The growing issue of worm resistance

Most people will be well aware about the growing issues with drench resistance. While vets and many farmers have been aware of this issue and dealing with these problems for 20 years, in the last few years there appears to have been an explosion in drench resistance cases, in particular in triple drench resistance.

Drench resistance is not only a costly issue in terms that once you have triple drench resistance you need to use a novel drench, but also in terms of animal performance. When drench doesn’t work as well as we expect, animals have a decreased live weight gain because they are diverting energy from growth to their immune system, which is constantly elevated in order to fight the parasite burden.

So how do you protect yourself from triple drench resistance:

  • Do a FECRT to check the status of your farm. This is the best way to determine the effectiveness of each drench against each worm species.
  • At least twice a year do a 10-day post drench check on lambs, 10 days after drenching all the samples should be zero.
  • Have an effective quarantine policy.
    • All animals should get drenched with a novel drench (Startect or Zolvix) plus a BZ/Lev combination (e.g. Corporal).
    • Ensure that all animals are kept off feed for a minimum of 24 hours, ideally 48 hours but be sensible with young lambs depending on where they are sourced from.
    • FEC these animals 10 days after arrival.
  • Ensure you have a refugia policy in place.
    • Refugia is very important to ensure that there is a susceptible worm population on the ground.
    • This can be achieved by leaving a percentage of the lambs undrenched, or running undrenched ewes (ideally 2ths) with lambs.
  • Consider cropping.
    • If you have the option of planting crops or renewing grasses, this is a great way to remove worms from the system but also to set up a fed system whereby animals can be grazed and less drench used.
  • Start talking about this more.
    • There is already lots of discussion about this topic which is great, and there are many people who have good advice to share re how to deal with this challenge.
  • Lamb sales policy.
    • Drench resistance is primarily a lamb problem as lambs are “worm factories” with no immunity to parasite, this immunity increases as they get old, but 2ths still don’t have full immunity.
    • The more lambs you have and the longer they stay the greater risk you have of developing drench resistance.
    • Consider selling more lambs store.
    • If you are buying lambs in try and source these from farmers with a good drench resistance status – yes these farmers do still exist! However, don’t take their word on it, ask to see their results, just as you would ask to see BVD results or B. Ovis certification.
    • As above, use a good quarantine drench, refugia and cropping.
  • Increase your monitoring.
    • FEC before you drench ewes is good to gain info. Is it worms causing them to look a bit off or is it the feed?
    • Larval cultures allow you to understand the proportion of each worm species on the property. There’s no point drenching for Barber’s Pole if you don’t have it. Cultures take around 10 to 12 days to get the information back.
    • Knowledge is power, it’s about building a picture and understanding your farm.
  • Ewe drenching policy.
    • Review your ewe drenching policy, ewes should need minimal drenching but there are times when they need to be drenched. As above, get some FECs done, don’t drench the entire mob and make sure you have a refugia policy.
  • Sheep:Cattle ratio.
    • The major sheep worms are not shared by cattle and vice versa. Therefore, cattle can be used to help groom pastures and provide clean feed.
    • Running a high number of one species increases the risk of resistance developing by 80%.
  • Pasture grazing.
    • Having a good grazing rotation helps create clean pasture and prevents a high build-up of larval contamination on the pasture.


Dealing with potential drench resistance issues can be daunting and sometimes it appears easier to just ignore it, however the issue won’t go away. Every journey starts with one small step. I encourage everyone to make their one small step – a post drench egg count, to get some information about your farm.