What exactly is resistance and how do Worm Egg Counts help?

July 13, 2018


The idea of resistance to wormers is not an entirely new concept, however, you wouldn’t be alone if you still weren't quite sure what it means, how it impacts you as a horse owner and lover, and how we can help to combat it. Hopefully I’m going to remove some of the confusion around these areas for you!

 So, what is resistance?

First things first, I should clarify what I mean when I say ‘wormers’. I am referring to the chemical drugs – scientifically known as anthelmintics - which are created to kill horse endoparasites; the worms which live inside your horse. There are several brands of wormers available, however they all contain one of 4 active ingredients; Ivermectin, Moxidectin, Pyrantel embonate or Fenbendazole. These drugs work in different ways and combat different variations of parasites.


When these wormers were first available on the market, in the 1960’s, the recommendation was to worm every horse with one of these drugs every 6-8 weeks, which remained common practice for many decades. This meant that horses were being wormed even if they did not have a worm burden, let alone one which posed a risk to their health. Blanket worming like this is what has promoted wormer-resistant parasites to evolve. The majority of the general population understands what antibiotic resistance is – when antibiotics lose their ability to kill bacteria, mostly due to overuse, and worm resistance to anthelmintics is a similar concept.



When we give our horses de-worming drugs, some percentage of the worms which occur in our horses will naturally have resistance to these drugs – mostly through random mutations. Often this ability to evade death by de-wormers is passed on to the next generation of parasites -  it is hereditary much like eye colour or hair colour in humans. The more frequently these worms come into contact with these drugs, the more important it is for the worm population to be able to survive contact with the chemicals; a selection pressure for resistance is created. This concept of selection pressures was first introduced to us by Charles Darwin to explain evolution and is the same reason why giraffes evolved long necks, or why humans learnt to talk; a trait which helps a species to survive will be favoured within a population, and it will become more and more common throughout every generation.

How does this impact me?

Resistance has been reported to every single class of wormer drug available. If more and more worms in a population become resistant to the wormers, then the drugs will gradually lose their efficiency in removing the worms from our horses. This means that, yes, there are 3 more types of drug available, but soon resistance will develop to these drugs too, and with no new anthelmintics likely to be introduced on to the market any time soon, we could be left with no way to effectively manage the worms in our horses which can be so detrimental to their health.

 An Equine Redworm egg as seen under the microscope during Worm Egg Counts


How can Worm Egg Counts help?

It is unrealistic to think that our horses will ever have absolutely no eggs within their system, and a small population does not pose a risk to them. When worms lay their eggs, they are excreted in the horses’ poo. These eggs then hatch and crawl out on to the grass, which is then grazed by the horse, putting the next generation of worm into their system; this is why regular poo picking is so important! Understanding this lifecycle (which varies slightly between different species of worms) can help us to combat worm burdens responsibly, therefore helping to combat the spread of wormer resistance.

Worm Egg Counts can help us to identify high, or risky burdens of worms in horses, so that wormers are only given when necessary, helping to stop wormer resistance, as well as giving us peace of mind that we are not putting chemicals into our horses unnecessarily!


Simplified lifecycle of the equine Redworm


As well as this, we can use worm egg counts to rum efficacy and resistance tests, to identify if the worms in your horse are developing a noticeable level of resistance to the drug, decreasing its efficacy. If resistance is identified, then you will know to avoid certain wormers so that you are not giving them to your horse unnecessarily if they are not going to have any affect on the worm population!

What next?

Ok, so you now understand what resistance is, and how we can combat it, its important to understand what your next steps should be!

If it has been 3 months or more since your horse last had a de-worming treatment, then it is recommended that your horse have a worm egg count carried out. At Target Worm Counts, we can carry out these worm counts for you, either by taking your samples by post, or by coming to your yard and analysing the samples on site the same day.

After your samples have been analysed you will be provided with a full results report, an explanation of the results, and recommendations for treatment. We will also be able to develop a full worming schedule for you and your horse, so you always know what your next step in responsible worming is!





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