Wormwise Programme Manager Ginny Dodunski discusses whether Faecal Egg Counts Reduction Tests are a useful tool in the face of widespread drench resistance and the value of drench checks.
The Faecal Egg Count Reduction test has long been considered the ‘gold standard’ on-farm test for getting to grips with which drenches are working, and which ones aren’t.
In this test, small groups of lambs or calves are treated with drenches from the different chemical families, to compare the performance of each of these against the worms on board.
Faecal egg counts are measured from the animals on the day they are drenched, and again one to two weeks later. At this point, if the drench has worked really well, there should be no eggs present because the worms have all been killed. Any eggs found in the samples are incubated and grown out into baby worms (a larval culture), so that we can identify which worm genera are surviving the drenches.
Given there is now such widespread drench resistance in New Zealand sheep flocks, one can be forgiven for wondering whether this test is still relevant?
The answer to this question is (you guessed it) – it depends.
If previous on-farm testing has shown you have significant resistance, including combination resistance, there may be little point in repeating an expensive and time-consuming test every few years to tell you what you already know.
In this case, your dollars and energy may be better spent on regular monitoring of the drenches that you can still use, while working your system towards one where you are relying on them less and less.
You can check the previously failed drenches along the way; there are cases now where performance of these has improved over time and they can be used in a limited way at certain times of the season. How this works in practice will be different for every farm. Get advice from someone who knows their stuff in this space.
However, if you have made significant system changes, you don’t know your drench status at all, or you have had a previously good status and want to check all is still well, a FECRT is still a great way to get a clear snapshot of where you’re at.
A word of warning about sheep FECRTs though – as you are probably aware, the dominant worm genera on sheep farms varies throughout the season. The worm mix in your lambs when you do your FECRT may be different to what it is later in the season. Occasionally farmers have been caught out thinking ‘all is well’ in December, only to have lambs sick with resistant Trichostrongylus the following autumn.
The message here is ‘continue to do drench checks all season’. In nearly all of these cases, the sick lamb outcome could have been prevented if regular post-drench FECs were undertaken and the problem would have been picked up earlier.
With calves, a full FECRT can be a difficult logistical challenge. You may be better to allocate a group of calves each month to doing a ‘one drench at a time’ testing regime. Testing drench performance in calves can be a challenge as they often have low or even zero FECs. Using 20-30 individually identified animals for the test (and identifying which poo comes from which calf) and using more sensitive methods to count the eggs than the standard ‘McMaster’ microscope test, are ways to get better data from calves. Talk to your vet or FEC testing provider.
For both lambs and calves, it’s getting harder to access the older, single active drenches required to do a ‘classic’ FECRT. You may be stuck with simply using the most common combination drenches available. While it’s nice to understand the efficacy of each of the components of your combination drenches, we have to work with what is available.
Finally, both faecal egg counts and larval cultures have a degree of variability. This means we shouldn’t place too much emphasis on the ‘exact’ reduction percentage calculated for each drench in a FECRT. A result of 86% one year and 92% the next year is not a clear indication that things are improving, unfortunately. There’s a very wide ‘grey zone’ from about 55% reduction to 94% reduction where individual results can bounce around a lot. All these numbers mean it’s likely that resistance is present and are a call to action – what that action is – you already know the answer to that question. It depends!