By Dr Clive Dalton
Thanks to Dr John Moffat, Schering-Plough Coopers for use of this case material.
- A family farming operation on a North Island hill country farm running 1200 ewes and 150 beef cows was concerned about the effectiveness of their sheep drenching programme, and suspected that maybe drench resistance had developed.
- This was because of poor growth rate, scouring and an unacceptably high death rate in their lambs (100 died between docking and weaning). They normally had to dag about a half of their ewes.
- Their suspicions were really alerted by a mean FEC of 800epg 10 days after drenching a mob of hoggets with ‘Closal’. Closal is a narrow-spectrum active which has roundworm-persistent activity against Haemonchus contortus but is also highly effective against liver fluke.
- No worms have shown drench resistance to it yet in New Zealand.
- Lambs were on a standard 7-drench programme of which two were given prior to weaning (10th September and 20th October), a weaning drench on 20th November followed by four drenches on 20th December, mid February, mid March and mid April.
- The first step was to arrange for a ‘drench test’ to see what was happening, so 60 hoggets were used to carry out a Faecal Egg Count Reduction test (FECRT) starting in May 2004. The results are shown in the table.
- The percent reduction in pre and post-drench faecal egg counts overall and by different parasite species was revealed.
- Note the wide range in individual animal’s FEC within the groups. This is expected.
- A drench is considered effective if the egg count is reduced by 95% or more.
- So the Lev, BZ, Combo drenches were not effective. Indeed the egg count was higher in the post-drench than the pre-drench FEC for the BZ group. This can happen.
- The MLs were 100% effective.
- Culturing larvae is an essential part of the FECRT to see which species are killed by which drench family.
- Look for the 100% kill rates and clearly the Lev drench was killing three species, the BZ killing one and the Combo was still killing three species of worms.
- Trichostrongylus parasites were resistant to both BZ and Lev drench, and so have had a reproductive advantage on this farm.
- Not enough parasites had developed resistance to ML to be detected as yet, i.e. ML drench was still killing all species cultured.
1. Develop and monitor the farm drenching programme
- Review carefully the current programme.
- Future drench usage must be warranted based on sound knowledge.
- Monitoring of parasitism in the flock is needed to ensure the on-going control programme is working.
- Do another FECRT in autumn to check for Haemonchus resistance, in case it is a threat when a narrow spectrum active should be used,
2. Drench active options are limited.
- Never use single action BZ or Lev drenches or Combination BZ or Lev drenches.
- Never use BZ capsules.
3. Protect the efficacy of ML family:
- Use other long-acting drenches very sparingly.
- In general minimise ewe treatments.
- Do not use long-acting drenches in ewes.
- Use preferably combination ML drenches or ML products on their own.
- Combination examples should contain ML+Lev or ML+BZ+Lev.
- Don’t drench about 5% of the animals in the mob (the best ones).
- Drench to the heaviest animal in the mob and regularly check the drench gun.
- Check FEC routinely before and after drenching using a composite sample.
4. Maintain Border Security
- Ensure there is no spread of drench resistant parasites onto your property from other farms or from your farm to others.
- Quarantine drenching.
5. Other Recommendations:
- Reduce farm parasite burden.
- Make use of cattle in cross-grazing programmes.
- Carefully consider post-weaning lamb management.
- Ensure good nutrition.
Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.