... and what will we do when the wormers stop working?

What does wormer resistance mean?

The term ‘Wormer Resistance’ describes how parasites (worms within our horses) have evolved to be no longer affected by the chosen worming treatment and the active ingredient within that treatment.

This hugely impacts and limits the effectiveness of the active ingredients we have available to control any parasite infections. There are currently no new wormers in development; if we end up with resistance to the wormers that we currently have available then we will be forced to keep our horses on infected land, which could be devastating or potentially fatal.

There are already signs of resistance in some worm species to certain active ingredients. Fenbendazole, for example, has shown high levels of resistance in treating adult and encysted stages of small redworm. A study carried out by Nielsen in 2023 has also shown apparent resistance in certain tapeworm species to Praziquantel and Pyrantel, which are the only two available active ingredients that are licenced to treat for tapeworm. You can read about this further here: https://www.sciencedirect.com/science/article/pii/S2211320723000222

For more information, Westgate Labs have put together this handy video below to help illustrate the risks we face with wormer resistance.

With winter in full swing, it is the time to be mindful of the Encysted Redworm!

There is a particular stage  of the redworm we need to be aware of during this time as this can pose significant risks. If your horse is classed as low risk, in a closed herd with good paddock management you can chat to your vet further regarding the Small Encysted Redworm blood test.

Remember, every horse that does not require treatment is helping contribute towards delaying wormer resistance. Please note, horses with high previous egg counts within the last year are considered high risk and therefore may not be appropriate for the innovative new test, so should continue routine treatment.

Just because you give a wormer it doesn’t mean it’s done the job we intended it to do. Resistance problems are rising across the five chemicals we have licenced to treat parasites in horses. Whichever drug you choose the winter dose is an ideal time to add in a reduction test to your programme.

However if you have any queries, please discuss with your vet before taking action.

What can I do to help prevent wormer resistance?

An easy way to prevent resistance is to protect our key active ingredients that we currently have available. Parasites can only build resistance when they are continuously exposed to the same active ingredients over time.

This is why regular testing according to an annual worming programme will help to diagnose if your horse does require treatment or not and so should reduce the frequency of the doses that they require throughout the year.

If your horse does require treatment following a diagnostic test, it is important to not underdose. Exposing a worm population to a dose of wormer that is not appropriate for your horse’s size is an easy way for these worms to develop resistance by not being killed with the treatment given. It is really important to accurately measure your horse’s weight and worm them appropriately, being sure to use slightly more wormer than your horse’s weight to ensure that the full parasite population is eradicated; for example, if your horse weighs 480kg, you would want to treat with 500kg worth of wormer.

When to speak to an SQP?

It is also important to speak to either your vet or a qualified J-SQP (equine only SQP) or E-SQP (equine and companion animal SQP) following a positive parasite diagnosis to ensure that you are using the correct active ingredient to target the worm population that your horse has. By targeting the drugs that we use, we are naturally preserving the active ingredients that we have available. Once your horse has been treated appropriately, it is advised to leave them out of the field for 24-48 hours and then turn them out on the same poo-picked pasture as before for a few days before rotating the pasture.

Another simple and effective way to reduce resistance is to reduce the worm challenge in the environment your horse lives in. The best way to do this is by regularly poo picking your fields and keeping stables and field shelters as clean as possible. By removing droppings from the pasture and stables, this naturally breaks the life cycle of the worms and so reduces the chance of your horse needing to be treated at all.

Worm larvae that are passed in droppings will develop and become mobile within around seven days and so can easily leave the droppings and infect the pasture. When the weather is mild and wet, this can allow the larvae to move even further than when conditions are dry. Therefore, it is a good rule-of-thumb to poo pick your horse’s fields at least twice per week all year-round, or more if you can! If you are able, it is also important to ensure that your muck heap is as far away from your horse’s grazing as possible.

What else can I do to manage things?

If you can, it is also important to keep your horses with the same grazing companions and be sure to not overcrowd your fields. One horse per acre is appropriate. You may also want to rest and rotate your fields throughout the year; a common way of doing this is by allocating specific Summer and Winter grazing. Some horse owners also have sheep and/or cattle which can be co-grazed with their horses. Worms are usually species specific (with some exceptions so it is important to double check with your vet or SQP) and so co-grazing can naturally reduce worm populations on the pasture for horses and the sheep and cattle they graze with.

Where you can, if you do buy a new horse or a new horse comes onto your yard, it is advised to keep them separated from the rest of the herd until they have been tested and treated appropriately. After treating them, keep this new horse off the pasture for 24-48 hours to prevent eco-toxic worming chemicals being leached into the soil.

When away at a competition or an arena hire, be mindful of where you allow your horse to eat. It may be best to just feed them hay/haylage in a net that is off the ground instead of letting them graze. As competition and training venues see many horses coming and going all the time, these can be a hotspot for worms to develop. Bear in mind that anything your horse may pick up from this land will come home with them!

These simple steps that all horse owners can take, will help us to preserve the efficiency of the wormers we have available.

How do I know if my horse has resistant worms?

It is advised to retest your horse after worming to test the efficiency of the worming treatment given; this is known as reduction testing. If you have treated for roundworm, carrying out a Faecal Egg Count test around 14 days after treatment is advised. If you have treated for tapeworm, carrying out an EquiSal test around 8-12 weeks after treatment is advised. By checking the efficiency of the wormer, you can see if the worming treatment you gave has worked. A general rule is that if the worm burden hasn’t reduced by 85-95%, then the worms are showing signs of resistance to the active ingredient that you have used. It is important to document this and use a different active ingredient to eradicate this resistant population. If you are worried or have any questions, please consult your vet or SQP for advice.

Parasite resistance is already showing signs in our horses – This is not something to delay for in the future, horse owners must act now!

*images and links taken from the Westgate Labs webpage 2025.