wide studies looking at the level of wormer resistance in sheep have shown alarming results.The level of resistance to the different wormer classes varies from farm to farm, depending on the population of worms present. Resistance levels can also vary depending on the time of year, as there are different species of worm which affect sheep seasonally – see our blog on diagnosis and treatment of gut worms in sheep.
What is wormer resistance in sheep gut worms?
You have probably heard of wormer resistance, but could you explain what it is? Wormer resistance, put simply, is the worm evolving to survive the treatment that would usually kill it. When a worm population inside an animal is first exposed to an effective new treatment, given at the right dose, most if not all the worms will be killed. With repeated exposure, or a sub-lethal dose, genetic mutations can happen within a small proportion of the worm population which will allow them to survive the treatment. These resistant worms pass on the resistance genes to their offspring. Because these worms have an advantage over the rest of the population, they can quickly become a significant proportion of the worms in your sheep and on your pasture.
Resistant worms pose a huge threat to your flock if not managed. Gut worms are one of the most significant challenges for the UK sheep flock, estimated to cost £84 million yearly.1 The direct costs of spending time and money on ineffective treatments are obvious,but the hidden costs will be where the real damage is done. Lambs infected with resistant worms that have survived the treatment will continue to have reduced growth and health.
All sheep farmers need to take resistance seriously, limiting the damage caused by arming yourself with the knowledge of which wormers work on your farm and how to control worms, whilst not encouraging further resistance to develop or be introduced onto your farm.
The scale of wormer resistance in sheep
There have been numerous studies to investigate the level of resistance on farms across the UK. In the past 10 years, there have been four studies conducted (2 in the SW of England and 2in Wales).2,3,4 The levels of resistance detected to White (Benzimidazole) wormers ranged from 89% - 100% of farms tested. 60-77% of farms tested had worms that were resistant to Yellow (Levamisole) wormers and between 25-100% of farms tested had wormer resistance to Clear (Ivermectin) wormers.
These levels show how widespread the problem of wormer resistance is. It is worth noting that if you have worms that are resistant to one wormer within e.g. the ‘White’ wormer class,then they will be resistant to ALL wormers within that class (even if the wormer is blue or green, as some are in this class of wormer). It is therefore imperative that you:
1.Know which wormer classes work on your farm and;
2.You are confident that you know which class of wormer you are buying.
How will I know if there is wormer resistance on my farm?
It is very likely that the worms currently present on your farm and in your sheep have some level of resistance to one or more classes of wormer. If you have advanced wormer resistance in the worms on your farm, you might notice that lambs are still dirty or sick even after treatment. This is a red flag and should be investigated urgently.
Lower levels of resistance can be more difficult to appreciate. If wormer resistance is in the earlier stages of development, lambs may appear to be clinically improved (reduced scour etc.)after treatment, but some worms will still be present, reducing growth rates and contaminating pasture with worm eggs.
Testing for resistance
It is a good idea to check the effectiveness of any wormer treatment you use. You should firstly monitor worm burdens by using faecal worm egg counts (FWECs). This will give you the pre-treatment FWEC. The crudest method for checking the effectiveness of the treatment is to take a post-drench muck sample. This needs to be 14 days after treatment with White (BZ) or Clear (ML) wormers, or 7-10 days after treatment with a yellow (LV) wormer. If the reduction in FWEC is <95%, this can be an indication that there is resistance present to that wormer.
A full faecal egg count reduction test should be used to determine the ‘resistance profile’ of the worms present on your farm. This is a more scientific, controlled study on your farm, which is relatively simple to do – ask your vet for more details on this but the general procedure is;
1. Groups of lambs are monitored with FWECs.
2. When the FWEC indicates that treatment is needed, around 15 lambs per group are randomly selected to be treated with either White (BZ), Yellow (LV), Clear (ML-Ivermectin), Clear (ML-Moxidectin)and a group of 15 are left as a ‘control’ group without treatment.
