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Why do faecal egg counts? Isn’t it simply good enough to have a regular drenching programme and take it as read that the strategy is working?

There is no doubt that a well-structured drenching programme is a must, but what faecal egg counts will do is provide you with much-needed information on the effectiveness of your programme, and suggest important tweaks that might make it even more effective – perhaps even using less drench at the same time.

It is well known that some horses harbour quite high parasite numbers while others consistently maintain a low burden.

By conducting faecal egg counts we can build a picture of each horse’s susceptibility to parasites and gauge the effectiveness of our programme.

It is possible, for example, that worms resistant to a particular drench are to be found on your property. In that case, the particular drench you’re delivering may be largely ineffective. Conversely, you may be drenching horses that have a low parasite burden.

The ultimate aim of any horse owner is to use as little drench as possible to obtain maximum, effective control.

What exactly is a faecal egg count? It’s a process whereby you take a sample of fresh dung and carefully take a precise amount of the material. To do this we add a simple solution which we can make at home and mix it up thoroughly.

Once mixed, we strain it and place a tiny portion of the discoloured liquid into a special slide that holds a specific volume of this fluid.

We place this slide in a microscope and, at quite low magnifications, we can count the eggs. Armed with this count, we can then do a simple calculation to find discover the number of eggs per gram (EPG) in the faeces.

This eggs-per-gram number is a standard measure and, depending on the number, we know whether the horse in question is carrying a low, moderate or high worm burden, and can then act accordingly.

The reason we want to quantify the level of infestation is that most horses exposed to pasture are infected by strongyles, and nearly all the eggs you are likely to see under the microscope will be strongyle eggs – probably 95% or more.

The fact your horse has strongyle eggs in its faeces is hardly front-page news. However, because strongyles are prolific egg producers, we can quantify the eggs and get a clear indication of just how heavy a burden each horse has.

Your main targets will be strongyle and ascarid eggs. It is your measure of these parasite eggs that will ultimately sway your drenching decisions.

Naturally, we are talking here about a do-it-yourself test. Don’t for a moment consider yourself a parasitologist! The pictures that follow will help you identify the key eggs but there is a real chance you will miss some, perhaps even count some air bubbles as eggs accidentally.

Consider yourself more of an amateur sleuth than a fully trained detective.

By performing faecal egg counts and conducting follow-up tests you begin to draw a more accurate picture of the parasite management issues on your property, and among your horses.

Repeated egg counts will quickly help you identify the horses most susceptible to parasites. Follow-up tests will show whether the drench you are using is proving effective, and will help you fully evaluate your overall drenching programme.

The other option is to send samples away to laboratories to have egg counts done.

Alternatively, if you lack confidence, why not do your own egg counts and send away part of the original sample collected from the paddock to see how well you did?

You will need to get some equipment to do the counts and will have to spend a few dollars. However, in the overall scheme of keeping horses, the cost is pretty modest.

The fact you are doing faecal egg counts also means you’ll probably save money through using less drench, or at the very least use drenches that are proving the most effective.

The equipment you need is as follows:

A microscope. This is the most expensive item on the list, but the good news is that you certainly don’t need a top-line model. In fact, you’ll need a magnification of only 100, which is right at the lower end of most microscopes. Even modestly priced microscopes can easily magnify to 400 times or more. You could easily find a scope to do the job for under $NZ100 and would be looking at a good one for about twice that. There are a couple of considerations in buying microscopes. Microscopes are lit by either a bulb (powered by the mains or by batteries) or a mirror which directs the available ambient light through the slide. If you intend doing your faecal egg counts in an outbuilding without power, ensure the microscope you choose runs on either batteries or is fitted with a mirror. Some are mains-powered only.

A McMaster slide. These are slides developed specially for doing eggs counts. They have two chambers that hold liquid and each chamber has a grid to help in your egg count. McMaster slides are readily available and cost around $NZ40. Your local rural stockist may carry them or you can source them through internet retailers. In New Zealand, they can be found in the Shoof catalogue.

Two beakers, graduated in millilitres. Those capable of holding about 100ml should do nicely. It would be great if you bought a measuring cylinder as this would allow you to measure quantities even more accurately.

A pipette or eye dropper which you can use to transfer your sample into the slide. These can be sourced from your local pharmacy. A small syringe tube without the needle will likewise do fine.

Something suitable for picking up the dung sample. You could set aside old spoons for this purpose or buy a packet of large wooden ice-block sticks, which you can dispose of after use. A $2 shop or your equivalent would be a good source.

While there, grab a few small plastic containers with lids, which will be handy for holding your dung samples, and some stick-on labels so you can write the name of the horse on each sample bottle. Alternatively, you can use small sealable plastic bags.

A flotation solution. This is something you add to the dung sample to dilute it. However, it serves another important purpose. The solution is of a specific density so that the eggs in the liquid float to the top, allowing you to count them in the slide. You must not use water, as the eggs will sink and you won’t see them. You can buy these solutions, but it’s cheaper to make them at home. See the side panel on how to make a suitable solution.

Disposable gloves. Dung contains bacteria and you don’t want to expose yourself to unnecessary risk. Wear these gloves when working with manure.

