This autumn as in 2023, we have seen the increasing effects of resistance of internal sheep parasites to drench. This combined with ideal weather conditions for the survival of the larval stages of these parasites on pasture, are leading to lambs being clinically affected despite their treatment.
While we have published various articles concerning drench resistance and how to manage this the scale of the issue is such that we pulled members of our vet and support teams across Totally Vets, Tararua Vets, and Levin and Horowhenua Vets to share information and ideas on worm resistance.
Key messages are as relevant now as they have ever been. Feeding your lambs well, use of crops, rotating pasture, refugia, not running lambs on pasture used for lambing, using cattle to help groom pastures and provide clean feed are all tools that can be used to decrease the risk of resistance. Drench resistance in cattle is also on the rise and many of the same principles apply.
As a minimum, post-drench faecal egg counting (FEC) is essential to determine drench efficacy. Faecal egg counting too early, and the drench will not have had time to reach full effect, and too late there could be new activity from worms taken up from the pasture after drenching and these are producing new eggs.
Faecal egg testing 10 to 12 days after drenching is the ideal time to bring in fresh faecal samples into us for testing. A FEC test around weaning and again in April is an absolute must.
If FECs come back with positive counts, larval cultures can be done to identify what worm is present. Historically Haemonchus has been blamed for high worm burdens in the early autumn period, but we are now seeing Trichostrongylus being identified as the main cause of poor performance in autumn. We recommend that clients undertake a full FECRT (Faecal Egg Reduction Test) which is a combination of FECs and larval cultures, to identify which drench families you should be using on your farm. The best time to do this is December – February as this is when all worm species are likely to be present.
If you are not already FEC testing and you want to receive more information please talk with one of our vets or commercial managers. As a vet practice we are committed to our farming community, and we will be continuing to look at this issue and ways that we can assist.