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National Drench Resistance - should you be concerned?

A recent nationwide survey1 was conducted in response to the need for up-to-date information on drench resistance in the Australian sheep industry to help inform management decisions, advisory practices and future research. Providers, including government and private parasitology laboratories, pharmaceutical companies and veterinarians who were known to have conducted Worm Egg Count Reduction Tests (WECRT’s) between 2009 and 2012, were asked to submit all test results that conformed to standards set by the World Association for the Advancement of Veterinary Parasitology. Of those submitted, 390 tests met these criteria.

The results (see Table 1. below for a summary) show that resistance to frequently used single active and combination drenches including macrocyclic lactone (ML) based combinations, is widespread and progressing in roundworm populations infecting Australian sheep. Although this is a concern it should not be surprising considering the age of the most common active ingredients- OPs, LEV, BZ and MLs, and their continuous use, ranging from 25 to 50 years. However the results should also not be seen as a disaster, but rather as identifying an opportunity for individual farmers to fine tune their worm control program to maximise both productivity and sustainability.

The percentage of farms on which <95% efficacy against small brown stomach, black scour or barbers pole worms (or any of these three worms) was found for single drench actives and combinations was:

Table 1: National summary of percentage of WECRTs with <95% WECR in sheep

*Any worm = one or more of the following: Brown stomach, Black scour or Barbers pole worms

BZ=Benzimidazole; LEV=Levamisole; NAP=Naphthalophos; IVM=Ivermectin; ABA=Abamectin; MOX=Moxidectin; MPL=Monepantel

The study also presented the range of efficacies found by active. Whilst drench resistance is defined as <95% efficacy of an active against a worm for which it has a label claim, it is clear from the results of the study that in many cases the actual efficacy on farm is significantly below the 95% threshold, as can be seen in the data presented for moxidectin (the most potent of the ML actives) in Figure1.

Figure1. Range of efficacy (%) and mean/average (%) efficacy for moxidectin against small brown stomach (Teladorsagia), black scour (Trichostrongylus) and barbers pole (Haemonchus) worms (combined data all farms).

These findings contrast sharply with the results of the last national drench resistance survey conducted in 1994 which found no evidence of ML resistance2. The current study therefore highlights the urgent need to better manage existing drench resources and to promote adoption of appropriate non-chemical control measures on individual farms as part of an Integrated Parasite Management (IPM) approach (see The authors of the study noted that in spite of the fact that there has been a new broad-spectrum class of active (AADs; active MPL) introduced in 2010, it is likely that the sheep industry will continue to rely heavily on the older drench classes (and especially the MLs) for some years to come.  

The good news is that there are a few simple things that farmers can do to better manage existing drench resources:

  • Know the resistance status of the drench actives used on your farm (not your neighbours or the district average) – conduct a WECRT every 3 years to determine efficacy
  • Use only drenches shown to be highly effective (>98%) on your farm – the more effective the drench the fewer resistant worms survive (simple!)
  • Rotate drench actives each time a mob of sheep is drenched
  • Use an effective quarantine drench/s whenever new stock are introduced to your farm
  • Use old actives in combination, not on their own
  • Adopt IPM principles as appropriate (grazing management, nutrition and genetics)
  • Integrate new drench classes into drench programs now to preserve the efficacy of older actives and maintain farm productivity. The value of doing this is supported by published modelling studies3.

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References: 1Playford et al., The prevalence of anthelmintic resistance on Australian sheep farms (2009 - 2012), WAAVP Perth, 2013.

2 Overend et al., Anthelmintic resistance in Australian sheep nematode populations. Aust. Vet. J. 1994;71:117-121.

3Dobson et al., Minimising the development of anthelmintic resistance and optimising the use of the novel anthelmintic monepantel, for the sustainable control of nematode parasites in Australian sheep grazing systems, Aust. Vet. J 2011;89:160-166.