3. Faeces are taken directly from the lamb, lambs are individually weighed, identified, and treated.
4. The same lambs are then re-visited 7 and 14 days later, where a post-drench sample of muck is taken. These samples have individual FWECs, which are used to estimate the level of resistance present to each class of wormer.
This ‘resistance profile’ will be the most important piece of information you have when deciding which wormers to use. You should repeat this resistance profile for both summer and winter,as the population of worms can be different at these times. Resistance profiles will change over time, so drench checks should continue to be used and the profile repeated as necessary.
Top tips for reducing the risk of wormer resistance
Worms will evolve to survive the treatments you use; this is out of your control. Managing the population of worms on your farm is, however, within your control to some degree. Strategies that you can use to reduce the speed of resistance developing on your farm include:
1. Keeping a susceptible population of worms ‘in refugia’. As discussed in ‘Gut worms in lambs – Diagnosis and treatment of parasitic gastroenteritis (PGE)’, treating the whole group at once can lead to the acceleration of resistance developing.If all the animals in the group are treated with a wormer at the same time,then all the worms inside those lambs are being exposed to the drug. ONLY worms that are resistant to the wormer will survive the treatment and continue to produce eggs, which are subsequently released onto your pasture. This new generation of worms will also be resistant. This means that ALL the eggs being released onto your pasture are likely to have the genes that are resistant to the wormer used.
We know that worms will affect lambs to varying degrees, so some animals that are clean and growing well may not need to be wormed. If you leave some animals in the group (e.g. the best 10%) untreated, you will be leaving a ‘normal’ population(resistant and susceptible) of worms inside these animals to release a ‘normal’population of eggs on your pasture. This reduces the selection pressure for resistance. This concept is called leaving a population ‘in refugia’.
Similarly,it is also good practice to send lambs back to their current pasture for 48 hours after worming before moving to ‘fresh’ pasture, to ensure they pick up a‘normal’ population of worms, rather than just taking the resistant population to the new pasture.
2. Ensure you use a wormer that works on your farm. Use your resistance profile to discuss which wormer to use with your Vet/SQP/RAMA. Rotate effective wormer classes as much as possible to reduce the level of exposure the worms have to a single wormer class.
The Chanelle anthelmintic range includes wormers for sheep across the three main wormer classes to help support responsible worming:
1. Group 1 BZ – Albex
2. Group 2 LV – Chanaverm
3. Group 3 ML – Animec
a. Group 3 ML (Moxidectin) – Moxodex,long-lasting residual activity for 8 weeks, with a meat withdrawal of just 14 days. Moxidectin was shown to have the least resistance in the studies conducted in Wales, with just 19% showing resistance to this active.4
There are also two ‘newer’ classes of wormer (Groups 4 & 5) which should be reserved for isolation drenches in purchased animals, or with care as a ‘clear out’ drench, especially if triple resistance (resistance to group 1-3) is present on your farm.
See Gut Worms in lambs – Diagnosis and treatment of parasitic gastroenteritis (PGE)’ for more detail on wormers.
3. Targeting treatments to only the animals that need it. Selective, targeted treatments are discussed in more detail in our Gut worms – diagnosis and treatment blog. Use FWECs, weight gains and cleanliness of lambs to selectively treat the animals that need worming, reducing the amount of wormer you use. Target wormer use in ewes by using FWECs, especially around lambing, to avoid blanket treatments as this can be an effective way to detrimentally select for resistant worms.
4. Reduce reliance on wormers. Our blog – ‘Gut worms in lambs – what to look out for’ discusses strategies to reduce reliance on wormers.
5. Follow best practice worming guidelines every time you use a wormer treatment.5 Ensuring you deliver an effective dose of wormer is imperative as sub-lethal doses will increase the chance of worms developing resistance. SCOPS (Sustainablecontrol of parasites in sheep) guidelines include;
a. Maintain and calibrate dosing guns before each use.
b. Make sure the wormer you are using is in date and has been stored properly.
c. Either dose to accurate weight of animal if you have it, or dose to the heaviest in the group (ensuring you do not over dose small lambs).
