Article summary
Faecal egg counts (FECs) are a useful way to monitor parasite egg shedding, but small mistakes can produce misleading results. Poor sampling, incorrect storage, inaccurate dilution, unsuitable flotation solutions and counting errors can all affect the final figure. Understanding these seven common mistakes, and how to avoid them, helps make faecal egg count results more consistent and reliable.
Why can faecal egg counts give inaccurate results?
A faecal egg count measures the number of parasite eggs in a sample of dung, with results expressed as eggs per gram (EPG). It is commonly used to monitor parasite egg shedding in animals such as horses, sheep and cattle.
The results can help inform parasite control programmes and assess the effectiveness of worming treatments when interpreted alongside veterinary guidance.
However, an FEC does not provide an exact count of the worms living inside an animal. Egg shedding can vary naturally, and even correctly performed tests may produce different results.
Testing technique also makes a difference. From collecting the sample to examining it under a microscope, each stage introduces opportunities for error.
For anyone carrying out their own testing, our introduction to faecal egg counting explains the basic equipment and process.
Seven common faecal egg count errors and how to avoid them
- Collecting an unrepresentative faecal sample
An accurate result starts with a good sample.
Parasite eggs are not always distributed evenly throughout faeces. Taking a small amount from just one part of a dropping may produce a result that does not adequately represent the material being tested.
Sampling becomes particularly important when monitoring groups of animals.
For example, the Sustainable Control of Parasites in Sheep (SCOPS) recommends collecting fresh samples from at least ten randomly selected sheep when assessing a group’s faecal egg count. Selecting only animals that appear unwell could give a misleading picture of the wider flock.
How to avoid this mistake:
- Collect fresh faeces from several parts of a dropping.
- Avoid contamination with soil or bedding.
- Clearly label individual samples.
- Follow the appropriate sampling procedure when testing groups.
- Storing samples incorrectly or leaving them too long
What happens to a sample between collection and examination can affect the result.
When faeces are left somewhere warm, certain parasite eggs may develop and hatch before testing. This can reduce the number of intact eggs available to count and potentially produce an artificially low result.
Poor storage can also cause samples to dry out or deteriorate.
For sheep samples, SCOPS recommends keeping fresh dung cool, but not frozen, in an airtight container and arranging examination or laboratory delivery within 48 hours. Requirements can differ depending on the animal and test.
How to avoid this mistake: Collect samples as close to testing as practical, use a sealed container and follow the storage and delivery instructions provided by the testing laboratory.
- Using the wrong dilution or failing to mix properly
Even an accurately collected sample can produce unreliable results if it is prepared incorrectly.
The proportions of faeces and flotation solution matter because they form part of the calculation used to report eggs per gram.
In our Vetlab McMaster counting method, the procedure for our two-cell acrylic counting slide involves:
- Adding flotation solution to the 26 ml mark of a measuring cylinder.
- Adding faeces until the liquid reaches the 30 ml mark.
- Straining the mixture and mixing the remaining suspension thoroughly.
- Immediately transferring the mixed suspension into both counting chambers.
These measurements are specific to this method and should not be substituted with quantities from another protocol.
Mixing is equally important. If the suspension is left standing, eggs may begin floating towards the surface. Taking liquid from the top without remixing can overestimate the count, while sampling lower down may underestimate it.
How to avoid this mistake: Follow the exact dilution instructions for your method and mix the suspension thoroughly immediately before filling the counting chambers.
- Using an unsuitable flotation solution
Flotation solutions allow parasite eggs to separate from faecal debris so they can be examined under a microscope.
Their effectiveness depends partly on specific gravity (SG), which measures the density of the solution.
If the solution is not sufficiently dense for the parasite eggs being investigated, some eggs may fail to float and could be missed. An unsuitable solution may also affect the appearance of delicate parasite stages.
Different parasites can require different flotation conditions. A solution suitable for routine worm egg counting in livestock should not automatically be assumed to be the best choice for examining protozoan cysts in dogs or cats.
At Vetlab Supplies, we offer ready-made faecal flotation solutions using sodium nitrate or zinc sulphate, both available at a standard specific gravity of 1.200. We can also supply solutions made to different specific gravity requirements.
How to avoid this mistake: Choose a flotation solution suited to the parasite and testing procedure. Check its specific gravity rather than assuming a homemade mixture has the correct density.
- Filling or examining the McMaster counting chamber incorrectly
Correct preparation can be undermined by mistakes when loading the counting slide.
