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Worming | Treatment & Control

May 2021

It is really important to look at multiple ways to control worm burdens in our horses and if a treatment against worms is required, ensure you use a targeted approach to worm control. Frequent interval worming has resulted in 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 dewormers available.

Which drug targets each worm:



  • Small Redworm*
  • Large Redworm
  • Roundworm
  • Pinworm
  • Lungworm
  • Threadworm


  • Small Redworm
  • Large Redworm
  • Roundworm*
  • Threadworm
  • Pinworm
  • Lungworm
  • Bots


  • Small Redworm*
  • Large Redworm
  • Roundworm
  • Tapeworm - Double dose required
  • Pinworm
  • Threadworm
  • Lungworm


  • Tapeworm only


  • Small Redworm
  • Large Redworm
  • Roundworm*
  • Threadworm
  • Pinworm
  • Lungworm
  • Bots

Praziquantel & Moxidectin

  • Small Redworm
  • Large Redworm
  • Roundworm*
  • Threadworm
  • Pinworm
  • Lungworm
  • Tapeworm
  • Bots

*Research has shown these worms are resistant to the drug base1.

Important questions to consider before the appropriate treatment decisions can be made include2:

  • Is there a medical reason for treating the horse?
  • What worm are you trying to reduce?
  • What level of that worm is present?
  • Why did you pick this wormer?
  • Will this wormer kill the specific worm you are targeting?
  • How confident are you that this drug will work as expected?
  • Are there better options ‐ is this the best choice for the horse currently?
  • Are there any additional management techniques, as highlighted below, that might help to achieve the goal of a reduced worm burden?

Worming your horse

If following the appropriate testing worming is necessary, the video below will take you through the correct steps to worm your horse safely and accurately.


Equine worming treatments may pose a risk to your pets

Although some of the ingredients routinely used to worm horses are also used to worm cats and dogs, they are usually given in far smaller doses in these species and can therefore prove poisonous if cats or dogs lick or eat even a small amount of equine worming product.

Cats or dogs who have ingested horse wormer may become unwell with signs including drooling, vomiting, difficulty breathing, poor coordination, trembling, blindness, seizures and in severe cases, they may not recover unless urgent veterinary treatment is received.

Dog Wormer 
Photo credit: Red Bluff Veterinary Clinic

The best way to keep your cats and dog’s safe is to keep them out of the way when your horses are being wormed and ideally until the stables are mucked out afterwards. It’s important to clean up any spillages or any wormer that your horse spits out. Always be sure to dispose of used worming syringes in a sealed bin and make sure any new or unused product is stored securely.

Whilst uncommon, it is also possible for dogs to become unwell from eating manure from a recently wormed horse. Dogs who tend to eat large quantities of horse manure are most at risk. However, some dogs belonging to certain breeds, for example Collies and other herding breeds, can carry a genetic mutation making them very sensitive to ivermectin-based wormers and these dogs can show signs of poisoning even when a small amount is eaten. Where possible, dogs should therefore be discouraged from eating horse manure to reduce this risk.

If you ever suspect your pet has eaten equine wormer or they seem unwell having been around your horse at the time of worming, contact your vet for advice early.

Always use a species-specific wormer as directed by a veterinary surgeon and even if the active ingredient appears to be the same, avoid sharing products between different species to avoid accidental overdose.

Written credit: Christina Kuhl, Veterinary Surgeon, VetPartners


Mare & Foal worming programme

The mare should be treated with a dose of an ivermectin or moxidectin based wormer 4 weeks prior to foaling, this will also provide passive immunity to the foal.

It is advised not to worm the mare in the first 2 weeks after foaling as this can affect the mare’s milk and as a result impact the foal. Continue testing the mare at 3 monthly intervals and worm where necessary.

Advice on the foal’s worming regime can vary so we would always recommend speaking to your vet for guidance on your specific circumstance. In general, the following principles apply:

  • At 4-6 weeks treat the foal with a dose of a fenbendazole or ivermectin based wormer. Continue to worm at intervals advised by your vet until the foal is 6 months old, monitoring with faecal worm egg counts when worming is due for best practice.
  • From 6 months of age a regular worm testing programme should be maintained, only worming if needed. Speak to your vet for advice on testing intervals.
  • Seek veterinary guidance on the appropriate wormers to use at each stage.

Minimising the worm burden

Worm control doesn’t just consist of targeted worm programmes. Careful management practices can massively reduce the risk of problems arising and break the lifecycle of the worms.

Some practices to follow include:

Manage the fields by picking up droppings at least twice a week, or ideally every day.
picking up

If possible, rotate paddocks for rest periods
Rotate Paddocks

Test new horses arriving on the yard and treat if necessary, prior to mixing with any other horses.
horse syringe
Photo credit: Jon Stroud

Putting foals onto ‘rested’ pasture (land that hasn’t had any other horses on it for a period of time, ideally 12 months).
mare and foal

Don’t over-stock your paddocks
Credit Charolotte Pardon
Photo credit: Charlotte Pardon

Dose foals correctly, based on veterinary advice.

Cross graze your pasture to help break the worm cycle. Sheep/cattle ingesting equine parasites will halt their lifecycle without having any adverse effect on those animals. All animals on the land will need a worming programme for their own welfare.
Cross Grazing

Treat each horse as an individual. Requirements may differ, even between horses on the same yard.
horse heads

Keep records and an up to date schedule.

Resistance to wormers

This is a state that occurs when a proportion of the worms picked up on the pasture are no longer killed by the wormer.

Just as unnecessary antibiotic treatment will promote drug resistance, the unnecessary use of wormers is leading to wormer resistance. It is important we move away from the practice of worming our horses at frequent intervals as a method of parasite control. This is a practice based on concepts which are decades old, and we now know that targeted worming and appropriate testing are the best methods for effective worm control in our horses1.


Why does resistance develop?

Horses naturally have a worm burden with a small number of resistant worms. This does not cause health issues, but rather helps improve immunity, to protect the horse from establishing a more serious worm burden.

A licenced wormer works to kill all but the small number of resistant worms.

We can measure resistance by using faecal worm egg counts and other tests to determine if any treatment is required.

Like all living species, worms are subject to natural selection. If we continuously worm our horses without testing, the number of resistant worms increases which are then able to continue to reproduce3.

As a result, worms on the horse’s pasture have become resistant to the wormers and no longer respond to treatment. Once this develops worms never regain their sensitivity to drugs.

The more we unnecessarily worm, the faster resistance will develop.

Impacts of resistance

  • A reduced stocking density, meaning less animals will be able to be kept on a certain area of pasture.
  • Only five chemical drugs are licenced to treat horses in the UK for worms. Widespread resistance is already documented to some chemicals.
  • If worms become resistant to all drugs, then horses would be at high risk of disease and death from untreatable worm burdens, for example intestinal related colic, colitis, haemorrhages and anaemia.

What can we do to slow resistance?


1) Dijk, J, V. (2021) Using worm egg count data to detect and counter trends in equine helminth abundance. Vet Record. 188(5).

2) Kaplan R. M & Nielsen, M.K. (2010) An evidence-based approach to equine parasite control: It ain’t the 60’s anymore. Equine Veterinary Education. 22(6) P.306-316.

3) Nielson, M.K., et al (2010). Practical aspects of equine parasite control: A review based on workshop discussion consensus. Equine Veterinary Journal. 42 (5) P 460-468.

4) O’Meara, B & Mulcahy, G (2002). A survey of helminth control practices in equine establishments in Ireland. Veterinary Parasitology. 109(1-2) P 101-110.


horses grazing


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