Worming: All horses require management to prevent worm burdens.  Understanding the lifecycles and different kinds of worms will aid in preventing these burdens.   Anthelmintic (worming treatment) resistance is growing so it is important to make informed decisions when treating our horses.


Horses generally become infected with worms, or endoparasites, directly when they graze and consume the worm larvae living in the grass.  A typical roundworm lifecycle is known as a direct life cycle.  Adults living inside an infected horse will lay upto 200,000 eggs a day.  These are deposited in the faeces.  If conditions are good (warmth and moisture) the eggs will develop into stage one larvae, L1, within a few hours.  The larvae live in the pasture, undergoing two moults to form a third stage larva, L3.  When the horse ingests L3 larvae, while grazing, the larvae can develop into adults within the horse and the lifecycle continues. Foals can be infected by mares either across the placenta with intestinal threadworms or through the mare’s milk.  Intestinal threadworm, can also penetrate through the skin to infect horses.

Small Stronglyes:

Small Stronglyes also known as Cyathstomes or red worms, are the most common worm to infect horses.  As the weather cools down the L3 and L4 larvae will become ‘encysted’ in the gut wall, waiting to emerge ‘en mass’ when conditions are good in the spring.  This process is called larval cyathostomosis and the synchronised emergence of large numbers of L4 can cause huge mucosal damage and the symptoms of colic.   While the larvae are encysted they are protected from most worm treatments.  Effective summer worm treatments to keep cyathstome infections low, and well-timed treatment of the encysted stage is important.

Large Stronglyes:

Large Stronglyes, S. vulgari, are also known as ‘blood worms’ as they are the most pathogenic horse parasite.  This is because they not only ingest mucosa and capillaries of the intestine but the L4 larvae can penetrate blood vessels of the intestines and travel these vessels causing major damage and even blockages. Fortunately, large Strongyles are well controlled with modern drenches and are not such a problem today as in the past.

Parascaris equorum are generally only a problem of foals and young horses as adults mount a strong immunity against them.  These are large worms, and can grow upto 50cm.   Another worm that mostly only affects foals is intestinal threadworm, Strongyloides westeri.

Female pinworms, Oxyuris equ, lay their eggs just outside the anus, causing loss of condition, poor appearance, rubbing and scratching of the tail region.  A parasite that will infect horses as well as cattle and sheep is the stomach worm, Trichostrongylus axei.  Lungworm, Dictyocaulus, as the name describes live in the lungs of infected horses.  Donkeys seem to be particularly susceptible and are often a source of infection for horses.


Tapeworms, Anoplocephala perfoliata, have an “indiect” lifecycle, as they develop through their larval stages in an intermediate host, the pasture mite.  When horses ingest the parasite mite, infected with tapeworm larvae, the adult tapeworm can develop.  Horses do not develop resistance to tapeworms so any age can be affected.  They are also intermittently shed in faeces, so tapeworm eggs may not be visible on a faecal egg count.  Mites are ingested when horses graze pasture low ie the end of winter and summer, so treatment at these times is important.


Summer time around horses and the annoying bot fly appears, Gasterophilus intestinal.  The adult flies buzz around and lays eggs, usually on the legs of horses.  The horse then licks the eggs which hatch into larvae and develop within the horse.  Bot eggs should ideally be removed from horse’s legs.


To help reduce worm burdens in horses, ideally faeces should be picked up at least two times a week.  These means larvae do not have time to start to move out of the faecal pile and onto pasture.  Harrowing is not as effective, as it can spread larvae through your pasture, meaning there is no “clean” pasture for your horse to graze.  Horses will naturally avoid faecal piles, and the surrounding grass where larval numbers are high, unless they are forced to overgraze.  Harrowing will be effective in hot and dry conditions, where the faeces are broken down small and any larvae will be desiccated.   Grazing with other species will help reduce burdens, as each species parasite will be consumed and die as they are not in the right host.  The exception is T.axei.

Unfortunately, resistance to drenches is emerging so targeted drench regimes should be used. During the warmer months when worms are completing their lifecycles, faecal egg counts (FEC’s) should be performed.  Ideally FEC should be done every 3 months to assess whether worm treatment is required.  Drenching is required if there are greater than 300 epg.  To test your drench effectiveness a second FEC should be done 10-14d later.  Horses also vary in their susceptibility to endoparasites, some horses tend to have lower FEC’s compared to others even if they are managed the same.  Therefore, horses should be drenched as individuals related to their FEC’s.

A suggested worming protocol:

  • September – Treatment for encysted cyathostomes and tapeworm eg Moxidectin (Equest + tape™ (has persistent activity for 16 weeks)) or drench with Fenbenazole (Panacur™) once daily for 5 days
  • Test your drench effectiveness with FEC, 14 days later
  • Perform FEC every 12-16 weeks and treat if required eg Ivemectin (Eqvalan™ or Promectin™) Abametin (eg Genesis™) or Fenbenazole (eg Panacur™)
  • May – Treat for Tapeworms and bots eg Promectin plus™, Triumph™, Genesis
  • Ideally a second treatment for encysted cyathostomes if infection is of concern eg Equest + tape™
  • Mares should be treated 2-3 weeks before foaling and foals from 8-10 weeks of age.  Foals should be wormed more regularly than other adult horses either every 6-8 weeks or ideally monitored with faecal egg counts.

Katherine Hansen BVSc