Air bubbles and incomplete filling can interfere with the volume of suspension being examined. Timing also matters because parasite eggs need an opportunity to rise towards the upper surface of the chamber.
For our Vetlab acrylic McMaster counting slides, the published procedure specifies filling both chambers and allowing the slide to stand for at least two minutes, but no longer than five minutes.
Leaving the slide too long may cause some delicate eggs to shrink or become distorted.
How to avoid this mistake: Fill both chambers steadily, avoid trapped air and follow the examination timing specified for your particular slide and method.
- Miscounting eggs or applying the wrong calculation
Once the slide reaches the microscope, accurate counting depends on correct focus, identifying the eggs and examining the specified counting area.
In the McMaster method, eggs float towards the upper surface of the chamber. Focusing too low may cause them to be overlooked.
Our method recommends initially using the microscope’s x10 objective and focusing sharply on a grid line to bring the eggs into view.
The calculation must also match the testing procedure.
For our published two-cell acrylic McMaster method, count the eggs within both grids and multiply the combined total by 25.
For example:
- Chamber one: 12 eggs
- Chamber two: 15 eggs
- Combined total: 27 eggs
- 27 × 25 = 675 EPG
With this method, each egg counted represents 25 EPG, making 25 EPG the smallest non-zero result that can be reported.
Importantly, other McMaster techniques may use different dilution ratios, counting arrangements and multiplication factors. Our glass counting slide also has slightly different counting instructions.
Applying a multiplier of 50 to the example above would incorrectly report 1,350 EPG.
How to avoid this mistake: Follow the counting instructions for your specific slide and preparation method. Never assume the same calculation applies to every McMaster procedure.
- Failing to clean equipment between samples
When several samples are processed consecutively, cleanliness becomes another important factor.
Parasite eggs or faecal residue left in counting chambers, mixing bowls, strainers or reusable equipment may contaminate the next sample. This can potentially produce an artificially high count.
Our acrylic McMaster slides feature a sonic-welded construction designed to prevent faecal residue becoming trapped between the slide components. However, the counting chambers still need cleaning after use.
Our published instructions recommend washing the chambers in warm water with domestic washing-up liquid, using a soft cloth or brush, before rinsing with clean water.
How to avoid this mistake: Thoroughly clean reusable equipment between samples, use clean transfer pipettes and take care not to damage the slide or counting grid.
Why a low or negative faecal egg count needs careful interpretation
Not every unexpected result means something has gone wrong with the testing procedure.
Parasite egg shedding naturally varies, and some worms may be present without producing detectable eggs. Immature stages of infection may also go undetected.
A result of zero EPG therefore means that no eggs were detected using that particular test. It does not guarantee that the animal is free from parasites.
The parasite being investigated also matters.
For example, routine faecal egg counts are not a reliable way to assess equine tapeworm infection because tapeworm eggs are shed intermittently. Alternative approaches include antibody-based tests, such as the EquiSal Tapeworm saliva test available from Vetlab Supplies.
Likewise, routine FECs cannot reliably identify encysted small redworm stages in horses.
Faecal egg counts are most useful when considered alongside the animal’s age, health, previous treatment and veterinary advice. If an animal is unwell or a result appears inconsistent with its clinical condition, further investigation may be necessary.
A simple faecal egg count accuracy checklist
Before recording or comparing results, check that you have:
- Collected a fresh, representative and correctly labelled sample.
- Stored the sample appropriately and avoided unnecessary delays.
- Followed the correct dilution and mixing instructions.
- Used a suitable flotation solution with the required specific gravity.
- Filled the counting chambers correctly and observed the recommended timing.
- Counted the correct grid areas and applied the appropriate EPG multiplier.
- Cleaned equipment thoroughly between samples.
For ongoing monitoring, use a consistent testing method wherever possible and record the sample date, preparation method and results.
Getting the right equipment for reliable faecal egg counts
Reliable results depend on careful sampling, correct technique and equipment suited to the testing method.
At Vetlab Supplies, we provide McMaster counting slides, ready-made flotation solutions, laboratory consumables and faecal egg count starter kits for a range of testing requirements.
Our starter kits are available with or without microscopes, making them suitable for customers setting up a new testing system or adding to equipment they already own.
Whether you need a replacement counting slide, a particular flotation solution or advice on selecting a complete kit, our team is happy to help you find the right equipment.
Contact Vetlab Supplies to discuss your parasitology testing requirements.
