Thoroughbred horses Clover and Heather at Burford rehoming centre

Managing and preventing worms in horses

We recommend a targeted deworming programme for your horse. This is based on testing, targeted worming treatments and good pasture management. This approach should apply to all horses on your yard.

Horse worms are internal parasites that can affect horses throughout their lives – while a low worm burden will not have a big impact on your horse, a high worm burden can cause serious health problems. If left untreated , this can result in permanent damage or even death.

Routine blanket deworming over the years has resulted in many parasites becoming resistant to the deworming drugs we use. There are currently no new deworming drugs in development so we could reach a point where there are no effective treatments available.


For specific worming advice for your horse, we strongly advise that you contact your vet or a suitably qualified person (SQP). If necessary, you can usually buy worming treatment from your vet, local feed merchant or tack shop, or online via a testing lab. Make sure you are purchasing from an SQP or a Registered Animal Medicines Advisor (RAMA). These are certified to prescribe medicines and give advice on worming.

Wormer resistance

Because of the frequent blanket worming treatments carried out in the past, parasites are currently at high risk of developing resistance to all available treatments. There are currently no new drugs in development so we may reach a point where most worms cannot be treated by the wormers currently available – the impact on horse health would be disastrous.

We can help to prevent wormer resistance by:

  • following a target led worming programme. This involves testing horses regularly and only worming for the specific worm burden they have at a given time.
  • worming at the right times of the year
  • making sure we administer the right dosage to be effective.
  • poo picking regularly and resting paddocks or cross grazing to break the lifecycle of worms

Controlling worms

Controlling the presence of worms in your horse takes a combination of a targeted worming programme and good pasture management.

Good pasture management

Since a horse’s worm burden will come from contaminated grazing, good pasture management is essential for reducing worm eggs in the field. To help keep your grazing as parasite-free as possible, you can:

Poo pick regularly

Removing droppings from the field is vital in preventing horses from becoming infected. Worm eggs in your horse’s droppings can quickly develop into larvae in 5 days. They can then travel up to three metres across the pasture.

This is why daily poo picking is best but should be done at least every three to four days. It’s also a good idea to keep your muck heap away from where your horses graze.


Do not harrow your field to spread droppings or use horse manure as a fertilizer on horse grazing. This will only spread eggs and larvae further. Research has shown that temperatures are rarely high enough in the UK (above 36 degrees) to kill off the eggs and larvae this way.

Cross graze and rest your pastures

Cross grazing your pastures with other animals (such as sheep) can help to break the lifecycle of worms. Most equine parasites and eggs cannot survive in sheep and cows.

You can also rest and rotate your paddocks to help prevent them from becoming overgrazed. Overgrazing can lead to the pasture becoming ‘horse sick’, making it rough and worm or weed infested.

Avoid wormer contamination

After worming do not move your horses onto new grazing for at least 48 hours. This will help to reduce the risk of contaminating new grazing and increasing wormer resistance. If possible, you can keep your horse stabled for 48 hours after worming to prevent contamination of other horses.

It’s helpful to keep horses with the same grazing companions and avoid changing groups. New horses should be kept separate from the herd until they have been tested and treated. This will reduce any parasite burdens they may be carrying to your grazing.

For mares and foals, you should alternate grazing every year where possible.

Testing for worms

Testing for worms is an important part of your horse’s worming programme. With regular testing you can monitor your horse’s worm count and treat in a targeted way.


Always talk to your vet, testing lab or SQP for specific advice on your horse’s FWEC results.

Faecal worm egg counts (FWEC)

A faecal worm egg count identifies the amount of worm eggs in your horse’s droppings. It detects the presence of strongyles (large and small redworms), which are the most common parasites that affect horses.

A FWEC is carried out by sending a fresh sample of your horse’s droppings to a laboratory, which is then checked under a microscope. The number of worm eggs are then counted. Since eggs are not spread evenly throughout the droppings, you’ll need to supply a sample from several nuggets of manure.

This test can be carried out by your vet or by a testing service such as Westgate Labs.

Your worm egg count results

Your horse’s test results will determine whether or not you need to deworm them, and what type of wormer you’ll need to use. A small number of eggs is acceptable, and can even be beneficial to stimulating your horse’s immune system.

  • 0 <50 EPG – No trace of worm eggs were found in the sample
  • Less than 200 EPG (low) – Less than 200 eggs per gram were found in the sample which is considered low. Worming will most probably not be necessary.
  • 200 EPG - 1175 EPG (medium) – A count between 200 and 1200 eggs per gram is a medium count. Your horse will need worming and your vet or testing lab will recommend which wormer to use.
  • 1200 EPG or more (high) – If the count is more than 1200 eggs per gram it is considered to be a high count. Your horse will need worming and a follow-on programme based on the advice of your vet or testing lab.
Saliva test (EquiSal)

The EquiSal test identifies the level of tapeworm burden by measuring the antibody levels produced in response to the tapeworm burden. It’s carried out by collecting a sample of your horse’s saliva. This is then sent to the EquiSal lab. The saliva test kit can be purchased directly from the manufacturer, or from a testing service such as Westgate Labs or Poo Post.

