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Sheep lice treatment trial Imidacloprid (Avenge) resistance or faulty product application?

Ian Masters, District Veterinarian, Gundagai

Posted Flock & Herd March 2012


In August 2011, a producer from the Illabo district discussed his concerns with DV Wagga about possible imidacloprid resistance in sheep that had been treated with Avenge off shears the previous year.

The sheep were showing signs of a lice infestation at shearing in 2010, treated off-shears with Avenge (imidacloprid) and were lousy again approaching shearing in 2011. The property had a protracted problem with lice control over several years and after IGR backline product failures the owner decided to try Avenge.

According to the owner, all merino sheep and lambs were treated off the board at shearing during the spring of 2010. Rubbing was noticed 1-2 months off shears and Craig Whiting, Bayer Area Manager was asked to investigate the owner's concerns. A light infestation was detected and the sheep were treated with a long wool product at crutching later that year but the infestation progressed to the point where the sheep were in poor condition and obviously lousy approaching shearing in 2011.The owner was willing to provide sheep for a trial to test the product which commenced at shearing on the 9th September 2011.

Image of sheep showing wool derangement
Figure 1: Typical sheep selected for trial. Photo Craig Whiting.


At shearing the owner had selected 20 badly rubbed sheep (CFA merino ewes). Craig Whiting and DV Gundagai carried out lice counts and ear tagged the sheep which were then shorn. Lice were counted at five sites along each side of the sheep. Average lice counts were 7+/opening. On most sheep there were a number of sites where the counts exceeded 20+. After shearing, Craig selected 10 ewes for the trial mainly looking for the cleaner shorn, plainer bodied types. These ranged in weight from 32 to 48kg and were treated with a 60mL dose of Avenge applied in two stripes either side of the midline from the mid neck to the butt of the tail.

The trial sheep were picked up the next day and transported to DV Gundagai's property where they could be held in isolation from other sheep for the duration of the trail. A small securely fenced paddock located in the middle of a larger double fenced paddock was used. Only one neighbour has sheep and there have been no issues with sheep straying onto the property.

Image of sheep showing off-shears backline treatment
Figure 2: Trial mob after off shears backline treatment. Photo Craig Whiting.

The first post treatment (60 days) inspection was conducted on the 8th November 2011. Craig Whiting and Neil Cooper from Bayer, DV Gundagai and Ranger Toby O'Brien inspected the sheep. This involved five openings at five sites along each side of each animal looking for lice. No live lice were detected at this inspection.

Image of sheep being inspected for lice
Figure 3: 60 day post treatment inspection. Photo Craig Whiting.


This is a work in progress. Next inspection will be carried out in early February 2012. At the time of writing the sheep were showing no signs to indicate that they may have a developing lice problem but time will tell. It would appear at this time that the product is effective if applied according to label recommendations on clean shorn sheep. The problem experienced on this property may have more to do with product application and shearing difficulties rather than a developing resistance to imidacloprid.


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