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Worming Advice - Updated Nov 2022

All horses have a degree of gastrointestinal parasitism which is more commonly known as a worm burden. High parasite burdens can cause poor body condition, loss of appetite, poor performance, weight loss, colic, and diarrhoea. If this is extremely severe, it can be fatal. A very low worm burden may even be beneficial for the horse’s immune system.

Traditional worming protocols have suggested worming all horses every 6-8 weeks regardless of worm burden. This means horses are treated unnecessarily with dewormers. Overuse of deworming drugs may make parasites resistant to the medication.

At Ashbrook Equine Hospital, we always recommend that a targeted deworming programme is used. In this programme, only horses with moderate or high worm burden shown on laboratory test and therefore those likely to become ill are treated. Within a population of horses, only 10-20% will require deworming.

The benefits of a targeted deworming programme include:

• Only deworming those that are at risk of disease

• resistant to deworming treatments

• Cost effective

• Lower environmental impact

• Only deworming those that are at risk of disease

• resistant to deworming treatments

• Cost effective

• Lower environmental impact

Testing for parasitism

At Ashbrook Equine Hospital, we recommend faecal worm egg counts (WECs) at approximately 3 monthly intervals during the year.

Sampling procedure

• Collect a golf ball sized faecal sample into a sample bag

• Label the sample bag

• Drop the sample into the clinic for laboratory analysis, along with contact details and date of last worming

• If there is delay from collection to laboratory analysis, the sample can be refrigerated

• One of our vets will then advise you of the results and whether deworming is required

Watch our short video of how to collect a sample here

Important points


Faecal worm egg counts do not assess for tapeworm burden due to intermittent shedding of their eggs. Tapeworm levels can be investigated using either a blood test or a saliva test. Routine treatment for tapeworm is no longer recommended due to increasing levels of worm resistance.

We recommend a saliva test in late autumn/early winter to determine if treatment is required. In higher risk situations, we may recommend doing this twice yearly. Testing must be carried out at least 4 months after the last tapeworm treatment.

Small redworm

Small redworm become encysted (burrowed) into the gut wall during the larval development phases. Emergence from the gut wall usually occurs in spring and can cause a potentially fatal condition called cyathostomiasis.

Cyathostomiasis is most commonly seen in youngsters and causes severe diarrhoea. Only moxidectin or a 5 day course of fenbendazole is licensed to treat encysted redworm. Historically, this has been given in late autumn/early winter, although due to new evidence, this should now be reserved for horses with increased risk. There is a blood test available to identify horses with high levels of red worm larvae, which can be discussed with your vet.

Special Considerations

Broodmares, youngstock and donkeys all have specific requirements and, as such, each individual situation should be discussed with a vet for appropriate advice. However, if they have had low worm egg counts through the year, their risk of disease will be low.

Management strategies

Paddock management is an essential part of parasite management on any yard. Collection of droppings twice weekly is vital for good worm control. Worms can migrate so siting the muck heap away from fields is also recommended. Rotational grazing and mixed species grazing (e.g. sharing with cattle or sheep) can help reduce the worm burden on a property. Harrowing or spreading muck can unfortunately spread the worm larvae further and increase the risk of horses eating large amounts of infective larvae.

Over-stocking or grazing with youngstock can increase the risk of higher worm burdens. Horses younger than 5 years are more likely to have higher worm burdens than adult horses as immunity to parasites increases with age.

Once wormed, horses should not be moved onto clean pasture for two weeks as this means all the resistant worms then infect the clean grazing.

New Arrivals

Horses that arrive with no known history of worming should be wormed with moxidectin and praziquantel combination and then stabled for 3 days (remember new arrivals should be isolated for infectious disease for approximately 2 weeks).


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