Every year there are discussions as to whether it is beneficial to treat ewes for worms at lambing. This subject was addressed in detail by Teagasc researcher Orla Keane at the recent lowland sheep conferences. Orla’s message on the subject was clear – healthy mature ewes should not receive worm treatment.
The reasoning was also equally clear. Irish sheep farms face serious challenges, with anthelmintic resistance developing at an alarming rate to the three most commonly used anthelmintic classes. Dosing healthy mature ewes will contribute to the rate of resistance developing.
Orla Keane, Teagasc researcher.
Pasture contamination
There were some questioning this advice, which is not surprising, given that some drug companies are currently advising farmers to treat ewes at lambing, citing reduced pasture contamination. In addressing this point, Orla explained how a population of susceptible (non-resistant) worms in ewes can actually be beneficial in helping to reduce the rate of resistance developing.
Increasing the population of susceptible worms is known as enhancing refugia. She explained how resistance occurs and how the rate can be accelerated or delayed.
Anthelmintic resistance refers to the ability of worms to survive a lethal dose that should kill them. Resistance is said to occur where treatment does not kill at least 95% of the worms present. This can range from treatments still achieving a relatively good kill and being close to the 95% kill mark, to the worst cases, where only 20% to 30% of the worm burden in an animal is killed.
There are five anthelmintic classes, with each having a different mode of action. These classes are detailed in Table 1. As shown, there are a number of chemical ingredients present in some classes. But within the same class, products share the same mode of action, meaning that if resistance occurs to albendazole for example, then it has also generally occurred to fenbendazole and oxfendazole.
Rate of resistance developing
When resistance is present and treatment occurs, then susceptible worms are killed leaving resistant worms present. Each time this is repeated, the number of resistant worms multiplies until they dominate the population in an environment.
Orla presented the results of research carried out from 2013 to 2015 and again in 2018 / 2019. In the first trial, resistance was identified on close to 70% of the 550 farms where white wormers were tested. In the region of 48% of 316 farms tested for yellow drenches showed resistance, while 36% of 568 farms (405 abamectin and 163 moxidectin) showed resistance to ivermectins.
“We followed this up in 2018 and 2019 with a smaller number of farms, but with a more specific, more detailed test. What we found was on every farm we tested, the white doses were not working, on 17% of the farms the yellow doses weren’t working and on 56% ivermectins were not working,” Orla said.
On every farm we tested, the white doses were not working, on 17% of the farms the yellow doses weren’t working and on 56% ivermectins were not working
She pointed out that as well as the incidence of resistance increasing, the gravity of the situation on individual farms was of huge concern. For example, there were some farms where resistance to an anthelmintic class had developed to such an extent that only 20% to 30% of worms may have been killed in a treatment.
It should be pointed out that resistance has developed to stomach and gut worms and that as of yet, no resistance has been identified with nematodirus. As such, the advice is to continue to only treat lambs with a white wormer for nematodirus.
Dosing ewes
Treatment where it is not warranted kills susceptible worms and increases the population of resistant worms. Orla detailed what pasture contamination is likely to look like for a mid-season flock, which is shown in Figure 1.
Figure 1: Pasture contamination of a typical mid-season flock.
The level of pasture contamination carried over from the previous year typically starts to decline early in the season, with mild weather at this stage of the year being the precursor to worms dying. Orla acknowledged that the worm burden in ewes increases in the run up to lambing and for six weeks after, due to a relaxation of immunity, but that this is normal and an important source of refugia before the main threat occurs. As seen in the graphic, the worm burden in ewes will reduce as normal immunity resumes.
The exception to this is treatment of individual sheep that may be immunocompromised, such as ewes in poor body condition or ewes with underlying health issues. These ewes can often give the impression of a high worm count in a seemingly healthy group of ewes and Orla advises that in such cases, further analysis is required to warrant treatment of healthy ewes.
Critical advice
Where a farmer still wants to treat their flock, then there are a few critical pieces of advice that should be followed to reduce the rate of resistance developing. The first is to investigate what anthelmintic class works and to ensure this is used to reduce the advantage given to resistant worms. This advice applies to all animals on the farm and not just ewes.
The two new worm classes can be used strategically in this regard, at times when there is likely be a greater threat from resistant worms.
If ewes are treated while outdoors, they should be retained on swards that are likely to have a high population of worms
The second piece of advice is to ensure that ewes receive the recommended volume of treatment for their weight. Heavily pregnant ewes in good body condition can weigh upwards of 15kg to 20kg heavier than their normal mature weight. Failing to take this into account will lead to under-dosing, which also promotes resistance. Another common occurrence leading to under-dosing is equipment not being calibrated correctly.
Finally, if ewes are treated while outdoors, they should be retained on swards that are likely to have a high population of worms and not moved to pastures that have a low burden where resistant worms will dominate quicker.
Product choice
It is also important to remember that there are times when animals can be treated for worms when treatment is targeting a different parasite – i.e. combination products. For example, treating sheep with an ivermectin for external parasites will also treat for worms.
Orla also advises not using wormer/flukicide products and avoiding the continual use of wormers from the same anthelmintic class.
Orla highlighted some practices that can be carried out on every farm to reduce the rate of resistance developing. These are all simple, cost-effective measures that farmers should have no issues adopting. They include:
Avoid dosing mature ewes unless there is a demonstrated need.Use a Benzimidazole (white dose) only to treat nematodirus.Implement strong biosecurity measures. Quarantine treat stock coming on to the farm using either of the two new (orange or purple) active ingredients.Regularly monitor faecal egg counts to appropriately time treatments.
