NEW STRATEGIES FOR WORM RESISTANCE
NEW STRATEGIES FOR WORM RESISTANCE
INCREASING levels of worm resistance to drugs over the past decade should prompt producers to look critically at their worming policy.
An anthelmintic survey, undertaken in Scotland by Moredun Research Institute last year, shows that resistance to white drench (benzimidazole) has doubled since the early 1990s.
Faecal material from ewes and lambs on 107 farms was analysed for worm resistance to drugs, says Frank Jackson of the institutes parasitic department. "Although most sheep showed no clinical signs of disease, the average prevalence of white drench resistance was more than 60%.
"But more than 80% of samples submitted from lowland farms showed resistance, compared with 55% and 45% on upland and hill farms, respectively," Dr Jackson adds.
In 1992, survey results showed benzimidazole resistance at 24% on lowland farms and there were no recorded cases on upland or hill farms.
This evolution of slow resistance to white drenches could be due to increased and prolonged use of them over recent years, says Dr Jackson.
A detailed questionnaire in the survey uncovered patterns of drug use and shows that farms with resistance tended to use more anthelmintic, particularly for their lambs. "On average, surveyed ewes received 2.8 doses/year and lambs 3.1 doses, but lambs on resistant farms had 3.5 doses."
Although the survey shows no evidence of resistance to the other two wormer families – levamisoles and avermectins – Dr Jackson believes resistance to avermectins could become a concern, as use of these drugs is increasing.
"Producers must be aware of resistance problems, particularly in wet, warm seasons, and plan their worming strategy to prevent this development. When resistance is suspected, seek vet advice." *
Resistance to white drenches has doubled in the last decade. But practical steps such as rotating drug families can help, says Frank Jackson.
WORMER RESISTANCE PREVENTION
• Rotate use of drug families annually and when resistance is suspected do not use the drug family again. Once a farm has resistance, it will be permanent.
• Withholding feed for 24 hours will increase efficacy of white drenches against tape worm when there is resistance. But do not withold feed from pregnant ewes.
• Worm incoming stock and isolate for 24 hours on a hard standing base.
• Do not underestimate dose rate of wormer. Calibrate the drenching gun correctly and check it is working properly.
• Use the correct drench for the type of parasite to be treated.
• After worming, put stock onto different grazing, aftermaths or re-seeds to prevent reinfection.
• Regularly check efficacy of drug families with a faecal egg count reduction test carried out by a vet or local VI centre.