None of us want our horses to be carrying a heavy worm burden. Unfortunately, the heavy use of wormers over the years has led to a significant amount of worm resistance within the equine population. This means that some of the wormers are potentially no longer effective at eliminating worm burdens in some of the horses. So, alternative means of managing worms have had to be found.

Problems with resistance can be overcome by monitoring faecal worm egg counts (FWEC) of all horses at regular intervals. This can ensure only those individual horses with a significant worm burden are treated using the appropriate wormers at the appropriate times. 

There is an increasing risk of resistance to anthelmintics (wormers), so we must use different strategies to protect our horses. 

How do horses get worms?

Worms naturally occur in horses and are present in most horses at varying levels of infestation.

How the horse gets infected by worms will differ from worm to worm depending on the lifecycle, but generally, eggs are ingested from infected pasture and then develop inside the intestinal tract (or, in some cases, the horses’ lungs) where they can cause disease.

These eggs produced by the adult worm will then be shed in the faeces to increase existing worm burdens on the pasture and to potentially infect new horses.

When the worms are present in low numbers, worms cause minimal problems. However, if they are present in larger numbers, they can severely affect the horses’ health and can result in poor body condition, colic, and general ill-health.

What worms can affect horses?

Large Redworms (Strongylus vulgaris):

The “large strongyles” or “large redworms” are potentially the most dangerous but are less common nowadays due to the widespread use of wormers. The adults feed on the gut wall which can result in extensive damage and bleeding and can block arteries leading to the gut, leading to gut death and severe surgical colic.

These worms live in the large intestine of horses and donkeys. There are various species and the developing larvae migrate around the body. How long it takes to migrate through the body depends upon the species. This may be as little as 7 months or 12 months. Eventually, they find their way back to the lumen (the inside) of the intestine.

Small redworms (cyathostomins) 

The “small redworms” are also known as cyathostomins. These worms are very common in horses. They go through a dormant phase, where they burrow into the gut wall, become encysted (dormant), and then emerge in spring. 

The hibernation of the small roundworm is particularly dangerous to the horse as in the spring the larvae emerge in large numbers. This mass emergence can cause severe damage to the gut wall, which can lead to weight loss, diarrhoea, and colic, all with potentially fatal consequences.

Roundworms (Ascarids – Parascaris equorum) 

Infection with these worms is quite common worldwide with adult roundworms growing up to 50cm in length. The adult female worm produces exceptionally large numbers of eggs which may persist in the environment for several years. They are also sticky, which may contribute to spread by attaching to the udder and teats of the pregnant mare. 

Roundworms migrate from the gut causing damage to the lungs and liver. In large numbers they can block the gut, causing it to rupture which could lead to death. Within the lungs, they are coughed up, swallowed, and returned to the intestine. 

These worms are dangerous to foals and yearlings that become infected from the first month of life – heavy infection is common in foals and yearlings: Some infected foals pass millions of eggs in the faeces. Some foals and older horses eventually develop immunity to roundworms, and this worm is less commonly a threat to adults. 

Pinworms (Oxyuris equi)

Pinworms lay their eggs around the outside of the anus causing intense itching and irritation. Persistent scratching will result in hair loss and open sores, around the tail head which can become infected.

Threadworm (Strongyloides westeri). 

These are found in the small intestine and are mainly a problem in young foals. They may cause diarrhoea, weakness, and severe weight loss. They transfer to the foal via the mother’s milk. Often foals on stud farms are given a wormer within the first couple of weeks of life. 

Like roundworms (Ascarids), they migrate through the lungs. Horses and foals older than 6 months tend to be immune to threadworms. Other than worming, control is by the removal of dung and provision of dry bedding.

Tapeworms (Anoplocephala spp.)

Tapeworms can grow up to about 20cm in length and have a width of 1.5cm. They form into clusters at the junction between the small and large intestines (where they can cause digestive disturbances, loss of condition, colic, and fatal blockages). Horses become infected through eating the intermediate host – the orbitid or Harvest mite – found on grass and forage. The lifecycle of a tapeworm is six months.

Lungworms (Dictyocaulus arnfieldi)

Lungworms prevail in pastures shared with donkeys – the lungworm’s natural host. These worms cause persistent coughing in horses as respiratory problems develop. Donkeys can tolerate very large worm burdens without showing any clinical signs.

Bots (Gastrophilus)

Bot flies are the most common irritant to horses during the summer grazing season. They lay sticky yellow eggs on the horse’s coat which are ingested as the horse grooms itself or another horse.

What is a faecal worm egg count (FWEC)?

A sample of your horse or ponies’ faeces is collected and then is viewed under a microscope to see whether any worm eggs are present in the dung. FWEC are generally performed at regular intervals through the grazing season (April-October).

Will a faecal worm egg count identify all horse worms?

A FWEC will show the eggs from the worms which lay as part of their lifecycle. This does not include pinworm, bots and tapeworm. In addition, an FWEC will not be able to identify immature worms that are not laying eggs and cannot identify the encysted small redworm.

How do I collect a faecal sample?

  1. Collect the faecal sample whilst it is still warm using a glove and place into a container or clean freezer bag which can be properly sealed. The sample needs to be fresh to ensure that the result is accurate.
  2. Eggs from worms are not evenly spread in the faeces so the sample should be taken from several faecal ‘balls’ within one pile of droppings. The total sample size needs to be at least 4g (but more is usually fine).
  3. Collect dung separately from each horse to be tested. 
  4. Ensure that the container or bag is clearly labelled with your horse’s name, your surname, your postcode and the date the sample was taken (you might want to do this before you put the sample in the container!).
  5. Ensure that the container/bag is properly sealed.

When should I take a faecal sample?

For the best results, a FEC should take place around 14 days after worming your horse. You can also perform a FEC prior to worming, this is to evaluate the effectiveness of your ongoing worming program.

What do the faecal worm egg count results mean?

 0 <50 EPG 

The sign < means ‘less than’, so a result of <50 epg means that the person who analysed the faeces could find very little or no trace of worm eggs when the sample was tested. (epg means ‘eggs per gram’). This means that there are very few reproducing worms of the types that can be detected by the test. Or that there was a problem with the sample collection.


If the count is less than 200 epg then it is a very low count and your worming strategies are doing the right thing.


If the count is between 200 epg and 1200 epg it is a medium count and your horse probably needs worming.


If the count is more than 1200 epg it is a high count, the horse needs worming and the worming programme you use needs to be discussed with your SQP or veterinary surgeon. 

Can FWECs help in resistance to wormers?

Resistance normally develops due to the small genetic changes that occur within a worm population, which lead to some worms naturally being more resistant to chemical wormers than others. 

When a horse is treated with a wormer, the worms which have not evolved with this advantage (susceptible worms) are killed, leaving only the worms which are resistant. These resistant worms then pass on their resistance, creating a wormer resistant population. This is an example of natural selection.

Other ways to help with resistance are: 

  • Use faecal worm egg counts to determine whether a wormer is needed and which one to use.
  • Weigh your horse(s) before dosing in order to avoid underdosing.
  • Use tapeworm and encysted redworm tests to determine whether a wormer is needed for either of these parasites.
  • Target specific worms with an effective product at the correct time of year.
  • Ensure pasture management: poo pick fields twice weekly.
  • Avoid over-crowding of fields.

Remember, faecal worm egg counts should form part of your horse worming programme and advice should be sought from your SQP or veterinary surgeon about best worming methods.

You might also like: