Equine Worming


Equine worms are parasites; organisms that live in or on another organism, the host (your horse) and benefits by taking nutrients at the host’s expense. The most common internal parasites affecting horses are small and large redworm, large roundworm, tapeworms, pinworm, bots and lungworm.

Small Strongyles (Redworms)

Cyathostomes pose the greatest threat to horses, especially with the increase in resistance to equine wormers developing.

The adults live in the large intestine and absorb nutrients from the food that the horse eats. If present in very large numbers they can cause weight loss and diarrhoea, however usually they do not cause a problem. More importantly the larvae form cysts inside the wall of the large intestine starting at the end of the summer. These accumulate over the autumn and then re-emerge in the late winter/spring causing a large amount of damage to the intestine wall. This re-emergence causes a large amount of damage to the intestinal wall with signs including severe diarrhoea, dehydration and often death.

This parasite can be controlled by routine worming from February to October at intervals dependent on the drug used. However this method of control encourages the development of drug resistance in equine parasites at a rapid rate and good pasture management and faecal worm egg counts should now be used.

Faecal worm egg counts: Faecal worm egg counts provide a way of determining whether or not a horse or donkey has adult Small Strongyles present in the intestine and needs worming with an appropriate wormer at an appropriate time. They help decrease the development of resistance in equine parasites to wormers by decreasing the routine usage of wormers in horses when they are not needed.

Faecal worm egg counts should be performed from approximately February to October. Faecal samples should be collected every 6 weeks to start with and then this may be decreased accordingly if the results are negative.

We perform in house faecal worm egg counts, please call for more information. Faeces should be freshly passed, packed in an airtight container/bag and then kept refrigerated.

Unfortunately faecal worm egg counts do not detect encysted larvae so good management and a good worming programme are needed. Poo-picking twice weekly will interrupt the lifecycle of the parasite and help prevent the development of encysted larvae. It is also advisable that all young horses less than 5 years old or others at particular risk are treated for this type of worm at least once a year, even with negative faecal egg count results.


Anoplocephala, the equine tapeworm can cause considerable problems in some horses. The adult parasites live in the ileo-caecal junction of the intestine where they attach to the gut wall causing inflammation and decreased gut motility. This can increase the risk of colic either due to intestinal obstruction or rupture.

The lifecycle of the parasite involves the ingestion of mites containing the parasite by horses. These mites are found on almost all pastures and therefore also hay and haylage so tapeworm should be considered for all horses even if they are not at pasture.

A blood test is now available to gauge a horse’s previous exposure to tapeworm. Reliable control can be achieved by using a wormer containing either praziquantel or using a double dose of pyrantel twice a year, once in the spring and once in the Autumn to kill the adult.

Large Strongyles

Although Large strongyles are a serious problem when a horse is infected, fortunately they are rarely seen now. The larvae burrow through the gut wall and migrate through blood vessels and the internal organs causing irritation and damage and then return to the intestines when they mature to adults. Signs of infection include colic, fever, anorexia and weight loss.

These parasites are sensitive to most types of wormer so that worming programmes designed to control other parasites will usually be sufficient to control them.


Oxyuris equi, the equine pinworm is usually seen in youngsters but is relatively uncommon. The adults live inside the rectum of the horse and lay their eggs in a very sticky, itchy material on the horse’s backside and tail. When scratching the horse rubs the eggs off onto all surfaces and the eggs are then ingested by other horses mouthing the same areas. The signs of infection associated with the severe itching are weight loss due to decreased eating and destruction of tail hairs.

This parasite is controlled by routine worming and cleaning of areas used by horses to scratch.


Gasterophilus flies lay their small yellow eggs on horse’s coats, usually on the legs in the spring and summer. When the horse is grooming it ingests the eggs which hatch into larvae and attach to the stomach. They usually don’t cause any clinical signs but can cause ulceration and mild colic.

Most worming programmes will control this parasite, however grooming and removal of eggs can also help to control the problem.


Parascaris equorum is the equine roundworm that usually affects youngsters as horses normally develop immunity to by 6 months old. Signs of infection include poor weight gain, a pot belly, colic and respiratory signs in very heavy infections.

The best method of control is prevention of infection by grazing horses under 6 months old on clean pasture (not grazed by foals for 1 or 2 years) and regular poo-picking. However foals should also be wormed from 2 months every 2 months through the first summer.


Dictyocaulus arnfeldi, is only likely to infect horses that graze with donkeys, causing respiratory signs.


  • Only worm when necessary:
    • Monitor your horses adult worm burden with faecal worm egg counts and only worm when needed.
    • Don’t worm at unnecessary times of year.
  • Use the correct dose:
    • Ensure you know the correct weight of your horse (use a weigh tape.)
    • Ensure that your horse is administered the whole dose.
  • Rotate the type of wormer used yearly (ensure the drug is different.)
  • Poo-picking should be done ideally twice weekly.
  • New horses on a yard should be stabled and wormed initially.
  • Co-grazing with sheep and/or cattle can help decrease the number of parasites on the pasture.

We can provide advice and information for your individual worming programme based on faecal worm egg count results, number and ages of animals and pasture management.

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