Every year there are discussions as to whether it is beneficial to treat ewes for worms at lambing. This subject was addressed in detail by Teagasc researcher Orla Keane at the recent lowland sheep conferences. Orla’s message on the subject was clear – healthy mature ewes should not receive worm treatment.
The reasoning was also equally clear. Irish sheep farms face serious challenges, with anthelmintic resistance developing at an alarming rate to the three most commonly used anthelmintic classes. Dosing healthy mature ewes will contribute to the rate of resistance developing.
Orla Keane, Teagasc researcher.
Pasture contamination
There were some questioning this advice, which is not surprising, given that some drug companies are currently advising farmers to treat ewes at lambing, citing reduced pasture contamination. In addressing this point, Orla explained how a population of susceptible (non-resistant) worms in ewes can actually be beneficial in helping to reduce the rate of resistance developing.
Increasing the population of susceptible worms is known as enhancing refugia. She explained how resistance occurs and how the rate can be accelerated or delayed.
Anthelmintic resistance refers to the ability of worms to survive a lethal dose that should kill them. Resistance is said to occur where treatment does not kill at least 95% of the worms present. This can range from treatments still achieving a relatively good kill and being close to the 95% kill mark, to the worst cases, where only 20% to 30% of the worm burden in an animal is killed.
There are five anthelmintic classes, with each having a different mode of action. These classes are detailed in Table 1. As shown, there are a number of chemical ingredients present in some classes. But within the same class, products share the same mode of action, meaning that if resistance occurs to albendazole for example, then it has also generally occurred to fenbendazole and oxfendazole.
Rate of resistance developing
When resistance is present and treatment occurs, then susceptible worms are killed leaving resistant worms present. Each time this is repeated, the number of resistant worms multiplies until they dominate the population in an environment.
Orla presented the results of research carried out from 2013 to 2015 and again in 2018 / 2019. In the first trial, resistance was identified on close to 70% of the 550 farms where white wormers were tested. In the region of 48% of 316 farms tested for yellow drenches showed resistance, while 36% of 568 farms (405 abamectin and 163 moxidectin) showed resistance to ivermectins.
“We followed this up in 2018 and 2019 with a smaller number of farms, but with a more specific, more detailed test. What we found was on every farm we tested, the white doses were not working, on 17% of the farms the yellow doses weren’t working and on 56% ivermectins were not working,” Orla said.
On every farm we tested, the white doses were not working, on 17% of the farms the yellow doses weren’t working and on 56% ivermectins were not working
She pointed out that as well as the incidence of resistance increasing, the gravity of the situation on individual farms was of huge concern. For example, there were some farms where resistance to an anthelmintic class had developed to such an extent that only 20% to 30% of worms may have been killed in a treatment.
It should be pointed out that resistance has developed to stomach and gut worms and that as of yet, no resistance has been identified with nematodirus. As such, the advice is to continue to only treat lambs with a white wormer for nematodirus.
Dosing ewes
Treatment where it is not warranted kills susceptible worms and increases the population of resistant worms. Orla detailed what pasture contamination is likely to look like for a mid-season flock, which is shown in Figure 1.
Figure 1: Pasture contamination of a typical mid-season flock.
The level of pasture contamination carried over from the previous year typically starts to decline early in the season, with mild weather at this stage of the year being the precursor to worms dying. Orla acknowledged that the worm burden in ewes increases in the run up to lambing and for six weeks after, due to a relaxation of immunity, but that this is normal and an important source of refugia before the main threat occurs. As seen in the graphic, the worm burden in ewes will reduce as normal immunity resumes.
The exception to this is treatment of individual sheep that may be immunocompromised, such as ewes in poor body condition or ewes with underlying health issues. These ewes can often give the impression of a high worm count in a seemingly healthy group of ewes and Orla advises that in such cases, further analysis is required to warrant treatment of healthy ewes.
Critical advice
Where a farmer still wants to treat their flock, then there are a few critical pieces of advice that should be followed to reduce the rate of resistance developing. The first is to investigate what anthelmintic class works and to ensure this is used to reduce the advantage given to resistant worms. This advice applies to all animals on the farm and not just ewes.
The two new worm classes can be used strategically in this regard, at times when there is likely be a greater threat from resistant worms.
If ewes are treated while outdoors, they should be retained on swards that are likely to have a high population of worms
The second piece of advice is to ensure that ewes receive the recommended volume of treatment for their weight. Heavily pregnant ewes in good body condition can weigh upwards of 15kg to 20kg heavier than their normal mature weight. Failing to take this into account will lead to under-dosing, which also promotes resistance. Another common occurrence leading to under-dosing is equipment not being calibrated correctly.
Finally, if ewes are treated while outdoors, they should be retained on swards that are likely to have a high population of worms and not moved to pastures that have a low burden where resistant worms will dominate quicker.
Product choice
It is also important to remember that there are times when animals can be treated for worms when treatment is targeting a different parasite – i.e. combination products. For example, treating sheep with an ivermectin for external parasites will also treat for worms.
Orla also advises not using wormer/flukicide products and avoiding the continual use of wormers from the same anthelmintic class.
Orla highlighted some practices that can be carried out on every farm to reduce the rate of resistance developing. These are all simple, cost-effective measures that farmers should have no issues adopting. They include:
Avoid dosing mature ewes unless there is a demonstrated need.Use a Benzimidazole (white dose) only to treat nematodirus.Implement strong biosecurity measures. Quarantine treat stock coming on to the farm using either of the two new (orange or purple) active ingredients.Regularly monitor faecal egg counts to appropriately time treatments. 
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