A complete guide to the types of worms affecting horses and the drugs effective against them

This article is intended as a guide only – please consult your veterinarian if you require further advice in order to determine the most appropriate programme for your horse. It is very important to work out an annual worming programme incorporating a variety of de-wormers to treat all the different types of worms listed below.

Effective parasite control depends upon both management of grazing to minimise worm egg and larval contamination and the use of wormers to remove parasites from the horses’ intestines. One cannot be adequately effective without the other.

Important tips for de-worming your horse:

• Worm your horse every 6-13 weeks (Tactical Routine Worming)
• Treat any new horse with Fenbendazole or Moxidectin and double dose Pyrantel on day six.
• Use one type of wormer for the entire 12 month grazing season to avoid build up and resistance to wormer use (beginning in autumn)  and change to a new chemical group for the following season (Moxidectin, Ivermectin or Membendazole)
• Worm foals at 4-6 weeks with Ivermectin then every eight weeks, before the worms reach maturity
• Read the dosing instructions on the wormer packet carefully.  Don't use more than the recommended amount for your horse’s weight.
• Use diagnostic tests to find out whether your horse needs worming.
• De-worming should be part of on-going grassland management programme (picking up droppings)

Types of common Equine worms (UK):

Bots (Gasterophilus species)

Bots are insect larvae of horse bot flies and are the most common parasite of the stomach which affects horses of any age. The eggs are laid on the legs or around the muzzle of your horse. The eggs are then ingested by the horse and burrow into the base of the tongue and below the gum line. They spend around a month within the mouth where they double in size, the larvae then move on and attach themselves to the stomach lining where they develop into adults. Bots can cause ulcers in the stomach wall and can even penetrate the wall with fatal results. 

 Left untreated bots remain inside the horse until spring when they pass out in the droppings, they then pupate underground until the adult flies emerge in the summer months and begin to lay their eggs (and the whole cycle starts again). Freezing temperatures (winter frost) can kill the adult fly which generally prevents re-infection occurring over the winter months.

• Use Ivermectin in December, ideally after the first frost.

Large Mouthed Stomach Worm (Habronema muscae)

Adults live in the stomach and lay eggs which are passed out in the droppings, they then hatch and are picked up by fly maggots feeding in the droppings. The worm larvae develop within the maggot and are then found in the mouth-parts of the fly. Horses become infected when they swallow the larvae that are released from the flies as they feed around the lips, nostrils and wounds of horses. Larvae can also cause sores if deposited around wounds, although they will be unable to complete their life cycle.

• Use Ivermectin or Fenbendazole every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

Large Redworms (Strongylus species)

Large redworms are not very common and have almost been eradicated in horses within the UK. The larvae are eaten during grazing and then burrow in and out of the walls of the arteries that supply blood to the gut of the horse. The resulting damage can lead to colic and sometimes death. The adult worms are approximately 2 cm in length and feed off the intestinal lining which can cause internal bleeding and anaemia. They are reddish in colour due to the blood they suck from the intestine. The complete life cycle is around 6 months and can affect horses of all ages.

• Use Ivermectin or Moxidectin (October – December)

Large Roundworms (Parascaris equorum)

The large roundworm is the largest parasite reaching up to 30cm in length. The eggs are protected by a thick sticky outer coat that enables them to survive for years in the field and stables, horses are infected by swallowing the eggs when grazing or ingesting them when inside the stable. The larvae migrate through the bloodstream to the liver and lungs before returning to the small intestine to develop into adults again, these can be present in the foal's small intestine from 12 weeks of age.

Adult worms can prevent the foal from putting on weight and can lead to emaciation. Heavy worm infestation can also block the gut causing digestive problems, and in rare cases a blockage or rupture of the gut which can be fatal. Migrating larval stages can cause fever, coughing and loss of appetite as they move through the lungs. Foals normally develop their own immunity to these worms at around 18 months of age.

• Use Ivermectin, Pyrantel, Fenbendazole or Moxidectin every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

Lungworms (Dictyocaulus arnfieldi)

Infective larvae are ingested when the horse is grazing and then migrate through the blood stream to the main airways where they develop into adults. Eggs are laid, then coughed up and swallowed before passing out through in the droppings.

Lungworm causes respiratory problems and persistent coughing in adult horses. Symptoms are rarely seen in foals or donkeys (which can act as carriers for the parasite).

• Use Ivermectin every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

Neck Threadworm (Onchocerca species)

The larvae (which are called microfilariae) live under the skin and are ingested by midges as they feed, they can also congregate in the eye tissue causing infections. The adult worms live in the tendons and ligaments.

• Use Ivermectin every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

Small Redworms (Cyathostome species)

Small redworms are extremely common due to their resistance to de-worming products. Small redworms are approximately 0.5- 1.5cm long and live in the large intestine, they are grey/white in colour but appear red when feeding due to the ingested blood. Without an effective preventive worming programme your horse’s turn out is likely to become heavily contaminated with infective larvae. Once eaten, the larvae of small redworms burrow into the gut lining until they mature. They then emerge from the gut wall to feed and move back into the gut to become egg-laying adults .The life cycle is complete in approximately 10 weeks.

As autumn approaches an increasing number of larvae become encysted in the gut wall where they stop developing and hibernate. These 'inhibited larvae' can make up a large proportion of the population in the gut wall at this time of year. When the larvae emerge (December-May) they can cause severe damage to the gut leading to diarrhoea, rapid weight-loss, colic and sometimes death. Small redworms can affect horses of any age.

• Use Fenbendazole (5 day course) in February to remove larvae acquired over the winter and in November (optimal time to treat).

Pinworm (Oxyuris equi)

Pinworms are small, white (round) worms which live in the large intestine and around the anus. The females are approximately 10cm in length, they lay their sticky (yellow) eggs on the skin around the outside of the anus which causes intense irritation. Horses become infected by ingesting the eggs that are rubbed off in the stable or field. Rubbing and biting at the base of the tail are often signs of pinworm infestation. Pinworms can affect horses of any age.

• Use Ivermectin, Pyrantel, Fenbendazole or Moxidectin every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

Stomach Hairworm (Trichostrongylus axei)

These worms are approximately 7mm in length and live in the stomach causing irritation and damage to the stomach lining. Heavy infestation causes diarrhoea and loss of appetite. Hairworm can be controlled by routine worming for redworms.

• Use Ivermectin or Fenbendazole every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

Tapeworm (Anoplocephala perfoliata)

Tapeworms are made up of segments containing eggs; they are cream in colour and can grow up to 10cm in length with an average life cycle of 3 months. The adult worms attach themselves at the junction of the small and large intestine. Tapeworm eggs are passed out in the droppings and eaten by a microscopic mite which lives in the field, horses become infected during grazing by eating the mites containing the immature tapeworms, which can lead to colic. The mites can remain active in the winter by surviving in hay and bedding, therefore, re-infestation can take place throughout the year. Tapeworms can affect horses of any age

• Double dose Pyrantel every six months (March & September)

Intestinal Threadworms (Strongyloides westeri)

Threadworms are small in size and live in the small intestine of young foals which can become infected soon after birth usually through the mare's milk. The life cycle is very short and foals as young as 4 weeks of age can develop heavy infestations which can damage the intestinal lining causing diarrhea, loss of appetite and dullness. Foals usually develop a natural immunity to infection with this worm by around 6 months of age.

• Use Ivermectin or Pyrantel every 6-12 weeks for the 12 month grazing season (Tactical Routine Worming)

The table below shows the ingredients in wormers that treat each of the different equine worms