Drench resistant worms in sheep – a farm with drench resistance

Sheep, farming, husbandry, disease, animal health, drench resistance, dealing with problem

By Dr Clive Dalton

What do you do to find out if your sheep have worms resistant to drench?

Acknowledgements
Thanks to Dr John Moffat, Schering-Plough Coopers for use of this case material.

The problem

• 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.
  

Action taken

• 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.
  

Key points from the table:

• 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.
  

Drenching Recommendations

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.