ANSC20003 Chap.7 Faecal Egg Counts, Drenches and Drench Resistance
Faecal Egg Counts, Drenches and Drench Resistance
Why a number is counted at all
Counting eggs is how worm burdens in grazing stock, and in sheep above all, are put on a scale. The count stands in for the size of the burden, and the decision about whether to treat is taken from it.
The counting chamber holds a fixed volume, so counting a chamber counts every egg in a known volume, and a known weight of faeces diluted to a known volume converts that into eggs per gram.
The multiplication factor follows from that arithmetic and differs between sheep and cattle only because the weight of faeces differs, and it is valid only for the chamber it was derived for.
The bench steps that decide the result
Several steps change the number without changing anything visible while it is being done.
The suspension is mixed side to side rather than in a circle and sampled immediately from the middle, because a circular mix spins eggs outwards and a pause lets them float. At least two chambers are loaded and counted per sample with re-mixing between them, because the multiplication factor assumes the two-chamber total.
Strongyle eggs are counted separately from other eggs and oocysts, because the published interpretation bands are strongyle bands.
Interpretation has no blanket rule
The handbook states this before it gives any numbers. Interpretation depends on the age of the animals, their physiological status, the host species, nutritional status, the worm species involved and any clinical signs consistent with parasitism.
The published bands describe rising production loss and clinical risk rather than issuing treatment instructions, and the guidance notes that a flock can pick up a serious burden faster than the count climbs to meet it, and that regular monitors may tolerate higher counts in order to preserve the working life of their drenches.
What the count cannot answer
Strongyle eggs of different genera cannot be told apart by eye, so a count cannot identify the genus, and faecal culture is what supplies it.
A single count also cannot say whether a drench is still working; that requires counts before and after treatment, with the drench classed as effective above a stated percentage reduction. Resistance has been reported against every major anthelmintic class, which is why the research question is new drugs and the farm question is fewer, better targeted drenches.
What this chapter covers
- 01
The counting chamber, the dilution, and where the multiplication factor comes from
- 02
The mixing, loading and counting steps that silently change a result
- 03
Counting strongyle eggs separately from other eggs and oocysts
- 04
The published interpretation bands, and the six factors that qualify them
- 05
Why a low count is not automatically reassurance
- 06
Faecal culture for genus, and the reduction test for drench efficacy
- 07
Anthelmintic resistance across drug classes, and what integrated control adds
Reporting a count and stopping at what it supports
- 1Apply the correct multiplication factor and report the result.
- 1Name what the count cannot identify and what would.
- 1Give a reason the flock decision may differ from this result.
Key terms
- Eggs per gram
- The standard unit of a faecal egg count, obtained by multiplying a chamber count by a factor derived from the sample weight and suspension volume.
- Strongyle egg
- The egg type counted in a routine sheep faecal examination, produced by several genera that cannot be distinguished by appearance.
- Faecal culture
- Incubation of a faecal sample so that eggs hatch into larvae, which can then be identified to genus.
- Reduction test
- Paired egg counts taken before and after treatment, used to establish whether a drench is still effective against the worms present.
- Anthelmintic resistance
- The persistence of a worm population after treatment with a product it was previously susceptible to.
Faecal Egg Counts, Drenches and Drench Resistance FAQ
Why is the multiplication factor different for sheep and cattle?
The suspension volume is the same for both, and the only thing that changes is the weight of faeces used. Twice as much faeces in the same volume halves the number of eggs per gram that each counted egg represents, so the factor is halved. It is arithmetic rather than a species correction.
Can a faecal egg count tell you which worm is present?
No. Strongyle eggs from different genera are not distinguishable by eye, which is why they are counted as one category. A faecal culture hatches the eggs to larvae that can be identified, and that is the step that supplies the genus.
Does a low count mean the animals do not need treatment?
Not on its own. The published guidance states that a flock can pick up a serious burden faster than the count climbs to meet it, and it qualifies every band with the age, physiological status, species, nutrition, worm species and clinical signs of the animals in question. A low count supports waiting and repeating rather than a conclusion of no burden.
How is a failing drench identified?
With paired counts: one before drenching and one ten to fourteen days afterwards. The drench is classed as effective if it reduces the count by at least about 95 per cent, and ideally 98 per cent or more. A reduction below that threshold indicates resistant worms are present.
Exam move
Practise the arithmetic until the factor and the two-chamber rule are automatic, because a calculation question here is quick marks and the error people make is applying the factor to one chamber. Learn the interpretation bands as a ladder of consequences rather than as treatment thresholds, and be able to state two of the six qualifying factors in any answer that quotes a band.
For resistance, keep the reduction test protocol and its threshold exact, and be ready to explain why repeating a treatment is the wrong first response to a flock that is not improving.
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