Vets and horse worming experts have warned of the devastating consequences the continued use of horse wormers without testing could have on the equine population. Equine health specialist Clare Wood said many owners are still worming their horses on a fixed schedule without doing a faecal worm egg count first, but that resistance to horse wormer is a serious issue which cannot be ignored.
Wormer resistance is similar to antibiotic resistance and describes where worms have evolved to be unaffected by a particular ingredient in wormers. This limits the number of effective drugs available to control infection and, if there are no replacement wormers to combat a worm burden, the results could be devastating. One such example is the ingredient benzimidazole, which encysted small redworm have become resistant to. Another is Equitape (a praziquantel-only wormer), which was discontinued due to tapeworm resistance, meaning there is no longer a product that targets only tapeworm.
“Over-worming is a major contributor to worm resistance,” said vet Sue Taylor MRCVS. “Because of this, owners should always test for the presence of worms using a faecal worm egg count before worming horses. This will help decide whether a horse needs worming as it shows the presence of most egg-laying adult worms, apart from bots, tapeworm and encysted small redworm. If the egg count is high, your vet will advise you to give the horse a wormer. If it’s lower, no wormer is necessary.”
Are mandatory worm counts the way forward?
Clare Wood, founder of Equine Faecal Egg Count Solutions (EFECS), told Your Horse that resistance is growing to such a level that in future “we may not have wormers that work and no way of treating horses”.
“It is a big and serious issue,” she said. “We have got to stop just worming, there is so much resistance and people have no idea how much of an issue this is. There are calls for worm counts to be mandatory before purchasing wormers in the not too distant future — that is how serious the situation is becoming and how seriously it is being taken.”
Clare added that some progress has been made in spreading this message, but it is slow. She said people on larger yards and studs can find testing for worms more tricky, with bringing multiple horses in for tests time-consuming.
“Education is key, but with increased costs of everything some people still think it would just be easier and cheaper to buy the wormer,” said Clare. “Owners of one or two horses are often very aware they should be doing worm counts, but this doesn’t always work out when they are on a livery yard and other owners are not doing the same thing. They are being penalised as the worm cycle has to be broken at the same time for all the horses.
“People can be very blasé about it, but If we don’t get it under control horses will die.”
Ways to prevent wormer resistance
Wormer resistance is when a drug doesn’t work as well as it did when it was first used against specific population of worms. The active ingredient in a horse wormer kills the sensitive parasites in the population, but those parasites not affected go on to create new generations of resistant parasites.
As well as frequent dosing (giving wormer too often), under-dosing (not giving the correct amount of wormer to a horse) can also cause resistance to occur. Dosing correctly and only when necessary is the key to managing resistance to horse wormers. Clare also suggests the following things that every owner can do to help combat the issue of resistance to horse wormers:
- Encourage your livery yard owner to start a faecal egg worm count testing programme for every horse on the yard and roll the price into livery fees. EFECS can work with yard owners to help ease the extra weight of admin.
- If you’re buying a new horse, ask that a worm count report is issued before taking them to a new home, or once your new horse has arrived do a worm count as soon as possible. Don’t allow the horse to share grazing with other horses until you have the results from the egg count and you’re given the all-clear, or you have wormed accordingly.
- Look into a subscription program so you never have to worry about when your horse is due for a worm count. EFECS offers this service and it is very popular.
- Poo picking is a major issue and needs to be tackled even in big herd turn out fields. Make it a regular (daily, ideally) part of your management routine. Do not spread horse manure on grazing paddocks.
- If you go on holiday and paddocks are shared, make sure pasture is poo-picked while you are away.
‘Disastrous for equine welfare’
The British Horse Society (BHS) launched its ‘Think Before You Worm’ campaign in collaboration with the British Equine Veterinary Association (BEVA) at BEVA Congress in September 2022. This followed members of the veterinary world launching Project WORMS earlier the same year, for which horse owners were asked to complete surveys about their worming habits. Analysis of this data is now underway.
“If we continuously deworm our horses without testing first, the number of resistant worms present will increase,” said Gabby Madders, welfare campaigns officer at the BHS. “This means worms on a horse’s pasture will become resistant to dewormers and no longer respond to treatment.
“Only five types of chemical drugs are available to treat horses in the UK for worms and there is now evidence of resistance to all of these chemicals,” continued Gabby. “Without action, we could reach a point in the future where all worms are resistant to the dewormers available, which will be disastrous for equine welfare.”
Choosing the correct horse wormer drug
If a faecal egg worm count result shows that your horse does need to be wormed, choosing the correct one to target the worms you need it to is key to getting rid of the burden and helping prevent resistance to horse wormers. It is also important to understand what worms are threat to horses in winter, summer and the rest of the year.
“When it comes to worming your horse, picking the right treatment depends on your ability to pick the right drug, not the right brand or wormer name,” says XL Equine vet Kirstie Pickles, who explains that there are three classes of worming drug:
There is widespread resistance to this drug. In fact 75% of cyathostomins (encysted small redworms) are resistant to it. Resistance to this drug is worse in the south of the country where yards over wormed on a large scale, for a long time. In rural Scotland, resistance is less of a problem. Fenbendazole treats encysted small redworm larvae, large and small redworm and large roundworm.
There is less resistance to this drug in the UK. However, a bizarre move in the US saw this drug introduced in a daily in-feed wormer which led to complete resistance. This would be like us taking a little bit of penicillin every day to ensure we never got an infection — a surefire way to ensure resistance and a very bad move. Pyrantel wormers treat adult redworm, large roundworm and a double dose will treat tapeworm.
Avermectins (includes ivermectin and moxidectin)
These tackle encysted worms, a job that was once shared with fenbendazole, but resistance to that drug means it is no longer useful for this. Avermectins are used to treat redworm and roundworm (Ivermectin), encysted redworm larvae and roundworm (Moxidectin).
What happens if I over-worm my horse?
Incorrectly worming a horse — either by worming too often or not giving them the right drug to treat a specific type of worm, as well as not administering enough of a drug for the horse’s weight — all leads to resistance. The active ingredient in a horse wormer kills the sensitive parasites in the population, but those parasites not affected go on to create new generations of resistant parasites.
Frequent dosing or under-dosing can cause resistance to occur as can the effectiveness of the wormer. Dosing correctly and only when necessary is the key to managing resistance in horses. Although there’s widespread resistance to some horse wormers, it’s not the same for all wormers and can vary greatly between yards.
If you’re worried about resistance to some horse wormers, the best way to ensure your wormers are working properly is to ask your vet to perform a faecal worm egg count before and two weeks after worming. That way you can assess the level of worm eggs being shed in your horse’s faeces.