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Drench Resistance - Faecal Egg Reduction Trials

At Crookwell Veterinary Hospital we regularly undertake faecal egg reduction trials (FERT) to determine the level, if any, of drench resistance on a given property. The process is straightforward and cost effective.  Knowing the drench resistance status of your property is essential for effectively managing internal parasites – FERTs have the potential to provide significant economic benefit through increased production, and less costs in labour and anthelmintics if resistance is identified.

What is drench resistance?

Under current definitions, parasitologists generally accept that at least 95% efficacy is required for a drench to be efficient.

Drench resistance occurs when a small number of worms inside an animal genetically adapt to survive drenching. These worms lay eggs and build up their numbers unaffected by the chemical group(s) being used. This can lead to a significant number of worms on the property being resistant to routine drenching.

Resistance is exacerbated by over-drenching and the continued use of ineffective drenches. The problem is already entrenched and increasing in Australian sheep flocks. One of the reasons for this is because by the time obvious drench failure has been identified, resistance will be well established in a property’s worm population.

How do producers determine if resistance exists in their flocks?

FERT – faecal egg reduction trials – are designed to test the efficacy of drenches and to detect drench resistance in sheep flocks. The aim of the trials is to test the efficacy of a single drench group or more commonly, a combination of chemical groups. CVH can guide you through the planning for a FERT, and organise the required chemicals for the trial.

Work out how many different chemicals or chemical combinations you want to test, based on the property’s history of known resistance – CVH can help with these decisions.

The process

  1. First, we need to carry out a routine FEC to establish that the worm burden on your property is sufficient to undertake the trial at that time.
  2. Drench 15 sheep for each chemical group being tested – collect faeces from 10 of these in 10-14 days after drenching.
  3. Retain 15 sheep not drenched as a control group.
  4. Identify the sheep in each (chemical) group with ear tags or spray mark
  5. All the sheep being tested can be grazed together until the faeces are collected 10-14 days post treatment.
  6. This procedure is usually carried out by the producer with our help and guidance, or we can undertake the trial if requested.


The results need to be interpreted carefully, in light of the species of worms present and the statistical evaluation of the results. This is where it is critical for CVH to be involved. Our staff have experience and expertise in undertaking and evaluating faecal egg reduction trials results.