Devising a successful worming programme for your horse

An effective worm control programme should form an essential part of every horse owner’s health care program. But the wide range of branded wormers on the market, with their different drug components and differing recommended dosing programmes can make it hard for us to understand exactly which wormers we should use at different times of the year. How frequently should we worm our horses? Should they all be dosed together? Which horses are at higher risk of worm-related disease? These and many more worming questions are addressed below by our veterinary expert – Nikki Walsh from Greenmount Equine Hospital.

Parasite control in horses is a complex issue with resistance to our mainstream drugs becoming increasingly common. Many studies documenting resistance on a farm have been associated with a history of very frequent deworming, especially with the same class of dewormer. Baseline FECs (faecal egg counts) are essential to making an informed worming programme and are necessary to establish whether there is resistance to parasites on your premises. If herds are “blanket treated” with dewormers, the only parasites left will be those who are resistant to the wormer used, thus inadvertently selecting for a resistant parasite population. Therefore, strategic worming with differentiated groups and in accordance with FECs is key to ensuring the efficacy of wormers in the future.

Firstly, there is an important distinction to be made when discussing equine deworming – the difference between treatment and prevention. All horses have a natural level of tolerated parasites in their system, it is only when that level becomes excessive that clinical signs manifest. Worming programmes are part of the prevention of clinical disease, not the treatment of it.


Horses who are clinically affected by worms can present with an acute or chronic form of the disease. Acute disease is associated with very heavy worm burdens that result in sudden onset of colic or colitis. It is commonly associated with horses with a poor worming history that are then treated resulting in damage to the gut or the system being overwhelmed with dead worms. Chronic disease is generally as a result of poor worming programmes that are ineffective, so that there is a high level of worms that cause dysfunction of the gut. It can also be associated with horses who have a very low tolerance of worms. It is usually attributed to small red worms or tapeworms in younger horses.



If your horse presents with clinical disease, I strongly recommend that you consult your vet in regards to a treatment regime tailored to your horse. Bloods can be taken to determine the severity of the disease and FEC can be performed which allows us to monitor the efficacy of treatment. Usually these horses need a more intense treatment programme, often using a combination of wormers and anti-inflammatories.



A good worming programme is about prevention of heavy burdens not the eradication of parasites, the aims of which are as follows:

* Minimise risk of parasitic disease
* Reduce transmission between horses
* Slow the development of resistance to worming drugs.

To allow targeted, effective worming, horses need to be sorted into groups in accordance with their level of risk of clinical disease. Also it is imperative that pasture contamination is kept to a minimum through good management.


High risk horses:

  • High density grazing
  • New horses introduced to the group often
  • Horses under 3 years in the group
  • Droppings are rarely picked
  • High shedders within the herd

NOTE: horses under 3 years and foals are always high risk

Low risk horses:

  • Mature horses (over 3 years old)
  • No more than 2 horses per acre
  • New horses are rarely or carefully introduced
  • Droppings are picked twice a week.

Once this is established the appropriate worming programmes can be implemented (see recommended horse worming programmes). To ensure the efficacy of the worming regime FEC can be used to confirm the success of the treatment. FECs are collected the day before and 10-14 days after deworming to determine if the drug is still effective against parasites.

An effective drug should reduce the egg count as measured in the “pre” sample by at least 90%. Genetics play an important role in determining a horse’s “natural” resistance or susceptibility to parasites. FECs can be used to identify which of your horses on pasture are your high, moderate and low egg shedders. This allows us to construct a strategic de-worming programme with the high egg shedders requiring more frequent deworming treatments than the low egg shedders.


The aim of good management is to reduce contamination of your pastures using the following measures.

  • Stable horses for 48 hours after worming.
  • If possible remove droppings from the pasture (twice weekly being the recommended frequency)
  • Resting periods for pastures of at least three months
  • Mixed species grazing (cattle/sheep)
  • Low numbers of horses grazing together, particularly younger horses.
  • Quarantine for new horses – Fenbendazole for 5 days, tapewormer on the 6 day and a moxidectin on the 7th Keep the horse stabled for 24 hours post last day of worming.

If you are unsure of a horse’s worming history, a 5 day fenbendazole course prior to worming with another wormer is strongly advised.

So the main take home message is that worming should be a targeted regime. It is about the prevention of clinical disease not the eradication of parasites. Consultation with your vet, good management and a strategic approach can not only save you money but can help slow the development of resistance in our worm population.

Read on for detailed horse worming programmes for low risk horses, high risk horses and details of the worming drug contained in leading brands of wormers. If you have any questions related to worming your horse, please leave a comment below and Nikki will do her best to answer it for you.



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