A wee set of scales for measuring your dung sample. You’ll be measuring only a few grams of dung, so the scales need to be reasonably accurate, perhaps to a tenth of a gram. I bought a new set of scales from an auction website for around $30. If you don’t want to spend this money, there is another way we can measure the sample, which we’ll discuss later.

A fine sieve or some cheesecloth. Your standard kitchen sieves vary considerably in the fineness of the mesh, so shop around until you find a fine one. Some people even use a tea strainer.

We’re now ready to roll.

To make a suitable flotation solution, we take water and dissolve another substance in it. This increases its specific gravity and provides the liquid density required to ensure the eggs we want to count float to the top.There are a whole raft of readily available chemicals we can dissolve in water for this purpose. Some behave in certain ways, by distorting eggs or in some cases the resulting fluid isn’t dense enough for certain kinds of eggs to float to the surface. We’ll therefore stick to a handful of general purpose solutions, which will be fine for our purposes. The commonest solution used by laboratories is sodium nitrate. Take 1000ml (one litre) of water and dissolve 400 grams of sodium nitrate in it. Stir to dissolve the chemical. This will give you a flotation solution with a specific gravity of 1.18. It can distort eggs if left mixed with the dung for more than 20 minutes, but that should provide you with plenty of time to conduct your test. It’s a perfect solution for strongyles. While sodium nitrate is quite cheap, it may be difficult to source – at least in the modest quantities that you require. Another common flotation solution is a saturated salt solution, which will give us a fluid with a specific gravity of 1.18 to 1.20. Warm a litre of water and stir in 400 grams of salt (sodium chloride) until dissolved. Allow to cool. A small amount may settle at the bottom once it cools, but this is no cause for concern. This solution works fine but may distort the eggs. Another good general purpose solution is a mix of salt and sugar with water. Mix as for the saturated salt solution above (400 grams of salt in a litre of water), but also add 500g of sugar. This provides a solution with the highest specific gravity of these three examples, at 1.28.

The first step is to collect your dung sample. You need to find a very fresh sample, particularly in warmer weather, as some eggs can hatch in a matter of hours and will therefore not show up to be counted. If too many hatch, it will make your count less accurate.

Collect a modest ball of dung, place it in your container, and label it with the horse’s name. If you can’t conduct the test immediately, store the sample in a refrigerator to prevent any eggs hatching. If you’re collecting the sample from pasture, ensure it’s not contaminated by soil or older dung deposits.

Don’t forget to wear your gloves when handling dung.

Place in your beaker 26ml of flotation solution.

Using one of your wooden ice-cream sticks, thoroughly mix the dung you collected and then weigh out precisely 4 grams. Add this to the 26ml of flotation solution and stir thoroughly. These precise quantities are important.

If you don’t have scales capable of measuring 4 grams, add small amounts of dung to the solution until the level comes up to the 30ml mark. The solution weighs roughly 1 gram per mil, so once the liquid is displaced up to the 30ml mark, you’ll be close enough to your 4 grams of faeces.

Stir this thoroughly so the manure breaks up and is even distributedly. Run this liquid through your fine sieve or several layers of cheesecloth into your second beaker to get rid of larger remaining particles. This is not an essential step, but it will certainly give you a much cleaner sample and will make your work under the microscope much easier.

Mix the strained solution well once again and immediately use your syringe or eyedropper (pipette) to draw up some of the mixture. Don’t leave the solution standing for any more than a moment before drawing up the fluid, as the eggs will begin floating to the top as soon as the mixture is left undisturbed.

Use your syringe or eye dropper to fill one chamber of the slide. Don’t allow large air bubbles to form in the chambers. If they do, suck some of the fluid out and refill.

When one chamber is filled. Empty the pipette back into the beaker and mix again, before drawing up some more of the mix to fill the second chamber.

Allow the slide to sit for five or so minutes to allow the eggs to float to the surface. Leaving the slide sitting will also provide time for some of the heavier debris in the sample to sink to the bottom of the slide, away from where you’ll be conducting your count. You can leave the slide for 30 minutes or more if you wish, although some flotation solutions will begin to distort some eggs after a time.

Place the slide gently under your microscope for examination. Don’t lock the slide into position as you will want to move it around to conduct your egg count.

Set the microscope at a magnification of 100. For most, the eyepiece will be at a fixed magnification of 10X, so all you need do to get your 100-times magnification is select the 10X lens immediately above the slide.

Be careful, especially with your initial use of the slide. It is much thicker than a standard microscope slide, and you don’t want the bottom lens to crunch into the slide and break it. You should be OK with the 10X bottom lens but be careful, just in case.

Using the higher magnification lenses on your microscope is not only unnecessary, but will likely result in a “collision”.

The grid in each of the slide chambers is at the same focal plane as the top of the fluid where the eggs will be accumulating. In other words, when you have the grid in sharp focus, you will also have the eggs in focus. You should see some dark circles, which are air bubbles that have also floated to the top layer.

