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Eliz Braddon, SDV Lachlan LHPA

Posted Flock & Herd March 2012


Within the Lachlan LHPA there is increasing concern, real or perceived, amongst ratepayers that the highly expensive anthelmintics they are using are failing to adequately control these parasites. In 2009 Dr John V Evers performed a macrocyclic lactones (ML) resistance survey in the Young district and found that 3/12 or 25% of flocks had developed/imported ML resistant strains of Teladorsagia (Ostertagia). He was concerned that the trend at that time was overuse of ML drenches and that 'major problems are likely to arise from that approach' (Evers 2009). With the increased rainfall and resultant internal parasite activity following spring/summer 2011, it was decided that now would be a good time to assess the drench performance for the district.


A randomised trial will be conducted using approx 60 sheep on each of 64 properties throughout the Lachlan LHPA. These animals will be of uniform age and type and will run together on their respective properties. The properties will be selected based on a sheep population of >1500.

The 60 animals will be divided into cohorts of 15 animals; they will have a baseline Faecal Egg count (FEC) established, with a minimum of 200epg being the entry point for the survey. Each cohort will be treated with different drenching regimes except the control. Faecal egg counts will be performed for numbers and types of larvae. This strategy will give 95% confidence based on a 25% estimated resistance prevalence of the total flocks involved (490) with a 10% confidence interval.

The drench groups chosen for evaluation were naphthalophos/BZ/Lev combination; abamectin alone and abamectin/BZ/Lev combination as these are the most commonly used rotations being used around the district.


At the time of writing, approximately ½ the properties have been initiated and completed. Worm numbers in the west of the Authority were slower to rise this year than the previous year so this has slowed the project initially. Evaluation of the current data shows most producers in the district have still got useful drenches to use in these three classes,though there is wide variation of efficacy between worm populations.

Table of drench efficacy

Each producer has also been asked to complete a one page survey capturing basic enterprise details and also previous 'current drenching practices'. This data has not yet been evaluated.


In the evaluation of district performance to date, 17/22 of the properties with full data sets still have good use of 2 of the drench classes based on the overall efficacy data. There were only 4 properties that showed resistance (eg. <95% efficacy) to 2 drench groups (usually the Napth/BZ/Lev and ABA); and only 1 that showed resistance to all three classes tested. In another 9/22 of the properties tested, the resistance was to one drench group only (Naph/BZ/Lev in most cases).

By far the most likely worm species to have developed resistance in this study was Teladorsagia . This species was the cause of resistance in 19/21 properties across all drench groups. The greatest resistance by Teladorsagia was to the Naph/BZ/Lev group and ABA group. This supports the concerns that John Evers had in his limited study in 2009.

Haemonchus spp. resistance to ABA was proven in 3 of the 8 properties with complete data sets. As this is not typically a large Haemonchus area, it is unknown at this stage whether these resistant populations were imported or 'homebred'. Further evaluation of this will occur once all the property data is complete.

Trichostrongylus spp are generally showing less resistance to common drenches, with most resistance (3/17) in the Naph/BZ/Lev group and only 1 in the ABA group.

As this is usually the predominant worm pathogen for the Lachlan area, the drench groups performance against this species is still encouraging.


Producers in the Lachlan LHPA will be advised of these results as they come to light. The future focus of worm control in this area will be to:


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