6. Keep your flock as healthy as possible. Healthy animals are less likely to be severely affected by worms in general as they can mount a stronger immune response.Investigate thin and poor doing animals, trace element deficiencies and disease outbreaks. Put robust preventative plans in place to give your flock the best chance of having a strong immune system. Similarly, make sure that the nutritional needs of your flock are always met.
7. Select replacement ewe lambs that are resilient to worm infection. We know that individual lambs are affected by worm infections to varying degrees.Recording weight gains, number of wormer treatments needed, and cleanliness in ewe lambs can help you select replacements which have resilience to worm infections.
8. Do everything you can to avoid buying in someone else’s resistant worms! Ensure your quarantine protocol is robust for introduced animals.
References:
1. Costs of the major endemic diseases of sheep in Great Britain and the potential benefits of reduction in disease impact —University of Edinburgh Research Explorer).
2. Bull K, Glover MJ, Rose Vineer H, Morgan ER.Increasing resistance to multiple anthelmintic classes in gastrointestinal nematodes on sheep farms in southwest England. Vet Rec. 2022 Jun;190(11):e1531.doi: 10.1002/vetr.1531. Epub 2022 Mar 26. PMID: 35338780; PMCID: PMC9310741.
3. Glover M, Clarke C, Nabb L, Schmidt J.Anthelmintic efficacy on sheep farms in south-west England. Vet Rec. 2017 Apr15;180(15):378. doi: 10.1136/vr.104151. Epub 2017 Feb 6. PMID: 28167646.
4. WAARD_FINAL_PROJECT_REPORT_1_-_19-11-15.pdf (meatpromotion.wales)
5. SCOPS
Is your wormer working? Resistance in gut worms in sheep
- What is wormer resistance in sheep?
- The scale of wormer resistance in sheep
- How will I know if there is wormer resistance on my farm?
- Top tips for reducing the risk of wormer resistance
Wormer resistance is an increasingly significant problem in the UK sheep flock. Various UK-wide studies looking at the level of wormer resistance in sheep have shown alarming results.The level of resistance to the different wormer classes varies from farm to farm, depending on the population of worms present. Resistance levels can also vary depending on the time of year, as there are different species of worm which affect sheep seasonally – see our blog on diagnosis and treatment of gut worms in sheep.
What is wormer resistance in sheep gut worms?
You have probably heard of wormer resistance, but could you explain what it is? Wormer resistance, put simply, is the worm evolving to survive the treatment that would usually kill it. When a worm population inside an animal is first exposed to an effective new treatment, given at the right dose, most if not all the worms will be killed. With repeated exposure, or a sub-lethal dose, genetic mutations can happen within a small proportion of the worm population which will allow them to survive the treatment. These resistant worms pass on the resistance genes to their offspring. Because these worms have an advantage over the rest of the population, they can quickly become a significant proportion of the worms in your sheep and on your pasture.
Resistant worms pose a huge threat to your flock if not managed. Gut worms are one of the most significant challenges for the UK sheep flock, estimated to cost £84 million yearly.1 The direct costs of spending time and money on ineffective treatments are obvious,but the hidden costs will be where the real damage is done. Lambs infected with resistant worms that have survived the treatment will continue to have reduced growth and health.
All sheep farmers need to take resistance seriously, limiting the damage caused by arming yourself with the knowledge of which wormers work on your farm and how to control worms, whilst not encouraging further resistance to develop or be introduced onto your farm.
The scale of wormer resistance in sheep
There have been numerous studies to investigate the level of resistance on farms across the UK. In the past 10 years, there have been four studies conducted (2 in the SW of England and 2in Wales).2,3,4 The levels of resistance detected to White (Benzimidazole) wormers ranged from 89% - 100% of farms tested. 60-77% of farms tested had worms that were resistant to Yellow (Levamisole) wormers and between 25-100% of farms tested had wormer resistance to Clear (Ivermectin) wormers.
