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How to worm your horseHow to worm your horse

How to worm your horse

Why you need to worm your horse: parasitic worms can adversely affect the health and well-being of horses and ponies of all ages. Internal parasites damage the gut and other organs and can be responsible for poor body condition, colic, and occasional fatalities.

General tips

Follow a strategic worming programme – Mole Valley Farmers produce calendars which outline a number of different programmes. These incorporate specific treatments required at certain times of the year for tapeworms, bots and encysted small redworms.  By following such a plan you will ensure that you are using the correct active ingredient and not just swapping brands. (If in doubt please consult the Suitably Qualified Person in Branch).

Worm all new arrivals – keep new horses stabled for 48 hours after worming to ensure that any eggs are passed before being turned out to avoid contamination of the pasture.

Follow the manufacturer’s recommendations – Dosing intervals vary from 4, 8 and 13 weeks depending on the active ingredient. Always read the box and enclosed product leaflet before use.

Dose your horse accurately according to its weight – Do not guess your horse's weight, a weighbridge is the most reliable method to determine the weight, but a weigh tape will give a rough guide.

Worm pregnant mares before and after foaling – Foals have a low resistance to worms and can quickly acquire massive worm burdens. Worming should start between 4 to 6 weeks of age.

Worm at the same time – all horses in the same yard/pasture should be wormed at the same time with the same product. Keep a record of when you worm and what you use.

General tipsGeneral tips


Targeted worming

Some horse owners are keen not to administer active ingredients to their horses if the horses' worm burden does not necessitate.

Tapeworm antibody tests

Tapeworm antibody tests are necessary to determine tapeworm burdens in your horse. A blood sample from your horse is required for this test.  Further information regarding these tests is available from your veterinary surgeon.

Faecal worm egg count kits

 It is important to worm only when necessary, as using less drugs will slow down the inevitable resistance that develops from overuse of the worming products.  


• If using an oral syringe, first identify the correct dose then remove the cap.

• Guide the syringe into the corner of the horse’s mouth and aim it towards the back of the tongue before administering the wormer.

• It may be necessary to hold the horse’s head up briefly to ensure that he has swallowed the wormer.

• If using granules, mix the dose into part of the horse’s ration and tempt him to eat it, using succulents if necessary.

• Once eaten, the remaining ration can be fed.

Pasture managementPasture management

Pasture management

Effective pasture management is a major part of worm control.  Remove droppings from your pasture regularly - at least twice a week during the grazing season. A 450kg horse produces 5-12 pats or approx 24kg of dung a day, that's around 10 tonnes a year.

Do not allow pasture to become overgrazed and horse sick.  Ideally, fields should only contain 1-2 horses per acre.

If possible divide paddocks into smaller areas so that they can be rested to reduce pressure on the pasture.  It also makes it easier to remove droppings.  Resting pasture is a good option but worm larvae can live for many years both on pasture and in the horse, so it does not guarantee it will be worm free.

Graze pasture with cattle/sheep. Worms that affect horses are generally host specific and cannot survive in cattle or sheep.

Any larvae eaten by sheep or cattle will be destroyed - known as the 'biological vacuum cleaning effect’.

Ideally, foals should not graze with older horses, as they are a major source of pasture contamination and require more regular worming.

Harrowing is only advisable in hot, dry conditions where exposed worms are killed by the heat. In damp conditions, it just spreads worm eggs and larvae over the pasture increasing their chance of being swallowed by the horse.

General husbandry

• Do not feed horses on the ground.

• Provide suitable feed containers.

• Do not re-feed hay that has been dragged through soiled bedding.

• Keep all feeding utensils clean.

• Keep water buckets clean.

• Remember worm eggs are hardy and can survive for many years in stables. Stables should be kept clean and ideally scrubbed or pressure washed regularly.

General husbandryGeneral husbandry

Types of parasite

Large Redworms
This parasite is potentially the most dangerous internal parasite to horses. The immature stages migrate through the blood vessels supplying the gut. Disruption of the blood supply to the gut as a result of larval damage can cause colic and in rare cases, sudden death.
Small Redworms
The most common and numerous of internal parasites. During the winter months some of these worms hibernate deep within the gut wall. The subsequent emergence of these worms in the spring can cause damage to the gut wall leading to loss of condition, diarrhoea, colic and in severe cases death.
Large Roundworms
The largest internal parasite to affect horses. Mainly found in young horses and foals. Horses develop a natural immunity to this worm by 18 months of age. ymptoms include coughing, poor growth rate and dull coat. The worm’s size can cause a fatal rupture or blockage of the gut. Large roundworms also migrate to the lungs and can cause respiratory problems in foals.
During the first few weeks of life foals are susceptible to this very small worm which can cause severe diarrhoea. Mares should be wormed around the time of foaling and foals may need worming from four weeks old.
The adult worms migrate to the horse’s rectum where they lay their eggs on the skin outside the anus. This causes intense irritation and scratching.
These commonly affect donkeys but can also affect horses, usually when grazing with donkeys. Infected horses show clinical respiratory signs such as persistent coughing.
Found in the large intestine and congregate around the narrow junction of the small and large intestine. Most infections do not necessarily produce obvious signs of ill health; however they can cause digestive disturbances, loss of condition and are strongly associated with colic.
Bot flies are a common irritant to horses at grass. They lay their sticky yellow eggs on the horse’s fore legs and around the head. The horse licks them off the legs and the larvae eventually reach the stomach where they attach to the lining. Large numbers can cause digestive disorders and occasionally perforation of the stomach.