13 Jan 2025
Joseph Henry
Job TitleAs a sheep specialist vet, I work with farmers to develop successful refugia strategies, which focus on maintaining a population of worms on the farm that aren’t exposed to a specific wormer treatment.
This is crucial to minimise the risk of developing anthelmintic resistance and to ensure sheep farmers retain the ability to achieve high levels of performance.
Mixing treated and untreated worm populations dilutes resistance genes, preventing them gaining a reproductive advantage. Without refugia, resistant worms would quickly dominate, increasing the rate anthelmintic resistance develops and making wormers ineffective.
When it comes to refugia strategies there are two areas to consider: managing which animals to treat and managing worms on the pasture.
I recommend only treating animals with high-worm burdens or clinical signs of parasitism, which may include lambs with lower than expected daily liveweight gain (DLWG). This could be a lower DLWG in comparison to the rest of the mob – providing lambs are grouped reasonably evenly in terms of size – or in comparison to historical weigh data if the system and breeding have remained fairly constant.
Alternatively, if farmers are taking more of a blanket treatment approach, I would advise leaving 10% to 20% of the flock untreated as a reservoir for susceptible worms.
The most simplistic way to do this is to drench 9 out of 10 animals. Alternatively, stock can be visually assessed, and big, tight-skinned, clean lambs that are thriving can be drafted out and left untreated.
For farms with auto-draft equipment, parting lambs out based on DLWG is a practical way to select those for treatment. I suggest running 30 lambs in the mob through the system to find the average growth rate and treating any lambs that fall under this threshold.
It’s essential to mark untreated stock to prevent confusion. Without marking, farmers might mistakenly assume that any dirty lambs are untreated or include faecal samples from untreated lambs in post-dose drench checks.
As well as helping to significantly slow down resistance, taking a selective approach to treatment can also save on drench costs, but it’s important to plan strategies based on the circumstances on individual farms.
Regular faecal egg counts to help monitor resistance levels are particularly important in informing treatment strategies incorporating refugia.
Grazing management should be considered alongside treatment strategies to successfully manage refugia.
This includes avoiding dosing and moving sheep straight on to clean pasture as only worms that have survived treatment will be passed out giving them a reproductive advantage.
If for any reason sheep must be moved to clean pasture straight away, it is even more vital that a percentage are left untreated.
Following best practice for quarantine treatments is also key.
Any incoming sheep, including those returning from keep elsewhere, should be treated with a newer group wormer and left on hard standing for 24 to 48 hours.
After this period, they can then be moved to pasture recently grazed, where they will pick up resident “good” worms.
There is a need for continuous education and support from veterinary professionals to ensure farmers understand and adopt refugia strategies.
While I appreciate changing farmers’ mindsets can be challenging, the benefits of managing refugia population to slow the development of wormer resistance outweighs the challenges. This approach is crucial to ensuring sheep farmers can maintain high levels of productivity, both now and in the future.
Many farmers are unaware of drench resistance, which is why industry-wide cooperation is important to preserve the efficacy of anthelmintics and ensure farmers retain the ability to achieve high levels of productivity.
Although it is challenging to measure how much resistance is slowing, refugia strategies have proven effective. Farmers implementing these have observed no negative impact on growth rates, even when treating only half the flock.
