Like all livestock, horses and donkeys have external and internal parasites that, if not managed, can cause serious or even fatal organ damage.

A parasite is an organism that lives in or on a living organism of another species (its host). The parasite draws its nutrition from the host and does so at the expense of the host.

The most common internal parasites in livestock generally are larvae and worms of various kinds.

Symptoms of infestation

In equines common signs of worm infestation are:

  • tail rubbing,
  • pale gums,
  • ill-thrift,
  • colic,
  • poor coats (called staring).

If left untreated worms might cause chronic diarrhoea and irreparable damage to internal organs.

parasites in equines
Donkeys and mules are affected by the same parasites as horses are.

Common parasites in equines

In donkeys and horses the most common parasitic species are

  • large and small strongyles (redworms),
  • ascarids (roundworms),
  • pinworms,
  • larvae of bot flies.

Large strongyles

The large strongyle larvae are eaten by the horse and then follow a complex migration, passing through the intestinal wall to the inner lining of small arteries, then moving up to the large arteries supplying the digestive tract.

The larval migrations last five to seven months and the larvae eventually return to the intestine where they become adults.

There they lay eggs which the host excretes. The cycle then to starts again.

Small strongyles

Small strongyles are small redworms usually found in the large intestine. Unfortunately, they are particularly good at developing resistance to deworming products.


Roundworms are a common problem in younger equines which should be treated for this parasite from one month old, so that they hopefully develop immunity by the time they are a year old.


Pinworms are most often a problem of stabled horses. Although pinworms are not a particularly serious parasite, horses that continually rub their tails can develop abrasions which might lead to infections.


Tapeworms have a life cycle that takes roughly six months to complete.


Bots are the larvae of the bot fly. Bot flies lay their eggs on different parts of the horse’s body. They are easy to see as little yellow spots attached to the hairs of the horse’s coat. The eggs are then ingested by the horse as it scratches itself with its teeth. The larvae spend approximately nine months attached to the stomach lining of the horse before passing out with the manure and maturing into adult bot flies.

parasites on equines
Bot eggs on a horse.

The owner should be on constant lookout for the eggs when grooming the horse, as it is easy to scrape the eggs off, using a specialist bot scraper.

Deworming medication for parasites in equines

There are many dewormers available for horses and donkeys. Some of them are broad-spectrum parasiticides, which bring about control for all major worms of equines.

The two main active ingredients found in equine dewormers are ivermectin and praziqantel, sometimes in combination. (It is not recommended to ingest horse dewormer containing ivermectin as a prevention or cure for Covid-19.)

In the “old days” it was recommended that a scheduled deworming programme be followed at regular intervals. However the thinking has changed, because resistance of equine worms to chemical dewormers has become much more common.

Nowadays, many equine owners are managing parasites through seasonal application of dewormers and through improved pasture management.

It makes sense to address the worm problem when infestations are likely to be higher, so the practice is to administer medication mainly in September/October, at the end of December and in April. Treatment against bots should be in June and November.

Consult your veterinarian about the administration of probiotics at the same time as chemical anthelmintic drugs, so as to restore the beneficial intestinal bacteria which are natural to the horse’s intestines.

Pasture management

Good pasture management in the fight against worms includes avoiding over-crowding of fields, rotating and resting paddocks and cross grazing pastures with other species such as sheep or goats.

Owners also need an effective plan for the management of manure. Because parasite eggs are passed out in manure, collecting faeces is by far the most effective way of breaking the parasite cycle. Obviously we clean stables every day. It is also good to collect dung from the fields, if possible twice weekly.

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