These levels show how widespread the problem of wormer resistance is. It is worth noting that if you have worms that are resistant to one wormer within e.g. the ‘White’ wormer class,then they will be resistant to ALL wormers within that class (even if the wormer is blue or green, as some are in this class of wormer). It is therefore imperative that you:
1. Know which wormer classes work on your farm and;
2.You are confident that you know which class of wormer you are buying.
How will I know if there is wormer resistance on my farm?
It is very likely that the worms currently present on your farm and in your sheep have some level of resistance to one or more classes of wormer. If you have advanced wormer resistance in the worms on your farm, you might notice that lambs are still dirty or sick even after treatment. This is a red flag and should be investigated urgently.
Lower levels of resistance can be more difficult to appreciate. If wormer resistance is in the earlier stages of development, lambs may appear to be clinically improved (reduced scour etc.)after treatment, but some worms will still be present, reducing growth rates and contaminating pasture with worm eggs.
Testing for resistance
It is a good idea to check the effectiveness of any wormer treatment you use. You should firstly monitor worm burdens by using faecal worm egg counts (FWECs). This will give you the pre-treatment FWEC. The crudest method for checking the effectiveness of the treatment is to take a post-drench muck sample. This needs to be 14 days after treatment with White (BZ) or Clear (ML) wormers, or 7-10 days after treatment with a yellow (LV) wormer. If the reduction in FWEC is <95%, this can be an indication that there is resistance present to that wormer.
A full faecal egg count reduction test should be used to determine the ‘resistance profile’ of the worms present on your farm. This is a more scientific, controlled study on your farm, which is relatively simple to do – ask your vet for more details on this but the general procedure is;
1. Groups of lambs are monitored with FWECs.
2. When the FWEC indicates that treatment is needed, around 15 lambs per group are randomly selected to be treated with either White (BZ), Yellow (LV), Clear (ML-Ivermectin), Clear (ML-Moxidectin)and a group of 15 are left as a ‘control’ group without treatment.
3. Faeces are taken directly from the lamb, lambs are individually weighed, identified, and treated.
4. The same lambs are then re-visited 7 and 14 days later, where a post-drench sample of muck is taken. These samples have individual FWECs, which are used to estimate the level of resistance present to each class of wormer.
This ‘resistance profile’ will be the most important piece of information you have when deciding which wormers to use. You should repeat this resistance profile for both summer and winter,as the population of worms can be different at these times. Resistance profiles will change over time, so drench checks should continue to be used and the profile repeated as necessary.
Top tips for reducing the risk of wormer resistance
Worms will evolve to survive the treatments you use; this is out of your control. Managing the population of worms on your farm is, however, within your control to some degree. Strategies that you can use to reduce the speed of resistance developing on your farm include:
1. Keeping a susceptible population of worms ‘in refugia’. As discussed in ‘Gut worms in lambs – Diagnosis and treatment of parasitic gastroenteritis (PGE)’, treating the whole group at once can lead to the acceleration of resistance developing.If all the animals in the group are treated with a wormer at the same time,then all the worms inside those lambs are being exposed to the drug. ONLY worms that are resistant to the wormer will survive the treatment and continue to produce eggs, which are subsequently released onto your pasture. This new generation of worms will also be resistant. This means that ALL the eggs being released onto your pasture are likely to have the genes that are resistant to the wormer used.
We know that worms will affect lambs to varying degrees, so some animals that are clean and growing well may not need to be wormed. If you leave some animals in the group (e.g. the best 10%) untreated, you will be leaving a ‘normal’ population(resistant and susceptible) of worms inside these animals to release a ‘normal’population of eggs on your pasture. This reduces the selection pressure for resistance. This concept is called leaving a population ‘in refugia’.
