FAQs on Testing Your Horse for Worms

What is a faecal egg count and what does it tell me?

A faecal egg count (abbreviated as ‘FEC’) is an estimate of the number of parasite eggs present in each gram of manure. The test, conducted in a laboratory, combines a sample of the manure with a flotation solution allowing eggs to be counted in a special chamber under a microscope. The result you receive from a FEC test is a number expressed as ‘eggs per gram of faeces’, sometimes abbreviated to ‘EPG’. A variety of different parasite eggs can be identified under the microscope, however, the FEC is most commonly used to monitor strongyle eggs shed by small and large strongyles (found in all ages of horse), and ascarid or roundworm eggs (found in foals, weanlings and possibly yearlings). A FEC result gives you an indication of the number of eggs being deposited onto your pasture by the tested horse, and managing this pasture contamination should be one of the key goals of your worm control program. While the FEC is an invaluable tool for parasite management, it is important to understand what the FEC is NOT useful for. A FEC result does not tell you how many worms are inside the horse – there are male worms and immature larvae that do not shed eggs and so are not accounted for by a FEC. A FEC result also cannot be used to explain a particular appearance of a horse or indicate its overall level of health – the two are not linked.

What threshold should I use for treatment and why?

If we are aiming to decrease the frequency of worming, we need to tolerate some level of egg-shedding by our horses. Completely preventing egg-shedding is neither possible nor sensible! A strongyle FEC result is typically categorised as ‘low’ if it is below 200 EPG, ‘moderate’ if it is between 200 and 500 EPG, and ‘high’ if it is above 500 EPG. The FEC threshold that justifies a treatment can be flexible depending on the circumstances, but a general rule of thumb for healthy adult horses is that a ‘high’ count (>500 EPG) would warrant a treatment. Remember, this is to limit the amount of pasture contamination, not because a high count indicates a risk of disease.

How do I use FEC results to inform my worming decisions?

You can get to know your horse/s egg-shedding pattern with just a few FECs throughout the year. Adult horses tend to be quite consistent, so once you know whether they are high- or low-shedders you can formulate a deworming plan to suit. To get started, we recommend doing three separate FEC’s during the seasons that are most favourable for parasite transmission – typically Spring and Autumn. Make sure you allow at least 6 weeks to pass since the last wormer was given so that you can get a good idea of the horse’s natural level of egg-shedding. If your horse tends to be in the ‘high’ category, he/she may require 3 or 4 treatments throughout the year to limit pasture contamination. However, if your horse is more of a ‘low’ or ‘moderate’ egg-shedder, you need only worm once or twice per year. Implementing any non-chemical strategies that are practical on your property, such as manure removal or pasture spelling will also help to keep your reliance on deworming chemicals to a minimum.

You can reliably assume that young horses, like yearlings and 2-year-olds, will be naturally high-shedders while their immune system is maturing, and they should be wormed as such. For foals, a couple of FECs undertaken during the first year of life (e.g. 3 months and weaning), will inform you of whether you have ascarid infections to deal with. This will influence the worming product you choose as ascarids are commonly resistant to the macrocyclic lactones (ivermectin, abamectin, moxidectin).

What is a FECRT and when should I use it?

FECRT stands for ‘Faecal Egg Count Reduction Test’, which is a test of how effective a wormer is against the particular worms on your property. It tells you whether there is an issue with resistance or not. As the name suggests, the FECRT is based on FEC’s – one at the time of worming and another (from the same horse/s) 14 days later. If a wormer has been effective, the reduction in FEC should be very close to 100%. Your vet or service provider will do this calculation for you and provide the result. We recommend that everyone checks the efficacy of the products they use every couple of years so that you can stay informed about which ones are effective.

Can I diagnose tapeworm infections with a FEC?

Diagnosing tapeworm infections can be a bit tricky. The eggs (if present) are sometimes visible under the microscope when a routine FEC is being performed and your vet or service provider should note their presence. However, there can be ‘false negative’ results – which means that a horse that is infected with tapeworm may not receive a positive result via FEC. This is because tapeworms don’t shed eggs directly into the manure the way strongyles and ascarids do. They shed whole body sections containing eggs, creating an irregular pattern of distribution. A modified test can be conducted to improve the detection of tapeworm eggs in faeces, but you would need to ask your service provider if this is possible.

How can I diagnose encysted cyathostomin infections?

In Australia, we have no tools or tests that can confirm the presence of encysted cyathostomins, but fortunately, cases of larval cyathostominosis are not common in well-managed horses. Provided you are using effective wormers (confirmed by FECRT) combined with any non-chemical strategies (outlined earlier), the risk of clinical consequences of encysted cyathostomins should remain low.

Using FEC results to guide your decision making can lead to a range of benefits including a reduced reliance on chemical worming, slowing the development of resistance, and in many cases, a reduction in the long-term cost of wormers! But one of the more interesting outcomes is that you will become more knowledgeable about your horse and the life cycles of important parasites.

Author: Dr. Anne Beasley Ph. D.