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Elizabeth Braddon, SDV Lachlan

Posted Flock & Herd April 2013


In September 2011, the Lachlan LHPA undertook a targeted surveillance project to assess the drench resistance profile of our commonly used drenches against the three sheep internal parasite pathogens (Trichostrongylus spp. Haemonchus spp. Teladorsagia spp.). The aim was to test 64 properties across the Lachlan LHPA using napthalophos/Bz/Lev; abamectin and abamectin/Bz/Lev drenches and then perform faecal egg count reduction tests on these flocks 10-14 days later. All the testing has now been performed and the final results have been analysed.


A total of 58 properties were tested successfully before the end of August 2012. It was then decided to stop the trial and use the data collected so far. Across the 58 properties there was a good representation of worm species with Haemonchus represented on 57%, Trichostrongylus on 97% and Teladorsagia on 91% of the properties tested.

The data was then analysed to reflect the effect of each drench used against the particular worm species rather than on a whole farm approach. Resistance in the paper is defined as <95% efficacy of the drench to reduce the worm population in the treated group when compared to a control group.

Teladorsagia showed significant resistance to abamectin at 34% of properties showing <95% efficacy with this drench. There were also 6% of properties that showed resistant populations to the abamectin combination drench. When the Naphalophos combination drench was used, Teladorsagia resistance was found on 68% of properties tested.

Chart of properties with resistance
Graph 1. Teladorsagia resistance results (<95% effective at reducing population)

Trichostrongylus showed resistance on 57% of properties to the naphthalophos combination. However, of greater concern is the apparent resistance on 18% of properties tested to abamectin. Only 2% of properties had apparent resistance to the abamectin combination.

Chart of properties with resistance
Graph 2. Trichostrongylus resistance results (<95% effective at reducing population)

Haemonchus showed resistance on 55% of properties in the abamectin drench group. Six (6%) percent of properties also showed resistance in this worm species to the abamectin combination. The naphthalophos combination was showing resistance in 6% of the properties tested.

Chart of properties with resistance
Graph 3. Haemonchus resistance results (<95% effective at reducing population)


The original premise for undertaking this drench resistance study in the Lachlan was that we suspected we had mectin resistance to Teladorsagia populations at levels of 25%. This current study confirmed our suspicions by showing that 34% of properties were showing a direct resistance (eg. <95% effectiveness of the drench).

In the Lachlan LHPA, the most common worm burdens are due to the scour worms Trichostrongylus and Teladorsagia. We have data that confirms that these worm species have been developing resistance to the most popular drench class - Mectins. Obviously Teladorsagia has developed a more significant level than Trichostrongylus but the fact that 18% of properties are suggesting mectin resistance in this population as well is concerning.

The surprise for all of us doing the project was the level of mectin resistance discovered in our Haemonchus populations. The majority of these farms are homebred animals and do not have a risky history of purchasing in sheep. This indicates that the resistance has been developing in the background despite the fact that most years Haemonchus is a short lived and usually subtle problem in our area of the State. I suspect that this is due to constant exposure of small populations during the drought years and with our recent rainfall, the only Haemonchus worms present have been those that have developed from this constant exposure. The end result is that Lachlan LHPA producers have a similar resistance picture in Haemonchus worms as do those in the northern tablelands.

Further analysis of the data will be done to look at the upper and lower confidence levels of efficacy. This will further breakdown just how severe the development of resistance has become. Also further investigations into the abamectin resistance in Trichostrongylus populations will occur. This will be to fully confirm that the populations are truly Trichostrongylus spp. and not an overlap between them and Teladorsagia.


I would like to thank the other veterinarians in the Lachlan LHPA for their support in collecting these samples, not to mention numerous veterinary interns over the trial and data collating periods. Rob Woodgate and Stephen Love provided technical and analytical support in working through project design and approaches to data analysis.

This trial would not have been possible without the generous support of Novartis Animal Health, Bayer Animal Health, and Virbac Animal Health who provided the drench for the project; the Department of Primary Industries who funded the lab testing and of course the willing producers, who provided the sheep.


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