Similarly,it is also good practice to send lambs back to their current pasture for 48 hours after worming before moving to ‘fresh’ pasture, to ensure they pick up a‘normal’ population of worms, rather than just taking the resistant population to the new pasture.
2. Ensure you use a wormer that works on your farm. Use your resistance profile to discuss which wormer to use with your Vet/SQP/RAMA. Rotate effective wormer classes as much as possible to reduce the level of exposure the worms have to a single wormer class.
The Chanelle anthelmintic range includes wormers for sheep across the three main wormer classes to help support responsible worming:
1. Group 1 BZ – Albex
2. Group 2 LV – Chanaverm
3. Group 3 ML – Animec
a. Group 3 ML (Moxidectin) – Moxodex,long-lasting residual activity for 8 weeks, with a meat withdrawal of just 14 days. Moxidectin was shown to have the least resistance in the studies conducted in Wales, with just 19% showing resistance to this active.4
There are also two ‘newer’ classes of wormer (Groups 4 & 5) which should be reserved for isolation drenches in purchased animals, or with care as a ‘clear out’ drench, especially if triple resistance (resistance to group 1-3) is present on your farm.
See Gut Worms in lambs – Diagnosis and treatment of parasitic gastroenteritis (PGE)’ for more detail on wormers.
3. Targeting treatments to only the animals that need it. Selective, targeted treatments are discussed in more detail in our Gut worms – diagnosis and treatment blog. Use FWECs, weight gains and cleanliness of lambs to selectively treat the animals that need worming, reducing the amount of wormer you use. Target wormer use in ewes by using FWECs, especially around lambing, to avoid blanket treatments as this can be an effective way to detrimentally select for resistant worms.
4. Reduce reliance on wormers. Our blog – ‘Gut worms in lambs – what to look out for’ discusses strategies to reduce reliance on wormers.
5. Follow best practice worming guidelines every time you use a wormer treatment.5 Ensuring you deliver an effective dose of wormer is imperative as sub-lethal doses will increase the chance of worms developing resistance. SCOPS (Sustainablecontrol of parasites in sheep) guidelines include;
a. Maintain and calibrate dosing guns before each use.
b. Make sure the wormer you are using is in date and has been stored properly.
c. Either dose to accurate weight of animal if you have it, or dose to the heaviest in the group (ensuring you do not over dose small lambs).
6. Keep your flock as healthy as possible. Healthy animals are less likely to be severely affected by worms in general as they can mount a stronger immune response.Investigate thin and poor doing animals, trace element deficiencies and disease outbreaks. Put robust preventative plans in place to give your flock the best chance of having a strong immune system. Similarly, make sure that the nutritional needs of your flock are always met.
7. Select replacement ewe lambs that are resilient to worm infection. We know that individual lambs are affected by worm infections to varying degrees.Recording weight gains, number of wormer treatments needed, and cleanliness in ewe lambs can help you select replacements which have resilience to worm infections.
8. Do everything you can to avoid buying in someone else’s resistant worms! Ensure your quarantine protocol is robust for introduced animals.
References:
1. Costs of the major endemic diseases of sheep in Great Britain and the potential benefits of reduction in disease impact —University of Edinburgh Research Explorer).
2. Bull K, Glover MJ, Rose Vineer H, Morgan ER.Increasing resistance to multiple anthelmintic classes in gastrointestinal nematodes on sheep farms in southwest England. Vet Rec. 2022 Jun;190(11):e1531.doi: 10.1002/vetr.1531. Epub 2022 Mar 26. PMID: 35338780; PMCID: PMC9310741.
3. Glover M, Clarke C, Nabb L, Schmidt J.Anthelmintic efficacy on sheep farms in south-west England. Vet Rec. 2017 Apr15;180(15):378. doi: 10.1136/vr.104151. Epub 2017 Feb 6. PMID: 28167646.
4. WAARD_FINAL_PROJECT_REPORT_1_-_19-11-15.pdf (meatpromotion.wales)
5. SCOPS