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Video 39 Roundworms and resistance in sheep

One of the big challenges in our flocks is parasites and dealing with worm resistance. Like every aspect of animal health we need to have a plan.

First, we must understand some key principles


The roundworms we commonly see in Irish flocks are nemadtodirus, teladorsagia and trichostrongylus. There are more roundworms in sheep but these are some of the main intestinal roundworms we see. I have also seen more lungworm in sheep for the last number of years in the autumn time. We also have case of Haemonchus or barber pole worm being diagnosed in Ireland.

The main clinical symptoms of roundworms are weight loss (lack of thrive) and scour. These scours can vary from severe watery brown scours to more subtle signs like dirty tails.

The products

A great tool in our arsenal is anthelmintics which control parasites in the animal. We have 5 classes of wormers in sheep

  • White benzimidazoles
  • Yellow levamisoles
  • Clear macrocyclic lactones
  • Orange Monepantel 
  • Purple (dual combinations Abmectin and Derquantel)


With high levels of resistance developing in some flocks across the first three wormers we must now be mindful to use these precious resources sparingly.

The orange and purple drenches should be used sparingly and only when absolutely needed. These are prescription medicines and come at a significant cost also. The purple class is currently only available in the UK.

We can still use wormers but we must use them in a way which slows down the development of resistance in our flocks.


Worm resistance develops when the parasites develop and change to adapt to the killing mechanisms of the anthelmintic products we use.

These parasites are smart so with repeated use of any wormers we risk developing resistance. Watch the video above as I explain how this can develop. In extreme cases, we can have large populations of resistant worms developing in our flocks.

So the risks are over usage of the same class of wormers or poor usage of any products must be avoided.

Effective usage is so important meaning using the right dose as directed per weight. Avoid underdosing and always calibrate dosing guns to ensure they are working correctly.

Have you got resistance issues?

Some farmers will know this, where doses simply don’t work. Always be aware of other issues like coccidiosis or mineral deficiencies. We need to get better at identifying resistance earlier. We can do this by using FECRT (faecal egg count reduction tests).

This can be done by taking samples just before dosing from animals and mark them. Then dose with our anthelmintic.

We repeat the faecal egg test at a certain time after worming depending on the product. We are looking for a 95% reduction in the numbers of faecal eggs in the 2nd samples.

  • For yellow drenches, this is 7 days
  • For white drenches, this is 10-14 days
  • For clear drenches, this is 14-16 days.

This can allow us to at least determine what products are working well. Then we need to look at what strategies your farm can adopt to tackle roundworms and reduce the risk of resistance.


Quarantine doses

One way to bring resistant worms onto your farm is by the purchase of new animals carrying the worms internally. This is why there should be a big focus on quarantine dosing for worms.

My current recommendation is an orange drench combined with an Ivermectin based product to clean out any intestinal worms for bought-in sheep. This is very important for flocks without worm resistance issues. Work these protocols out with your own vet.

These new classes of wormers can be used here for this important job but should not be used for widespread usage.

Grazing strategies

With grazing playing a key role in the lifecycle of the parasite outside the sheep we can look at options around management to help slow resistance. With rotating grazing and paddocks with lambs every spring at turnout we can help reduce pasture larval burdens or at least slow them down. With mixed species swards also showing some exciting potential for the reduction on parasite issues.

Using some science

Fecal egg counts work really well in young lambs in their first grazing season to help monitor parasite burdens. These need to be taken early and regularly.

We can utilize pooled samples to save costs. They must be combined with clinical signs and thrive (weight gain).

So use FECs in combination with monitoring daily live weight gain and clinical signs of worms.

A rough guide for FEC readings in eggs per gram

  • < 200 epg risk is low
  • 200-700 epg risk is medium and must be combined with clinical symptoms
  • >700epg risk is high and discussions should be had on dosing

Ensure you take fresh fecal samples and psot them quickly to a registered lab. Some farmers perform these tests on the farm allowing for very regular monitoring of egg counts.

As the season progresses and in older sheep, they may be less reliable.


This is one that every farmer can be confused about. Our traditional drench and move approach is now less popular with the risk of spreading only resistant worms to new pastures.

We now adopt a dose and stay policy on sheep farms. Leaving sheep on the pasture for 24-48 hours after dosing allowing them to pick up susceptible worms and slowing down the development of resistant worms. This is basically about keeping a susceptible population of worms in our flocks.

We can also look at strategies where we don’t dose lambs which are thriving and achieving weights.


Putting it all together

So every flock should look and ask the question have they resistance issues

Consider less drenching with more effective timing and management.

Rotate products between seasons or maybe during the year.

New drugs like the orange and purple drenches must be used only when necessary. This means for quarantine dosing and also where resistance issues have been diagnosed to the other 3 wormers. Use them at peak worm activity time to clean out lambs where resistance has been diagnosed.

Look at your grazing strategies to reduce the larval levels at pasture on lambs.

Dose properly and consider lambs hitting their weight targets not being dosed

A good parasite control program should involve minimal dosing of adult sheep. At-risk times like the periparturient period we can select at risk ewes like thin ewes and those carrying triplets.


Watch the video above as I talk through the key aspects of tackling resistance.


Thought for the day 

Be better, not bitter!


Huge thanks to Nettex for their support in making #50in50 happen for more information click here http://www.net-tex.co.uk


Happy safe farming

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