At the 100 magnification, your view should encompass the width of a grid. Start in one corner and count the eggs by gently moving the slide so your view proceeds down the first of the six grid areas on each chamber. At the bottom, gently move the slide across and count the next grid going up. Proceed in this zigzag fashion until you’ve counted all 12 chambers.

Take a note of the tally of each chamber and remember which one you are counting so you don’t lose your way.

To be consistent, count only eggs where more than half fall within each grid. This ensures that an egg sitting on a line between a grid is not accidentally counted twice.

Once finished, tally up the number of eggs you’ve collected. You’re now ready to make the simple calculation to arrive at an eggs per gram figure.

All you need do is multiple the number of eggs seen by 25 and this will give you the eggs per gram figure.

How does this work? It’s simple mathematics. Each chamber holds exactly 0.15 millilitres of fluid under the grid. Across both grids that’s 0.3 millitres, which just happens to be 1 per cent of the total amount of faeces/fluid mixture that we started with.

The method described here involved 4 grams of faeces mixed with 26ml of our flotation solution. Now that we know the maths involved, we can choose to alter the quantities if we like, as long as we do the calculation correctly. For example, we could take our 4 grams of faeces and mix it with 56ml of flotation solution. We know that we will count the number of eggs seen in .3 of a milliliter in our McMaster slide, meaning we have counted the eggs seen in one-two-hundredths of the total dung-solution mix. But, remember, we started with 4 grams of dung, which means we have to divide our four into the 200, to give us a factor of 50. This means, using the quantities here, every egg we see indicates 50 eggs per gram in the faeces.

However, you may recall we started with a 4 gram dung sample, not 1 gram, so to get an accurate result we multiply the egg count not by 100, but by 25.

So, if, for example, your total egg count for the slide was 10, you multiply it by 25 to get a “moderate” worm burden of 250 eggs per gram of faeces.

Essentially, each egg you see under the grid on the slide represents 25 eggs per gram in the faeces. It’s a broad measure with a considerable margin for error. If, for example, you see no eggs, it doesn’t mean the horse has no worms. It means its egg count is probably below 25 eggs per gram.

For your purposes, all you need to understand is that the horse’s count is obviously low and it does not require drenching.

While your counting may not match the performance of the experts, the information you glean will help you paint a more accurate picture of the worm burden among your horses and the effectiveness or otherwise of your drenching programme.

Keep a careful written record and, by continuing to do egg counts, you’ll be able to keep a tight rein on the worms silly enough to hang around your property.

What does my egg count mean?

You can classify your worm count as either low, moderate, or high. A faecal egg count of fewer than 100 eggs per gram of faeces is considered low. A count of between of 100 and 500 is considered in the moderate range and anything above 500 is considered high.

Identifying parasites under the microscope

A picture is worth a thousand words, so study the images closely to help you identify what you’re looking for through the microscope.

The eggs are more likely to see are those of strongyles and ascarids. More than likely, it will be strongyle egg count that will persuade you it’s time to drench a horse. Remember, strongyle eggs are ubiquitous in pasture, meaning your horse’s re-infection by this parasite is likely to begin in its first few mouthfuls.

It is important to realise the limitation of a faecal egg count – and your own limitations.

You might be lucky enough to know a friendly veterinarian or laboratory technician who will help explain what you see through your microscope.

If you don’t, you’re relying on pictures and your identifying skills should gradually improve.

Given the DIY nature of your egg count, what particular value is it? Firstly, if you perform the test properly and your count indicates a high worm burden, that’s as much as you need to know. The next step is to organise a drenching for the horse or horses in question.

Once your testing programme is under way, it has great value as a means of assessing your deworming programme. Are you seeing lower egg counts in subsequent tests? Are some parasites coming back faster than you might expect, which indicates the parasite might have some resistance to the particular drench you’re using? Which of your horses tend to have higher eggs counts and which ones enjoy lower numbers?

With every test – or every round of tests – you build a clearer picture and can work even smarter.

Other limitations exist. For example, the above test is not an effective way of detecting tapeworms. Mature tapeworms do not release a consistent flood of eggs as strongyles do. Tapeworms release little packets of eggs from their tail section only at intermittent times, so there is a good chance you won’t find them in the dung sample. Even if tapeworm eggs are present, they will be distributed unevenly and there is every chance you will not see any in the small sample you selected for testing.

Naturally, the more tests you undertake the greater the likelihood of finding a tapeworm egg or two. You’re best to assume your horse is infected with tapeworms and drench when appropriate. Studies involving post mortems on adult horses have pointed to an infection rate of up to 50%.

It is also important to realise that while an egg count will provide you with some guidance as to worm burden, it provides only part of the answer. Only mature worms will be producing eggs. An egg count will, for example, provide you with no data on the numbers of encysted strongyles or immature ascarids.

Another limitation relates to the mathematics. We already know that every egg we see under the microscope equates to 25 eggs per gram in the faeces. Given this equation, just because you don’t see any eggs in your test does not mean the horse is free from worms. It simply means that the horse’s egg count is under 25 eggs per gram — certainly at a level that should cause you no concern.

» Next: Fighting against worm resistance

» Refrigeration is best, say researchers

First published on Horsetalk.co.nz in February, 2009

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