Worms: how to control them in horses

How do I stop my horse getting worms? 

Horses and ponies can have a variety of internal and external parasites throughout their lifetime. While having a low worm burden will have little or no effect on a horse’s health, a high parasite burden can cause serious health problems if left untreated. 

If owners follow a worm control programme and exercise good pasture management, most horses will not suffer from worm infestation damage.  

There are a range of internal parasites that can affect horses in the UK, and these can include: 

• Small redworms 
• Large redworms 
• Tapeworms 
• Roundworms 
• Pinworms 
• Lungworm 
• Liver fluke 
• Bots (Gasterophilus fly larvae) 

What steps can be taken to prevent worm damage? 

To control worms effectively, you need to keep your horse’s pasture clean and use a targeted approach to de-worming. You should test each horse before making the decision to de-worm them. The test provides an estimate of how many eggs are present and identifies the type of parasite eggs the horse is shedding in its droppings to allow treatment of those horses who need it. You can use our Annual Horse Health Plan to help you put together a targeted de-worming programme.

‘Blanket’ de-worming has resulted in some parasites becoming resistant to many of the drugs that we use. This could mean that in the future we reach a point where all worms are resistant to the de-wormers available – which makes it even more important to look at other ways we can control worms in our horses, starting with targeted de-worming programmes. To determine your horse’s risk profile, why not check out CANTER’s risk tool. 

How does pasture management help with worm control? 

The worms that end up in our horses come from contaminated pasture. A redworm can develop from an egg in a dung pile to the larval stage in just five days. In dry conditions, it can travel a metre in that time but in wet weather they can travel up to three metres. Wet, mild weather just helps worms to spread further, making it even more important to remove droppings regularly in such conditions. 

See our Pasture management for horse paddocks guidance for more information on how to reduce the number of worm eggs in your pastures.  

How do I worm my horse correctly? 

Key points to remember: 

• Weigh your horse before you de-worm them to make sure they get the correct dose. You can use a weigh bridge to obtain their exact weight, but using a weigh tape will give you a good enough estimate. If you use a weigh tape, you should add 10% to the weight shown by the tape. 
• Make sure all the de-wormer goes down the horse’s throat – tilt their head up after you have given the wormer to stop them spitting it out. If your horse spits out even a small amount the dose received can be significantly lower than needed, which increases the risk of resistance. 
• If you’re not confident de-worming your horse, enlist help from someone experienced. You can also practise with a clean, empty syringe to get your horse used to it. Find out more here.

For queries about which de-wormer to use, contact your vet or another Suitably Qualified Person (SQP)/Registered Animal Medicines Advisor (RAMA). 

If you suspect your horse may have a significant worm burden or they are showing clinical signs, please call your vet straight away. If you rescue, rehome or buy a horse without knowing their health/de-worming history, you should keep them isolated until they have been tested, and if necessary, treated for a worm burden. 

In addition to the parasites guide above, we have additional information on redworm infection, also known as cyathastomiasis.