Worm control in horses is becoming a real cause for concern in the UK, with increased resistance reported for most currently available anthelmintics (‘wormers’). For many years it was rare to see clinical disease associated with worm infestation, but over the past few years it has again become increasingly common. Together as vets and horse owners we urgently need to respond and alter our approach to worm control before we reach crisis point.

This is a confusing topic for vets and owners alike…. a literal ‘can of worms!’.  Sadly, there is no ‘one size fits all’ approach as age, breed, health status and management practices all need to be considered for each horse. There are multiple names for different worms and each has their own specific life cycle that must be considered in order to target control effectively.

What worms do we have?

  • Small Redworm (Cyathostomins/small strongyles): These worms are our biggest concern. As part of their life cycle the larvae burrow into the gut wall in winter (encyst) and emerge as adults in the spring. If there are high numbers, this can result in a massive inflammatory response (Cyathostomiasis) which causes diarrhea and weight loss and can prove fatal. Youngstock and elderly horses are most at risk.
  • Large Redworm (Large Strongyles): The larvae of these worms migrate through various organs and blood vessels and can cause a variety of symptoms including colic. Their numbers have been vastly reduced over the past forty years due to modern worming practices but although still a rare cause of disease, they are gradually making a return.
  • Tapeworms: These rarely cause disease but may be associated with colic, particularly in youngsters.
  • Ascarids (Parascaris/large roundworm): These large white worms can cause disease in youngstock while adult horses acquire good immunity. The larvae migrate through the lungs and can cause coughing. Adult worms reach up to 40cm in length in the gut and high numbers can cause weight loss and impaction.

What wormers do we have?

  • Moxidectin (e.g Equest and Equest Pramox): The only horse wormer with efficacy against encysted small redworm. However, resistance to Moxidectin has recently been proven in the UK.
  • Ivermectin (e.g Eqvalan): This has good efficacy against adult redworms but no efficacy against encysted redworms. Resistance is increasing in Ascarids.
  • Fenbendazole (e.g Panacur 10%): Historically used to treat encysted redworms, however, there is proven resistance in the UK. It is now very unlikely that the five-day course has efficacy against encysted larvae. However, it does have good efficacy against Ascarids in youngsters at a dose of 1ml/10kg as a single dose.
  • Pyrantel (e.g Strongid-P): This has reasonable efficacy against all adult worms but does not treat encysted redworms. At double the dose it has good efficacy against tapeworms.
  • Praziquantel: This has efficacy against tapeworms only.

What’s the problem?

  • Due to years of overuse, there is now proven resistance to ALL of the wormers that we have available to treat small redworms. There is also resistance in Ascarids.
  • If Moxidectin resistance becomes more widespread we will have no way to treat larval Cyathostomiasis which may prove fatal.
  • There are no new drug formulations on the horizon.
  • Climate change means that worm lifecycles are becoming less seasonal and therefore harder to predict.
  • Drug residues in the environment are damaging invertebrates and aquaculture that are essential to ecosystem

What must we do?

We must limit the use of wormers by targeting those horses that actually need them.

In the past the accepted advice was to give all horses a dose of Moxidectin in the winter to remove any encysted redworms. Now we are seeing evidence that this approach has made the problem of resistance worse. We need to target only animals at risk.

To assess the risk, we can carry out regular worm egg counts every 2-3 months throughout the year and only treat if counts are high. If egg numbers are low all year, then larval numbers will also be low come the winter and Moxidectin treatment may well be unnecessary.

There are also now blood tests available that measure antibodies to small redworms and tapeworms. These give us a picture of the horse’s exposure to worms in recent months and aid in our ability to risk assess each horse.

Ideally all adult horses would have a worm egg count and a blood test in the Autumn. This approach should greatly reduce unnecessary treatment.

Youngstock (<3 years old) and elderly horses, particularly those with Cushing’s disease, should continue to have targeted Moxidectin treatment in the winter unless advised otherwise.


  • PICK UP THE POO: The importance of this simple action cannot be overstated. The only way to combat resistant worms is to break their life-cycle by removing the droppings from the grazing frequently. This is essential especially where youngstock are concerned.
  • Winter means winter: If you are advised to use Moxidectin then only use it once it has been cold and frosty. If it is still mild the larvae will not be encysted.
  • Don’t believe the hype: The spectrum of action on the wormer box may not reflect current research.
  • Avoid multi drug products. If treatment is needed, it should be targeted as specifically as possible. Praziquantel for tapeworm in single drug form is available through our practice.
  • In most cases we would only treat egg counts of >400 eggs per gram in adults.
  • Don’t be tempted to treat ‘just in case’. In adult horses worms are an unlikely cause of weight loss. Give us a call prior to treating.

We are always happy to chat about worm control with you. To make it as easy as possible we have a number of worm testing packs available, including yard packs and annual packs. These contain all the pots, gloves, packaging and postage required for regular worm egg count testing.  If you would like to discuss a worming plan for an individual horse or for a yard of horses, then please do get in touch.