The results will identify if your horse has a low burden, a borderline burden (one adult tapeworm) or a moderate or high burden (nine or more adult tapeworms). If the results are moderate or high, your vet or SQP will advise an appropriate wormer, as well as a follow-on programme.

Blood test (ELISA)

The ELISA blood test can detect encysted redworm larvae, and is carried out by your vet. It’s important to assess whether individual horses and herds are suitable candidates for this test. You should consider the age of your horse, the likelihood of small redworm infection, and their previous FWEC results. Results are reported as ‘serum scores’. They should be assessed by your vet along with the above factors to determine whether a moxidectin treatment should be administered.

Reduction testing

If you’ve administered wormer, you should follow up with another test to check if the number of worms in your horse has reduced. After treating for roundworm or redworm, carry out another FWEC 10 to 14 days later. After treating for tapeworm, carry out another saliva test two months later.

When to test for worms

Worm egg counts should generally be carried out every eight to 12 weeks. However, as worm egg counts do not pick up encysted redworm or tapeworm burdens, you’ll need to carry out separate tests for these, or use a suitable wormer to cover these worms. All horses in one paddock should be tested at the same time.


Below is a guideline of when to test your horse – this will need to be followed up with suitable treatment as advised by your vet or SQP. Once you’ve tested your horse and received the results, a record of the date and result. This will help you to track the success of worming treatments and wormer resistance.

Season Test
Spring FWEC for redworm and ascarids
  Saliva test for tapeworm
Summer FWEC for redworm and ascarids
Autumn FWEC for redworm and ascarids
  Saliva test for tapeworm
Winter Blood test for encysted redworm larvae

Worming your horse

Once you’ve tested your horse for worms, you’ll know if you need to worm your horse and which worms you need to target. If your horse has a medium or high egg count, your vet, testing lab or SQP will recommend which worming product to use based on your results.

Below are the different chemicals found in de-worming treatments with the parasites they target. Your vet or SQP will advise you on a suitable brand of de-worming treatment for your horse’s infection.

Chemical Worms targeted
Praziquantel Tapeworm
Pyrantel Large strongyles, small strongyles, roundworms, pinworms, tapeworms (when given a double dose)
Moxidectin Large strongyles, small strongyles, encysted strongyles, roundworms, pinworms, bots, lungworms, threadworms
Fenbendazole Large strongyles, small strongyles, roundworms, pinworms, threadworms
Ivermectin Large strongyles, small strongyles, roundworms, pinworms, bots, lungworms, threadworms
Oxibendazole Large strongyles, small strongyles, roundworms, pinworms

How to introduce a horse to wormer

If your horse is new to worming or is nervous around syringes, you’ll need to be patient when giving worming treatment:

  • Practice placing the syringe near your horse, starting from their shoulder. Reward them with a scratch or a tasty treat when they hold still and do not move away. Slowly, you can place the syringe closer to their mouth each time until it's inside their mouth.
  • Use a syringe filled with apple sauce while you practice worming. This creates a positive association with the syringe being placed in their mouth.
  • Do not practice worming just before you intend to administer wormer. This may encourage your horse to spit it out. If your horse spits out any wormer, this may mean the treatment will not be effective as it is not the correct dose.
  • Depending on the type of wormer you use, you may be able to find flavoured wormers for horses who are reluctant to be wormed
  • Ask a friend for help if necessary as you may need more than two hands

How to administer wormer

Before you administer the wormer, make sure to weigh your horse as accurately as possible. This will help you to calculate the correct, most effective dose of treatment. Administering wormer is easier when you have prepared the dose beforehand and kept it out of sight.

  1. Put a headcollar on your horse and hold them in a relatively confined area. Do not tie them up in case they pull back when you give the wormer and injure themselves.
  2. Stand on the left side of your horse by their shoulder, placing your right hand on their nose to keep it steady
  3. Using your left hand, place the syringe into the corner of their mouth and push the plunger down
  4. You may need to hold your horse’s head up until they swallow as some horses may try to spit out the wormer

If you’re struggling to give your horse wormer, you can try adding it to a small handful of hard feed. This may help to mask the taste and avoids wasting wormer if your horse is prone to spitting it out from the syringe.

Worming mares and foals

Preventing a high worm burden in young horses starts during pregnancy. Because of their weaker immune system, foals and young horses are especially at risk of worm infection.

While general worming advice is to test, target worm and maintain good pasture management, due to the higher risk posed to this age group, we advise you speak with your vet or testing lab about the best programme of testing and worming for your brood mares and youngsters.

Effects of wormers on the environment

When a horse drops some wormer on the ground, or when horse droppings contain traces of wormer, it can be ingested by wildlife. Chemical wormers are dangerous for many animals and insects. For some, they can be fatal. Small amounts of ivermectin is known to be toxic for some dog breeds – you can find out more about which breeds are at risk of ivermectin poisoning.

Some wildlife species, such as dung beetles, can also be killed by wormer poisoning. Dung beetles are incredibly useful in breaking down droppings, which can help to keep pastures clean and nourish the soil. To prevent dung beetles and other wildlife from coming into contact with worming treatments, it’s recommended to keep your horse stabled for at least 48 hours after worming if possible.

Page details


• 13 October 2023

Next review

• 13 October 2026

Approved by
Ruth Court

Horse Welfare Manager